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Re: [Amps] IMD and spurious measurements

Subject: Re: [Amps] IMD and spurious measurements
From: Manfred Mornhinweg <>
Date: Tue, 11 Aug 2015 15:49:50 +0000
List-post: <">>
Hi Gary, Karl-Arne, Steve, Bill, Ron, Peter, etc,

As promised, here is the follow-up of my strange spurs measurements.

Indeed it would be quite strange to see DX broadcast signals, that come in at about 1 millivolt on my antenna, be mixed by a transmitter and amplified to a few volts. And those were in fact the actual levels! So I checked each part of the setup.

Mixer saturation was not involved. The dynamic range of a diode mixer with strong local oscillator drive, followed by a 16 bit computer sound card, is close to 80dB, and it's limited by the sound card rather than the mixer. While doing the measurements I was staying 15 to 20dB below the saturation point. Further attenuating the input to my mixer, so that the main signals were 40dB clear of saturation, didn't change the ratio between the spurs and the main signals at all.

I didn't have a directional coupler, so I built one. Taking my test signal off the coax cable, via the coupler, instead of capturing it with a tiny antenna, made a major difference: The strange spurs were pushed down nearly to the noise floor, with only the stronger ones being visible. So it's clear that I was getting who-knows-what noise into my mixer, via the small pickup antenna.

I then returned to a small pickup antenna, but a balanced one: A small loop, connected through a common mode choke to the mixer. This produced almost the same results as using the directional coupler. So I'm pretty sure that when using the monopole pickup antenna, I'm getting noise into the mixer because the other pole of the antenna is the computer wiring.

Anyway I couldn't make sense of the spurs, and I don't know where they come from. Their frequency relationships don't seem related to line frequency, power supply switching frequencies, nor any other I can recognize. And I live in the woods, far away from any neighbors, let alone cities, so there aren't any strong local signals here, except for those I generate myself.

Another interesting note about the directional coupler: It didn't matter whether I took my sample from the forward or the reflected signal port. Other than the amplitude of the whole thing, nothing changed. The ratio between spurs and main signals remained the same. This probably means that the spurs are not caused by anything from the outside world mixing in my mixer. And it also means that a simple nondirectional pickoff from the coax line should be as good as the directional coupler, for this kind of test.

Next I inserted a 1:1 audio transformer between my mixer and the computer sound input. The spurs got slightly smaller, about 1 to 2dB further down from the main tones. The improvement isn't a lot, but since there was some improvement, I left the transformer in place.

Next I inserted a 1:1 RF transformer, wound on a small ferrite toroid, between the DDS signal generator and the mixer. The DDS is powered and controlled through an USB interface, so it does have a potential for coupling lots of noise. At this point my mixer was galvanically connected to the coax line only, with both the oscillator input and audio output galvanically insulated. It seems to have improved the situation another little bit, but only marginally, so it looks like USB noise getting through the DDS wasn't much of a problem.

I can't conveniently run my computer from batteries to break any ground loops, because it's a desktop computer.

At this point the strongest of the strange spurs were about 65 to 70dB below the main tones, close to the dynamic range limit of my spectrum analysis setup, so I won't worry any more about them!

I can now conveniently measure the IMD of my transceiver and amplifier, with a valid dynamic range of greater than 60dB. That should be enough for all ham purposes.

About Steve's funny way to tell me to get on the air, well, trampling on others isn't my style! :-) But seriously, I tend to enhoy more playing with radios and electronics, than talking to "typical" hams. I do sometimes engage in long-winded rag chewing, if any interesting matter comes up, and I happen to find someone interesting to talk with. But that's preciously rare these days! Most QSOs revolve around the weather, and what brand and model of radio the guy bought. When it does get technical, it's at the level of which of his 25 mikes sounds brighter, duller, more or less boxy, and so on. Those tests might be interesting for mike collectors, but they tend to bore me to the point of signing off.

And DXing? That's the dullest thing I know in ham radio! Invariably most DXers just want to exchange callsign, a fixed 5-9 report, maybe tell me the name and location, and then it's "thanks for a new prefix, bye bye!" And after three such contacts, at most, somebody puts me in a DX cluster, and the next thing that happens is a pile-up of guys calling, crying, shouting, without even making sure they can actually hear me. So I quietly slip away.

Compared to that, it's still far more fun to spend my time measuring the IMD and efficiency of an amplifier at different bias settings, locking my transceiver to GPS, or playing any other of the games that involve a soldering iron and some instruments, rather than a mike.


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