Noise Blanker Testbox

[Chris Parry]

This unit was built in 1991, at a time when I was especially interested in Noise Blankers. Published documentation seemed very lacking, so I decided to make my own tesbox which hopefully would produce repeatable results on a range of different radios.

The testbox can also be used on older radios having a simple noise limiter rather than a blanker.

The photos should be almost self explanatory. To get some realism, a genuine 12V car ignition coil is used, and the testbox is fed with 12VDC at about 3A from a beefy external bench supply. The original version of this testbox used a real sparkplug, nowadays replaced by a round-headed 6BA setscrew arcing onto an earthed soldertag as photographed here. The spark length is some 75thou'. The toggleswitch is a centre-OFF type. One side gives slow sparking, to represent an idling lawnmower engine. The other side runs much faster, to represent a 4-cyl car engine at about 3000rpm. These were the two problem scenarios I was most interested in evaluating.

The "points" comprise a rattling 12V relay, with a 100nF/400V "condenser" connected across its contacts. No nasty electronics anywhere.

There are 4 coax sockets. The outer shells of all of these, and also the body of the ignition coil, are connected to the -12VDC pole.

* The two centre BNC sockets are wired in parallel and not connected to anything else. They go in the coax line between the receiver and its aerial (or signal generator). The wire link between these two sockets receives some energy from the spark 40mm away. The radio is fed with this unwanted pulse energy, superimposed on the signal it is trying to receive.

* The other BNC socket comes from the points via 100nF/400V, to be used as a trigger source for my Cossor CDU150/CT531 scope. The Y-input gets connected to the loudspeaker output, so I can see what the blanker is doing.

* The TNC socket provides a 47R source impedance for the Collins 136B-2 (an option for the KWM-2/2A) or other noise blanker requiring a dedicated noise-collecting whip antenna. The 47R resistor is exposed to spark energy and thus provides a disturbance source which hopefully, can be "ignored" by the radio at an certain setting of its blanker threshold pot.

My tesbox is built into an ABS enclosure. I didn't want to use a metal diecast box because that would stop me evaluating portable transistor radios fitted with integral whips.

>>>SAFETY<<< Please be careful if you replicate this project! If you get the circuit diagram or construction wrong, you could hurt yourself or cause damage to the radio.

What are the results in practice?

1. The original blanker design was the Lamb type fitted to the Hallicrafters SX28. It works very well but there again, it does represent a very large proportion of the total complexity in this radio.

2. The Collins 136B-2 blanker module requires its own 40MHz whip antenna. It works very well, especially if pump diode CR2 is reversed. This appears to be an uncorrected design error which made it all the way to production.

3. Yaesu had much trouble with the blanker for the popular FT101 series. My example is a late FT101E fitted with the PB1582B blanker PCB. This works OK as built, but can be improved with a few tweaks. The earlier PB1292 blanker module is largely ineffective whatever you do to it.

4. Drake listed the NB4 (aka 4NB) as an option on the R4C. The later R7/TR7 families had their own blanker PCB, called option NB7A. All of these Drake blankers work well straight out of the box.

5. An unlikely goodie is the humble Belcom Liner 2 VHF SSB transceiver. The blanker circuit is extremely simple, but it works well. Then Japanese industry seems to have gone for a long period producing blankers of ever-increasing compexity leading to...

6. Icom R71E, generally the best conventional HF ham receiver that I have ever used, with a noise blanker that is highly effective.

7. As for older radios fitted with noise limiters, the RCA AR88, GEC BRT400 and Marconi Atalanta designs work well. The CR100 also works well once the AP56703 module has its design error taken out. (Move 6H6 anode bias resistor R60 one step further up the cathode chain of the audio triode, to junction R25/R28/C87.) Most other old radios have limiters that seem of little use.

8. The MFJ1026 can work well enough, but I find it hard to adjust.

I hope this lot proves thought provoking, and does something to address the shortage of information about how to test and compare noise blankers in a way that gives repeatable results. I have deliberately excluded consideration of DSP designs because that's too close to how I earn my living, so not interesting to me as a hobby.

Chris

[Chris Parry]

Photos for this project - Chris.

[Bazz4CQJ]

Chris, what is the difference between a blanker and a limiter? I'm working on a major rebuild of my HRO which has no original noise circuitry. I guess that you'd suggest it should have some added to it?

[Radio Wrangler]

Bazz, you probably wouldn't want to add a noise blanker to an HRO. It's going to be as complex as the rest of the receiver put together. You'd probably wind up with an outboard box, and you'd need to add a few connectors to the HRO

A noise limiter is little more than a pair of diodes used to clip any spikes larger than the average signal level. Simple ones clip at a fixed level, fancier ones let you vary the level, and some make an attempt to follow the current received level. The purpose is to stop ignition pulses, distant lightning, fridge clicks etc damaging the operator's hearing. Very important with headphones.

A blanker goes two steps further.

The first idea is that shutting off the receiver path for a few dozen microseconds is far less obtrusive a response to a noise spike. The second idea is that cutting out the spike with a fast switch located before the receiver's main IF filter will stop things being made worse by the filter ringing in response to the spike. These things really do fix ignition noise quite well.

Then along came the 'Moscow woodpecker'. The Russians built a massively powerful HF RADAR system. It used ionospheric propagation and immense power to be able to see over the horizon. They were looking for incoming missiles and bombers. This system rode roughshod over all other HF users, and they tuned it to follow the MUF which is right where all the interesting stuff tends to be for precisely the same reason.

It became pretty important for HF receivers to have a blanker which worked on the 'woodpecker' - named from its sound. Many had switched blanking widths, one optimised for ignition, one optimised for the woodpecker.

A blanker starts with a second receiver. Some take a signal from the RF or early IF stages, others have a separate antenna on a low VHF frequency to find pulse interference. Because the Russian transmitters were tuned, their disruption only spread over a few hundred kilohertz, and the separate frequency blankers didn't work.

The blanker is a race with time. The idea is to receive a signal, wider band than the main receiver and amplify it enough to be detected and trigger a monostable circuit which makes a pulse which opens a switch in the main signal path. The noise path must be fast enough compared to the receive path that the blanker opens its switch before the noise spike energy gets to a narrow filter. Otherwise the filter would ring and make the interference far more intrusive and you'd need a longer blanking period.

Some designers used wide-ish roofing filters in their radios to not only do their prime job, but to act as a delay line in the main signal path, to give their blanker a chance to get ahead of the spikes. Some designers didn't like the thought of this filter ringing and put proper delay lines in the signal path ahead of the blanker switch.

If you take a radio mobile, then you have use of a blanker. Big classic receivers aren't used much on the road. Aircraft have carefully shielded Magnetos and ignition systems. You might live near a road with a proportion of badly suppressed cars going by, but they tend to be older classics nowadays.

The russian woodpecker closed down a long time ago. The Americans also built one (Cold-war woodpecker race?) They called it CONUS-B. It seems that whenever they operated it, the Russians got blamed anyway https://www.vintage-radio.net/images...es2/angry1.gif

I've got an HF radio here with a good blanker with adjustable trigger level and switched blanking widths to handle Bosch or Breznev. I can't remember when I last needed the blanker.

It's one of those bits of circuitry where we now (fortunately) need to build testers to find out if they still work.

Cheers
David

[Chris Parry]

A noise limiter is one or two diodes which chop the tops off audio clicks caused by static or car ignition interference so that they don't sound dramatically louder than the wanted signal, as they would otherwise. This makes the audio less fatiguing for the operator. Simple noise limiters use just a single diode to stop the output valve being driven into gross overload. More complex noise limiters use a pair of diodes to chop the tops >and bottoms< off the audio pulses. The operating threshold of the limiter may be preset, or it may have a front panel knob. The most complicated noise limiters are "self-following" so that the operating threshold will always follow the tops of the speech envelope. As the signal fades up & down, so the noise limiter operating threshold goes up & down in sympathy. These are called "automatic noise limiters", often abbreviated to ANL. These types often have no user-adjustable threshold pot, just a simple ON/OFF switch on the front panel. All noise limiters introduce distortion. The best of them are not too obtrusive though, especially in CW mode. Noise limiter diodes often suffer from cathode poisoning, unless silicon types are used. Apart from that, the only thing that goes wrong with noise limiters is leaky tubular paper capacitors.

It is perfectly possible to modify an old radio to include a noise limiter. For example, National published 2 different noise limiter circuits for the HRO. The first one was a simple preset limiter; the second was a more complicated ANL type.

A noise blanker is a more complicated circuit which inserts a "hole" into the audio where the noise pulse would otherwise be. The point is that a short audio hole sounds a lot less noticable than a noise pulse, even one that's been cut down to size by a limiter. The blanker works by introducing an extra signal path into the receiver. The original signal path is unchanged except that one of the valves is now a type which can be quickly disabled by a blanking pulse. For example, the SX28 uses a 6L7 heptode as one of its IF amplifiers instead of the 6K7 pentode you'd expect to find doing that job normally. The signal through the 6L7 can be "blanked" by a short negative pulse fed into one of its two control grids. OK, so where does the this negative blanking pulse come from? It comes from a special little IF amplifier strip with a detector diode on the end, which drives the 6L7 blanking grid. The bandwidth of this mini-IF is kept as wide as possible, otherwise the blanking pulse might arrive too late to hide the unwanted noise pulse. The front panel has an ON/OFF switch and a pot to set the signal threshold above which which blanking pulses start to be generated. Usually, the width of the blanking pulses is fixed. If too narrow, not all of a big long static click will be successfully removed. If too wide, then interferance from a high revving petrol engine could result in a chain of blanking pulses which join up one into the next, so that the entire receiver ends up being completely shut off. Then the operator can hear nothing at all! In addition to the "blanking threshold" pot, some receivers have a "blanking width" pot on the front panel which allows the user to select a blanking period just slightly longer than the noise click.

In general, it is impractical to modify an old receiver to include a noise blanker. There are two noteworthy exceptions: Collins made a retrofittable blanker module for the KWM-2 and 75A-4 receivers, containing qty=4 6U8A valves working as a 40MHz broadband TRF receiver which drove a diode IF switch. A separate external 40MHz noise-collecting whip antenna was required. These units are rare, and long out of production. Today it is possible to buy the MFJ model 1026 blanker unit, which is a stand-alone box inserted into the aerial path of your receiver. This device works best with an external noise-collecting antenna, and is very adjustable. (Lots of knobs & switches!) The only problem is... there is a design error which means the MFJ1026 will not reliably blank all incoming noise pulses. The web has details of the modification needed to overcome this deficiency.

Hope this helps to demystify.

Chris.

[Chris Parry]

David - It took me so long to type that we doubled on our replies to that one! Chris.

[Radio Wrangler]

Same here, Chris. We must have been writing in parallel.

Good job we both agreed on everything

David

[Bazz4CQJ]

Thanks for the info guys; I knew the basics of limiters but the concept of blankers was entirely new. The rebuild of the HRO includes an audio processor comprising of a bandpass filter and an enhancer; perhaps I should think about adding a limiter, but I live out in the sticks and it may not do much for me.

[Chris Parry]

Depends what goes on around you. I've found stock control fences and milking machines extremely noisy. Long runs of overhead AC power transmission lines also have an evil reputation for generating high levels of local noise. Good luck! Chris