About Line Noise:

One of the banes of the VLF/LF/MF listener is powerline noise.  Down at these frequencies (several MHz and below) the energies of various electronic switching devices (such as light dimmers, motor controllers, fluorescent lights, etc.) are very strong.  There are several reasons for this:  Harmonics tend to decrease in energy with increasing frequency, and at these low frequencies, their energy hasn't dropped off by much.  Another reason is that the filters built into devices such as light dimmers just aren't effective below several hundred kilohertz owing to the small effect of the capacitive and inductive reactance at these frequencies.  Finally, these devices are typically connected directly to the house wiring and are therefore conducted into equipment and radiated on inside wiring and powerlines.

Efforts to rid yourself of this type of noise may be hard-fought battles, particularly at LF and below:  A filter that works effectively at LF frequencies is likely to be too large to fit within the fixture or enclosure of the device for which it is intended.

There are several other ways to minimize the impact of powerline-related noise on your listening:

• Listen only during power failures.  (Rare occurrences for most of us...)
• Listen well away from powerlines.  (Not convenient at most of our homes.)
• Careful location of the receive antenna.  Locating the antenna away from noise-generating devices and powerlines will minimize reception of such noise.
• The use of a shielded loop or other H-field antenna.  These types of antenna are less-responsive to near-field E-field noise, often a major component of received noise.  This type of antenna can also be rotated to null the noise source - but an obscure corollary to Murphy's Law states that the desired signal is always in the direction of the noise source.

Assuming that you have tried all of these but you need more help, then a noise blanker may be for you.

Most of today's radios contain something referred to as a "noise blanker."  Those who have used these noise blankers also know that most of them have only limited effect on powerline-type noise and many of them will also cause large amounts of intermodulation distortion in their operation, obscuring weaker signals.  A few noise blankers (such as the one in the Drake TR-7/R-7 or R-4 lines) do work well, but these blankers (as well as others) are affected by nearby strong signals and furthermore, they don't usually work on but the first "layer" of line noise (more on this later.)

A Brief Analysis of Line Noise:
 

Figure 1:
From top to bottom:  Dimmer noise pulses in a 12 kHz, 2.2 kHz, and a 300 Hz filter.  Note the widening of the pulses with progressively narrower filters.  (Division are 2 ms)

Line noise typically occurs on both sides of the power line sinusoid -  that is, it produces pulses at twice the line frequency (we'll assume throughout this page that we are talking about the 60 Hz U.S. power system) at 120 Hz but even so, this noise often has strong 60 Hz components as well.  The result of this is that powerline-related noise produces energy every 60 Hz across the spectrum.  To further complicate matters, AC power is distributed by the utility in three phases, each being 120 degrees from the other phase:  This can seem to "multiply" the number of noise pulses that occur during each cycle of the powerline's waveform.

The noise is produced by the abrupt switching of some device (i.e. a triac in a light dimmer) and this fast switching produces noise pulses extremely rich in harmonics.  The timing of this pulse with respect to the "beginning" of the sinusoid (let's say that this is where it crosses zero on the positive-going portion) can vary, depending upon when the offending device triggers.  This can also be offset in timing if, say, the device is operated on a different phase of a three-phase power system, for example.  This fast switching produces a pulse that is very narrow (a few tens of microseconds or less by the time it reaches your antenna.)

What about filters?  While it is true that with a narrower filter less noise energy will be intercepted, there is another problem:  Narrower filters tend to stretch noise pulses.  The three images to the right demonstrate this:  These images are from a line-triggered oscilloscope connected to the audio output of a Drake TR-7.  Three filter settings are used (top to bottom) with the first one being a 12 kHz filter (only the first IF filters are used) followed by a 2.2 kHz SSB filter and a 300 Hz CW filter.

With the 12 kHz filter (the top image) the vast majority of the noise pulse energy occurs within 1/8 of a division (about 250 microseconds) with a bit of ringing extending through 1 millisecond.  Contrast this with the middle image depicting the same pulse, but through a 2.4 kHz SSB filter.  Notice that the pulses now drag out to longer than 1 millisecond, about 4 or 5 times longer than before.  The bottom image shows the same pulse through a 300 Hz CW filter:  The pulses are actually beginning to run together.
It is true that most receiver noise blankers are placed earlier in the IF of a receiver - before the main bandwidth-determining filters, but even these receivers do have (wider) bandpass filter that tend to stretch out the pulses somewhat.

All is not lost:  With powerline noise, we have an advantage over random noise in that we know very precisely its repetition rate - 60 or 120 Hz (in the U.S. and a few other countries, at least.)  Furthermore, this noise source is most likely powered from the same power grid as your equipment (even if it isn't on the same phase as you) and thus you have a ready-made reference for the noise-pulse frequency.  What is not known is the precise position and width of the offending pulse.

(By the way, the noise blanker described reduced the above "light dimmer noise" by well over 35 db.)

A few comments on DSP noise reduction:

At first glance, one might think that a modern DSP-based noise reduction circuit would go a long way toward eliminating line noise - but this isn't necessarily the case.

Take Audio-based DSP, for example:  This type of DSP is that which is most commonly found on lower and moderately-priced amateur rigs.  There is a problem, however:  As seen in the example, the noise pulse is often badly distorted and "smeared" by the radio's filters even before they get to the DSP circuit - a fact that makes it extremely difficult to get rid of the noise pulse.

Another type of DSP commonly found is IF-based DSP.  Rather than being placed in the audio path, this DSP circuit is placed in an Intermediate Frequency (IF) stage.  On most radios with IF DSP, this is actually at or just above the audio frequency range, typically in the 10-20 kHz range.  Placing the last IF in this low frequency range reduces the computational power required to process the signals - but still is high enough that the IF bandwidth coming into the DSP filter can be wide enough to prevent excessive distortion of the pulse.  If designed correctly, these types of systems can be very effective in removing power line noise - but in very serious cases, the dynamic range of the DSP hardware and software can be pushed to its limits.

In short:  With audio-based DSP, you will still likely benefit from having an antenna-based noise blanker.  Even if you do have an IF-based noise blanker, an antenna-based blanker may still provide some benefit.

Deleting the offending noise pulse(s)...
 

Figure 2:
The effects of the blanker on a received signal.  The top image shows the "holes" punched in the received signal with one blanking channel, the bottom image shows two active blanking channels.  The blanking widths are exaggerated for clarity.

As can be observed above, simply muting the pulses at audio is not a very good solution due to the stretching of the pulses by the receiver's filters.  If this fact weren't the major concern, there is the problem of the receiver's AGC action attenuating the desired signal:  The noise will set the AGC to levels based on the strongest signal being received (even noise) - but if the desired signal is, say, 15 db below the noise peaks, the AGC will bury that signal 15 db in the audio... amongst the stretched noise pulses...

What we need to do is blank the RF during the noise pulse, starting just the blanking action before it gets to any  of the receiver's filters:  Since the noise pulse itself is (usually) of very short duration, we don't need to blank it for very long.  The most obvious place to do this is at the antenna connection and this may be done several ways.  One type of blanking gate described here uses PIN diodes in an attenuator arrangement and this provides attenuation that is related to the current applied to its diodes and is fairly "linear" even when only partial attenuation is occurring, which is important if you want to minimize the amount of intermodulation that the noise blanker will cause.  The use of MPN3404 PIN diodes is intentional here:  While this circuit will work with ordinary 1N914/1N4148-type diodes, you can expect performance to be worse as garden-variety diodes produce more nonlinarity when "partially" turned off.  As mentioned below, another type of blanking gate that reportedly works somewhat better at HF uses a diode-ring mixer.

Most important to intermodulation reduction is careful control of the slew rate of the blanking pulse:  If one were to simply employ a switch that turned the antenna on and off, the switch itself would cause significant harmonics and the blanker itself would cause what sounds like line noise.  If one (relatively) slowly turns the attenuation on and off during the blanking, you can greatly reduce the possibility of intermodulation and reduce the bandwidth and intensity of the "blanking sidebands" that are necessarily created during blanking (we are amplitude modulating our received signals with the blanking pulse, remember...)

As it turns out, while there is typically one major noise pulse causing most of the buzzing, but there may be several other noise pulses occurring during different times -
that is, if you blank just one set of the offending pulses, there may be another set that you are now able to hear that also obscures the desired signal and for this reason, you may wish to have the ability to set up individual blanking pulses for the various "layers" of noise.

Design philosophy:

There are several schools of thought when designing noise blankers:  "Normal" noise blankers (such as those found in receivers) have fast-acting blanking gates - a necessity because these blankers are typically "asynchronous," operating by first detecting the rising edge of the pulse, and then blanking it before much of its energy finds its way into the receiver.  Clearly, if you are going to blank a pulse that you have just detected, you need to act quickly.

With a synchronous noise blanker such as the one described here, you have the luxury of knowing precisely when a pulse is going to appear and because of this, you could use a fast-acting blanking gate or you could use a slower one.  What's the difference, then?  Which one is better?

The answer isn't necessarily a simple one and each has its own set of tradeoffs.  About fast-acting blanking gates:

• Because the "attack" and "decay" of the blanking pulse occurs very quickly, it is a small percentage of the total time of the blanking pulse.  If the total blanking pulse is centered on the offending noise pulse, the entire "blanking window" produced by a "fast" rise/fall blanking generator will be narrower than the windows produced by a generator that creates a "shaped" blanking pulse:  This can reduce the size of the "hole" punched in the signal.
• The act of blanking a signal is simply modulating the received signal with the blanking pulse.  The spectral components of the blanking pulse will, therefore, be mixed in with the received signal.  In the case of a fast-acting blanking generator, these spectral components may be hundreds of kilohertz (or even megahertz) wide, encompassing a very wide bandwidth of signals.  The effect of this may be the generation of noise or intermod throughout the receive spectrum.

• The "attack" and "decay" of the blanking pulse are shaped and can be quite long in duration compared to the overall time of the blanking pulse.  This means that the entire blanking window may be significantly longer than the duration of the pulse itself.  In this case, you would need to center the middle of the blanking pulse (the time at which the attenuator is at maximum) directly atop that instant that the offending noise pulse is occurring.
  
• The "shape" of the blanking pulse is a sinusoid, designed to minimize the spectral content of the blanking pulse.  When this blanking pulse is "mixed" with the entire spectrum of received signals, the modulation sidebands produced by this blanking pulse are confined much more closely within the close proximity of each signal, rather than "smeared" over a very large bandwidth.  This can have the effect of reducing the generation of noise and intermod.

The use of a doubly-balanced diode-ring mixer (used as a variable attenuator) can be used to mitigate this problem as their intrinsic balance reduces the likelihood if intermodulation distortion.  The problem with using a diode ring mixer instead of the PIN diodes is that their cost is much higher, and they have a very definite low-end frequency response which may make them unsuitable if very low LF or VLF reception is desired unless the proper device is used.  If you only intend to use this blanker on, say 160 meters and/or the AM broadcast band, you may wish to consider using a DBM - see the details here.

Description of the circuit:
 

Figure 3:
The schematic of the unit:  Click on the image for a larger version.

What we need is a device that will produce a blanking pulse that has two adjustable parameters:  Position and width.  The position of this pulse is relative (in this case) to the zero crossings of the 60 Hz line frequency (which occur at a rate of 120 Hz) and the width is, well, width...

Schematic versions:

• Version 1.00 - Original version of schematic
• Version 1.01 - Added pin numbers to ICs, fixed minor errors - 4/2000
• Version 1.02 - Corrected pin numbers on U1B (e.g. pins 9 & 11 were swapped) - 1/2001
• Version 1.03 - Added designation of diode on blanking generator output, made schematic file smaller - 11/2001
• Version 1.04 - Added some waveforms and nomenclature to switches - 12/2001
• Version 1.05 - Added "dots" to connection points on schematic - 4/2002
• Version 1.06 - Added details pertaining to the use of a diode-ring mixer as a blanking gate - 2/2004
• Version 1.07 - Pins 12 and 13 of U2D were swapped:  Corrected 12/2004
• Version 1.08 - Pins 1 and 9 of U1 should be grounded - but had been shown connected to +5V - sorry.

Figure 4:
A view of the proto board containing the pulse and blanking generators.
(Click on the picture for  a larger view)

The blanker is powered by an 18 volt AC power cube and the voltage is filtered and isolated (at RF frequencies) with a bifilar-wound inductor removed from a defunct computer switching power supply.  This inductor has several millihenries of self-inductance and it prevents noise that might be conducted from the power cube (from the power line) into the receiver itself.

This power is then half-wave rectified and filtered and applied first to a 7818 regulator for a stable 18 volt DC supply (for the blanking drivers and for powering the receive antenna on the coaxial cable) and then filtered and regulated to 5 volts for the timing logic:  The capacitor and series resistor across the diode are used to suppress any "hash" that might be produced by its nonlinearities.

A portion of the "pre-rectified AC" is extracted, filtered, AC coupled (to remove any DC bias) and applied to U2C which is wired as a comparator to produce a 60 Hz 18 volt square wave.  This square wave is resistively divided down to approximately 5 volts and applied to U3B, a 74HC86 which buffers the 60 Hz square wave and applies it to U3A which is wired as an edge-detector.  The resistor/capacitor combination on its input cause this section to produce a narrow pulse on each transition of the 60 Hz square wave, thus producing a 120 Hz pulse train which is buffered by U3D and is made available for the blanking-pulse generators.  The remaining section (U3C) is simply tied to U3D so that it's input isn't floating and is therefore unused. (Yes, I should have tied it to to either the ground or the 5 volt supply, but I was lazy...)

The 120 Hz pulse train is applied to U1B, a 74HC123, a dual one-shot timer and is the "position" timer and has a period that is adjustable from a few microseconds to the entire duration of the 120 Hz pulse repetition interval to allow blanking to occur at any point in the period of the pulse train.  The output of this is first section is sent to U1A.

Since U1A is edge-triggered, it responds only to the rising edge of the pulse from U1B - the end of the timing period.  The section of U1A operates exactly like the U1B section, except that its timing range is restricted to approximately one-third of the interval of the 120 Hz pulse train.  This section functions as the "width" generator for the blanking pulse.

The finished blanking pulse appears on pin 4 of U1A and it is at this point that this blanker generator may be "diode-ORed" with the outputs of other pulse generators.  If it is desired that more than one blanking pulse is needed (at least two are recommended) then the next section would be connected at this point.  For enabling/disabling a blanking generator, a switch (which may be mounted on the rear of the "width" control) is placed on pin 3 of U1A to enable/disable the timer.  For a single section blanker (or for the first of several blanking channels) this may be simply tied to +5V.

U2D, an Op-Amp section, is used as a comparator and it takes the diode-ORed input from the pulse generator(s) (using a 10k pullup resistor) and generates inverted 18 volt blanking pulses.  This pulse is then shaped by U2A which is wired as a 3rd order lowpass filter.  A 100k resistor limits the maximum output to prevent clipping of the amplifier at the 18 volt rail and thus prevents distortion (and the ensuing harmonics) from the blanking pulse.  A switchable 0.018 microfarad capacitor allows selection of a lower slew rate to allow the blanker to be used at lower LF frequencies (more on this later.)

The filtered output of the blanking generator is applied to the PIN diodes through a resistor (for current limiting) and across a capacitor and through an inductor (for RF decoupling.)  When the voltage is high, current flows through the two PIN diodes, turning them on.  When the voltage is off, the diodes do not conduct and high isolation is obtained.

Important Notes relating to previous versions of the schematic:

• It was (eventually) noticed that pins 12 and 13 were swapped on U2D.  While the blanker could still work, the controls would not work exactly as intended.  Although several others have built this circuit, no-one else has mentioned it...
• It was also noticed that pins 1 and 9 of U1 were shown as being tied to +5 volts while they should have been tied to ground.  This would have, unfortunately, inhibited operation of these one-shots.  Sorry for any inconvenience.

Figure 5:
The PIN diode attenuator section of the noise blanker.  (Note that there are 2 extra coils shown - these were originally used in the prototype, but bypassed - but not removed - in the "final" version.)
(Click on the picture for a larger view)

Using a Doubly-Balanced Diode-Ring Mixer ("DBM") as a blanking gate in lieu of PIN diodes:

If you do not intend to use the noise blanker at very low frequencies, it is possible to replace the PIN diode blanking gate with a diode-ring doubly-balanced mixer of either homebrew or commercial manufacture.  An example if where you may wish to do this is if you were to use this blanker exclusively on, say, 160 meters.

There are many suitable diode-ring mixers that may be used - the selection depending upon the desired frequency range.  An inexpensive and readily-available diode-ring mixer is the venerable SBL-1 by MiniCircuits.  This mixer is rated for use from 1 to 500 MHz (although it will work just fine across the entire AM broadcast band) and is typically sold for well under $10.00 U.S. and may be found for sale using an internet search.

Alternatively, you can construct your very own diode-ring mixer fairly easily using two trifilar-wound ferrite toroids and four matched diodes.  Details how diode-ring mixers may be constructed can be found in the ARRL Radio Amateur's Handbook as well as by doing some internet searches.  A few salient points on constructing a diode-ring mixer, however:

• PIN diodes are preferred, but you can do quite well using matched 1N4148 diodes.  To match the diodes (either PIN or '4148's) one would simply use a digital voltmeter in the "diode-test" mode and, out of a dozen or more diodes, pick four of them with voltage drops as close to each other as possible - but even if you don't do this, you'll probably do OK.  If you use 1N4148 diodes, placing a 47 ohm resistor in series with each diode can (reportedly) improve its intermodulation specs. and dynamic range.  (Even if the added resistors don't help, they don't add much loss - and you can pretend that they made a lot of difference...)
  
• The self-inductance of the trifilar winding should present a reactance of at least 10 times the input/output impedance at the lowest operating frequency - or 500 ohms if you are using a 50 ohm antenna and receiver.  (If you don't mind a small amount of extra loss at the low end, you can get away with just 3 times the reactance, or 150 ohms.)
• If you have one, by all means use a DBM-type attenuator.  These are just mixers that have been optimized for use as attenuators in terms of insertion loss (both minimum and maximum) and minimum distortion.
• If you are willing to spend the money - or if you are inclined to "roll your own" - it is possible to obtain a DBM that will work at VLF frequencies.  Bought new commercially, these mixers are somewhat specialized and are fairly expensive but it is perflectly practical wind your own trifilar-wound toroidal transformers and make a mixer that works even at audio frequencies, provided that one uses the right core material.
• There are also devices (also sold by Mini-Circuits) referred to as "attenuators."  These are actually just DBMs - but they have been optimized/selected for operation as an attenuator.
  

How it works:

First of all:  Yes, the "output" of the mixer is the "LO" port.  Because the mixer is being used as a variable attenuator, it isn't wired exactly the same way as you would wire a mixer.

When there is no current on the "IF" port of the DBM (this is the port that has direct DC connection to the four internal diodes) all of the diodes are turned OFF.  As the current increases, however, the diode starts to conduct, allowing RF to flow through.  Owing to the symmetric nature of the circuit, most of the diode's nonlinearity is canceled out internally, reducing the potential for intermodulation distortion.

The signal from the blanking gate goes from about 0 volts (if you use an LM324 or another op-amp that can swing to its negative supply rail) to about 15 volts. The 3.3k resistor limits the maximum current to about 5 mA and, importantly, the 51 ohm resistor (a 47 ohm resistor could be used) and the 0.01 uF capacitor set the terminating impedance at the IF port (at RF frequencies) at 51 ohms for best performance.

Note:  Because the attenuation of the DBM (and the PIN diode, for that matter) is somewhat proportional to the diode current (over portions of the curve) it is imperative that the diode current go to zero in order to maximize the amount of attenuation during the "off" portion of the blanking pulse.  In the case of the DBM, it also must not go negative, or else the signal will re-appear, but in the opposite phase.  It is also important that the diode current just reach zero at the bottom of the blanking pulse in order to mimize the intensity of the sidebands imparted on received signals by the blanking action:  Built according to the schemtic, you will be pretty close to optimum, but if you are of the sort to fiddle and tinker...

A few comments:

• The diode-ring mixer is fairly sensitive and fragile - more-so than the (relatively rugged) MPN3404 PIN diodes.  If you have some AM broadcast stations nearby, it is likely that the attenuator will be overloaded, resulting in audible intermod products - especially during the blanking.  If this is the case, you will have to put a simple bandpass filter on the input of the blanking network.  Note that a bandpass filter is, in this case, preferable to a highpass in that it also limits the amount of energy that the attenuator would intercept from higher frequencies.
• Other builders report that the DBM works somewhat better on low HF bands than the PIN diode switch - especially in the presence of strong AM broadcast signals.  I haven't tried it personally, but since more than one builder has said this...
• It is advisable that some means of static protection be placed at the input of the DBM.  If a bandpass filter is used, this will offer a reasonable degree of protection as it will intercept the vast majority of energy from, say, a nearby lightning strike.
• If your receiver is also a transceiver, make certain that you do not attempt to transmit through the mixer:  It won't like that!
• The value of the 27 mH inductors used in the power insertion/steering is not critical.  What is important that the reactance of the inductor used is 3 to 10 times (the latter preferred) the impedance of the line at the lowest frequency being used (e.g. a "Z" of at least 150 ohms.)
• The 27 mH inductor in series with the 2.2k resistor on the blanking input (to the diodes) is not critical.  It tends to reduce IMD at lower frequencies and, in some cases, in the presence of higher levels of BCB interference:  Try it with and without, if you have such a choke available.

The prototype unit (shown in the pictures) is a three-channel synchronous noise blanker.  Although two channels usually suffice, there are (rare) instances where all three are required.  

Figure 6:
Front panel view of the Synchronous noise blanker.
(Click on the picture for a larger view)

The front panel shows six potentiometers:  One for position, and another for width for each of the three channels.  Channels 2 and 3 (the middle and the one on the right, respectively) have switches mounted the rear of their width control pots and these switches are used to disable their respective channels.  They are placed on the width control so that the channel may be disabled without disturbing the pulse position settings.  Each of these pots is wired such that counterclockwise rotation corresponds to the shorter timing interval for the associated timer (i.e. "earlier" blanking pulse for the position control, or "narrower" blanking pulse for the width control.)
 

Figure 7:
Rear view of the front panel showing the wiring of the potentiometers.
(Click on the picture for a larger view)

The PIN diode attenuator is constructed on a piece of circuit board material and is mounted using the antenna connectors (RCA connectors, in this case.)  This provides a good "RF" ground, mechanical rigidity, and minimizes extraneous RF pickup or radiation.  1.5 and 2.2 microfarad ceramic capacitors (precise values are not critical) are used for DC decoupling and these values are sufficiently large enough to allow for minimal attenuation at VLF frequencies.  If such large-value ceramic capacitors are not available, electrolytic capacitors may be used (observe polarity!)  If response at such low frequencies (below 10 kHz) is not needed, lower-value capacitors may be used.  It is recommended that electrolytic capacitors (if used) are paralleled with some 0.1 uF ceramic capacitors to minimize losses that electrolytics often exhibit at higher frequencies.

  With my receiver (a Drake TR-7) I have an "LF Interface box" that puts power on the LF antenna input connector to power an active antenna.  Diodes are employed to steer power from the receiver to the antenna and from the noise blanker.  A switch is used to enable/disable this power - useful if using a passive antenna.
 

Figure 8:
A view of the rear panel of the synchronous noise blanker.  A banana jack (left) is for monitoring the blanking pulse with an oscilloscope.
(Click on the picture for a larger view)

It is recommended that the bodies of the potentiometers and switches be grounded to reduce "hand effects" and to protect the CMOS circuitry from static discharges.   It is also strongly recommended that the timing capacitors on U1A and U1B (the 0.068 and 0.022 uF units) be (relatively) temperature-stable devices such as mylar or polyester:  Ceramic disk types in this capacitance range are not stable with temperature and you will be plagued with temperature-related drift of the position and width adjustments if you use them.

Finally, it is necessary to choose op amps that are capable of swinging all the way to the negative power supply rail.  The LM324 specified in the schematic is capable of doing this - but many op amps are not.  If the op amp that you use cannot go all the way to the negative rail (in this case, ground) it is possible that the PIN diodes in the blanker gate can never be turned completely off and blanking performance will suffer.  In other words, just use the LM324!

Obtaining MPN3404 PIN diodes:
 
Several people have reported difficulty in finding MPN3404 PIN diodes.  While not ubiqutious, there are a number of places where they may be obtained in small quantity:

• Newark Electronics:  Note that Newark has an added $5.00 charge if the order is less than $25.00.  (Newark P/N for the MPN3404 is:  07F9506)
  
• Downeast Microwave:  These guys usually have MPN3404's in stock for a very reasonable price.
• Dan's Small Parts and Kits:  This place as a lot of these and other esoteric parts...
• Kits and Parts dot Com:  Not only do they have the MPN3404's, but lots of toroids as well as other parts useful to the homebrewer.
  

Operation of the unit:

Most of the time, operation of just one blanking channel will eliminate most of the noise.  Even if two (or three) blanking channels are ultimately required, you start out with just one.  The noise blanker is set up thusly:

1. Turn off the receiver's own noise blanker.  Since the idea is to null the noise, you don't want the radio's blanker to make it harder to hear the noise while you are doing your adjustments.
2. Make sure that only one of the synchronous noise blanker's channels is on.
3. Set the receiver's bandwidth to a fairly wide setting (usually an SSB or AM filter.)  This will allow the noise to be heard more clearly as you null it out.  If your receiver has it, you might find it easier to fine-tune adjustments with the receiver set to AM as it will be easier to hear the "buzz" go away as you null it out.
   
4. Set the receiver's AGC to SLOW, or turn it OFF (adjusting the RF gain to set the signal to a comfortable level.)  This will allow easier nulling of the noise.
5. Set the width control to to about mid-rotation.
6. Slowly, carefully rotate the position control.  A definite decrease in the powerline noise (the "buzz") should be noted at some position - unless you just happen to have two noise sources of different timing and similar strength.  Keep an eye on your S-meter as you do this, as the AGC might simply compensate for reduction in the amplitude of one noise source and make another one more audible!
   
7. Adjust the width control to narrow the pulse slightly.  Readjust the position control to re-null the noise.  Repeat the previous steps.  If you adjust the width control too narrow, a deep null cannot be obtained while adjusting the width control too wide can result in needless incidental AM modulation of desired signals (more on this later...) - but remember that some pulses (especially those from switcing power supplies) may simply be "wide" by nature.
8. At this point it may become obvious that there is more than one noise pulse and a second (or third!) blanker channel may be activated.  It is adjusted in the same way as the first.  It may be advantageous to "touch up" the first (or second) channel a time or two to achieve maximum noise reduction.  Remember:  Set the "width" control to the minimum required to achieve the best blanking!
   
9. If you are trying to listen to VLF signals (i.e. those below 30 kHz) it is possible that the switching of the PIN diodes themselves may cause some audible harmonics at the switching rate.  At (and below) these frequencies, one would set the slew rate switch to the slow position.  Note that changing the switch setting will usually necessitate readjustment of all width and position controls on all enabled channels.
10. The AGC, filter, mode, and noise blanker settings on the receiver may be returned to their normal positions.

Operational notes and comments:

Here are a few comments about a synchronous noise blanker - what the blanker will and will not do:

• A noise blanker such as this will work only on impulse-type noise that is created by a line-powered device.  It will do absolutely nothing for random atmospheric/lightning noise or "white" (hissy) type noise.
• When the blanking pulse is active, the receiver is muted briefly.  This has approximately the same effect as amplitude-modulating the received signal with the blanking pulses and because of this, even a CW note will acquire a slight (to severe) "buzz" as sidebands will be generated.  The intensity and "bandwidth" of these added sidebands (i.e. the added "buzz") will vary with the setting of the width control and the slew rate of the blanking pulse, as well as the number of blanking channels that are active.  Usually, the resultant buzzing is far less offensive than the noise you are blanking.
• It may be useful to use the radio's built-in noise blanker as well - once you have adjusted the blanker for maximum effectiveness.  These are often effective against the random impulse noise pulses (such as lightning, etc.) and can prevent these pulses from causing AGC action that might momentarily "deafen" the receiver - causing you to miss a portion of an ID, for example.
• There is an "enable-disable" switch shown on the noise blanker.  This does not disable the blanking generator, but rather it turns the PIN diodes fully on (i.e. set to minimum attenuation.)  It was convenient to disable the blanker in this manner, but one could do it by disabling the blanking generators as well (making sure that they are disabled in the "on" state, effectively "un-muting" the receiver) or even bypassing the blanker gate at RF with a relay/switch.
  
• Most active antennas get their power through the coaxial cable.  The "power passthrough" switch enables/disables this supply.  18 volts is applied for the active antenna's power, but you should build this such that it suits the requirements for your active antenna.
• If the the blanker is used on a wire antenna it is possible that very strong signals (such as local AM stations) can cause the PIN diodes to produce intermodulation.  This can be prevented by using an antenna matching network (which can often provide an effective bandpass response) a bandpass filter, or even a lowpass filter.  This is not usually a problem on active antennas as they typically have sufficient rolloff characteristics to prevent such overload.
• The effectiveness of a synchronous noise blanker (and more traditional types) is somewhat less at higher frequencies.  This is because the noise tends to become "hissy" due to some "smearing" of the original noise source - probably due to incidental phase modulation and/or the possiblity of the noise coming from multiple, distant sources.  For this reason, a blanker such as this is most useful below a few megahertz.
• Even though these descriptions are based on using the device on a U.S. (60 Hz) powerline system, there is no reason why it wouldn't work equally well (if not better!) on a 50 Hz power system.  The capacitor and/or resistor values on the "position" timers will have to be adjusted slightly to accommodate the longer periods associated with 50 Hz power.  (That's to say, the value of the 0.022uF capacitor between pins 6 and 7 should be increased to the 0.033-0.039uF area.)
  
• You may notice that the "null" seems to change with receive frequency.  For example, I've noticed that the settings for the best null at, say, 20 kHz are different for the best null at 200 kHz.  Why?  It could be that different noise sources appear at the different frequencies.  It could also be that the group delay in the active antenna (or noise source) changes with frequency, causing the apparent position to change accordingly. At higher frequencies it may be that a different device that is being heard -  There are many possibilities that come to mind.
• There is an inherent 4-6 db loss in the noise blanker when it is not blanking.  This is due to the loading of the RF lines due from the 470 ohm resistors used, as well as other miscellaneous losses.  Usually, this isn't too much of a problem as the loss of 6 db or so isn't usually enough to put the normal atmospheric noise below the receiver's threshold - even on a quiet night.  If you are using a passive antenna or if the receiver you use is very "deaf" at these frequencies (many of them are) then this extra attenuation may adversely affect your receive ability.  In other words:  If you always get an "S-Meter" reading on atmospheric noise with the noise blanker in line, you probably aren't missing any signals...
• A minor improvement could be made to the performance of this blanker at extremely low frequencies (i.e. <10 Khz.)  At the moment, the slew rate of the blanking pulse is adjustable simply by placing more capacitance on the line.  To do it properly, one would change the characteristics of the entire shaping filter to provide a more sinusoidal waveform at the lower rate rather than simply tack a capacitor across the line.  This would probably mean building a second filter with the lower frequency response, thereby minimizing the harmonic energies present in the blanking pulse that might appear in the receiver passband.  (Did that make sense?)
• The DC blocking capacitors shown on the schematic are what  I happened to have laying around (1.5 uF and 2.2 uF ceramic) and aren't critical in value except that they should be more-or-less "transparent" at the operating frequency.  For VLF operation, they need to be fairly large, but if you are planning to use this only above, say, 150 kHz, then some 0.47 uF capacitors would suffice, and for BCB-AM or use at 160 or 80 meters, you could get away with 0.1 uF caps.  If you use electrolytics, be sure to observe polarity - and parallel them with some 0.1 uF units to make sure that any losses in the electrolytics at higher frequencies don't show up.  The same is true for the 1.5 uF ceramic across the resistor/diode on the power supply input (next to the bifilar choke) used to supress potential diode switching noise- at MF frequencies a 0.1 uF should be more than adequate.
• "Deleting" that annoying electric fence:  I received the following email from a French amateur:
"In case of electric-fence noise (I am in the country side) I just added a wideband opamp using a part of LM324 drived by a noise antenna (10 meters of wire 2 meters high running near the offending site.)  The time constants of your schematic seem just right for blanking."
I don't (yet) have a schematic describing this, but the idea makes perfect sense - especially if the offending electric fence noise is generated when the high voltage pulse collapses.  If you try this - and it works (or, if it doesn't...) - please let me know what you've done.

The noise blanker in action:

I have received several emails over the years about the operation of the noise blanker and how effective it may be.  The latter question is very difficult to answer, as it all depends on the qualities of the QRM with which you are being afflicted.  To demonstrate some of the various properties, here are some audio clips with analyses:

• Blanking at 100 kHz - In this brief clip, the receiver was tuned to 100 kHz, set to AM and set to a wide (>10kHz) bandwidth.  In the AM mode, the nature of the offending noise pulses is pretty obvious as brief, sharp pulses such as those shown in the top picture of Figure 1.  At the beginning of the clip, you can hear the buzz of the offending noise source (in this case, a triac light dimmer) but you soon hear the blanker brought to bear as the "width" and "position" controls are adjusted to (mostly) remove the buzz, allowing the listener to more-clearly hear the Loran-C pulses.  Of course, this blanker can't do anything to remove Loran-C pulses!
• Beacon YCD-251 - Because it is unlikely that you would build a noise blanker just to listen to Loran-C blasting away, this is a more-practical example.  In this clip, the same, nasty light dimmer is present, wreaking havoc with reception - this time, with the receiver tuned to 250 kHz in USB mode.  At the beginning, you hear the buzz, but the blanker is quickly adjusted to remove it, allowing the listener to hear what is really on-frequency - in this case, the "YCD" beacon in Nanaimo, near Vancouver, BC, Canada.
• Noise on WWVB - This represents about the worst case in which one may be helped with the synchronous blanker!
• For this test, I turned on some track lighting with four dimmable, compact-fluorescent lamps on a triac dimmer - two potentially offensive noise sources.
• To make things worse, these lamps have built-in inverters operating at about 30 kHz - and I'd tuned to WWVB at 60 kHz, the second harmonic of the inverter.
• What is heard is a combination of the brief pulse from the triac dimmer itself (remember - although dimmers are supposed to have built-in filtering, they have no real efficacy at such a low frequency!) plus the FM/AM modulated noise from the lamps' inverters.  The QRM from the inverter is particularly bad because, unlike the brief pulse from the triac, the inverter rapidly shifts frequency and amplitude as it follows the ripple from its power supply, creating a relatively long burst of noise instead of a short pulse.
• For the first 8-10 seconds of this clip, nothing is being done, but at this point you start to hear some changes as the first blanking channel is adjusted to reduce the S-meter reading.  At about the 30 second mark, the second blanking channel is activated and a pronounced reduction of noise is obvious at about 34 seconds - followed by a bit of fine-tuning.  At around 40 seconds, the third blanking channel was activated and at 45 seconds, you can hear even more reduction in QRM from the light dimmer and lamps.  Following this is more twiddling and fine-tuning of the "position" and "width" controls of the three channels to minimize the size of the width and the position.  Finally, at 1:23-1:29 the blanker is disabled - and re-enabled for comparison.
• In this case, the "slow" setting was used as the harmonics of the blanking pulse itself are detectable at 60 kHz.
• The use of the "slow" setting, the fact that all three blanking channels were active (which is quite rare, actually) plus the fact that one or two of the blanking channels required quite wide blanking pulses to blank the "slow" noise pulses from the lamps punches many "holes" in the received signal:  Even though the majority of the QRM from the light dimmer is gone, the normally-pure CW note from WWVB is rendered as a buzz - but at least you can hear it and some of the atmospheric noise in the background!

Other pages at this site:

The CT MedFER Beacon page - This page describes a PSK31 MedFER beacon.

The CT LowFER Beacon Archive -  Some pictures/info about the "CT" LowFER beacon of the late 1980's.  (Includes QSLs and sounds from some other beacons of the time.)

"QRSS and you..." - Using absurdly low-speed CW for "communications"

Using your computer to ambush unsuspecting NDBs - A brief description of how Spectran may be used when trying to receive NDB

Any comments or questions?  Send an email!