Curing Common Mode Hum in the VRX-1

As I’ve previously noted, the VRX-1 is a nifty little basic direct conversion receiver, but it has some shortcomings that could be problematic under certain circumstances. Here’s a story of one of those issues and the cure that was found.

Dave AA7EE purchased and built a VRX-1 kit a while ago but was never fully satisfied with the performance due to an annoying 60 Hz hum. He and I had briefly traded comments on the topic via Twitter, but I never really seriously took the time to think about it until just recently. Dave had built and placed a peaked lowpass audio filter into the receiver thinking that would help with the hum, but unfortunately it did virtually nothing to help with it.

I was a bit surprised to hear of the hum problem, since I had never encountered any significant amount of hum, nor had I had other complaints of hum. The eureka moment came when Dave had mentioned that the hum went away when he disconnected the antenna, or it decreased in signal strength when he moved away from his home. I had assumed that the hum was a glitch in his audio circuitry, but this reminded me of the problem known as common mode hum. The best description of this phenomena is found on pages 8.8 – 8.9 of Experimental Methods in RF Design, but I can provide a brief overview. Common mode hum is the result of the LO leakage getting out of the antenna port, modulated by a mains power supply (like an old-fashioned model with rectifiers), and then re-received by the radio.

Due to the simple, single-ended mixer design in the VRX-1, I knew that LO-RF isolation was very poor. So the first suggestion to pop in my mind was to tell Dave to try a common-gate JFET preamp on the front end. Although these type of mixers have modest gain, they have a low noise figure, and even more importantly for us, excellent reverse isolation (on the order of 30 dB). This should be enough to kill any significant amounts of LO leakage.

Dave built a circuit from master homebrew experimenter, Todd VE7BPO. It’s the last circuit on this page, and it looks rock-solid. A double-tuned circuit on the front and a single-tuned circuit on the output. Sure enough, that ended up doing the trick. Rather than trying to reinterpret Dave’s thoughts, go visit that last link, then watch his YouTube video so you can hear the results for yourself:

I’m really pleased to hear that Dave’s annoying problem is finally fixed. This makes me wonder, in retrospect, whether I should have just designed in a preamp to the VRX-1. It certainly isn’t needed for noise figure purposes, but as you can see it can make a huge difference with those who might have problems with hum. There’s also a well-documented problem of a loud impulse generated when the antenna is connected or disconnected during operation. I suspect at the reverse isolation of the preamp would also help this. Hindsight is certainly 20/20. If there is ever a VRX-2, then you can bet that it will get a stock common-gate preamp.

5 thoughts on “Curing Common Mode Hum in the VRX-1

  1. I just spent 45 minutes writing a long comment here, only to lose it when your site couldn’t read the captcha I entered. I knew I should have done a cut and paste on the text before attempting to submit it.

    I’ll re-comment once I’ve regained my composure!

  2. OK, deep breath, reminder to self to cut and paste the text before submitting just in case I goof up again, and here we go:

    First of all Jason, allow me to say that I am very much in awe of people like yourself who design circuits. The amount of toil, sweat and grief that is involved in the creating of anything is huge and often goes unnoticed by us end-consumers. I know that you are a fan of the open-source way of doing things, so I hope you don’t mind if I make my comments here.

    I very much with agree with Bob KE7GKM that the VRX-1 has amazing performance for such a simple receiver. Bear in mind that I live in a particularly harsh environment for radio, living in an old (over 100 years) house that has wiring inadequate for the needs of the occupants; every time my fridge kicks in, the lights dim! Furthermore, there is no electrical ground, I doubt whether much f the wiring is carried in metal conduits and if it is, they aren’t grounded. Also, to put my comments in context, know (as I think you do) that I listen and observe very critically. I do this not to be a poopy and grumpy old man, but in the interests of always reaching for something better!

    The common-gate preamp almost completely eliminated the hum. The majority of listeners would say that there is no hum. My super-critical ears hear an extremely faint hum way down below the noise that is at the same level at which you would hear the hum in a good commercially-made receiver running from an AC supply. That is how critically I listen. So for all intents and purposes, the hum has left the building.

    In addition, there is less of a click on connecting and disconnecting the antenna. It is still there, but much reduced, and doesn’t hurt the ears like it used to. I guess this means we have less LO leakage now?

    The audio filtering (especially with a nice gentle peak at around 500Hz) definitely helps, but ideally I’d like several poles of passive audio filtering with a filter bandwidth of 500Hz. Why passive? Well, I think the circuit already has enough gain, and can overcome small losses from the addition of a passive filter. On top of that, there is a beauty to well-designed passive circuitry. Note that this comment is coming from a guy who has harbored a secret love for crystal sets since boyhood 🙂

    There is one issue remaining that does bother me a little, and that is the one of microphony. I am experiencing quite a bit of it in my VRX-1 and would like to see less in any future editions of this hot and crackin’ little receiver. I wonder if the increased isolation between the ports of a DBM package would help? I know this adds to the cost, but the ADE-1 is only $2.29 if you buy in bulk, and if there were a significant increase in performance, I think it would be justified.

    While typing this, I’ve had the ear-buds in and have heard 2 JA stations one of whom is our friend JA1NUT, ZL2AGY, WA5PFJ and N5PHT as well as numerous stations whose ID’s I missed. This receiver is definitely working!

    So here are my suggestions for a possible VRX-2:

    *common-gate preamp
    *several poles of non-ringing passive audio filtering with b/w of around 500Hz
    *less microphony (no concrete suggestions on how to achieve, but would a commercial DBM package help?)
    *I’m even thinking of a simple one-sided PC board with the components soldered to the tracks on the top (no through-hole). This could be a lot cheaper to produce than a 2-sided plated-through board, but would ensure that all builders adopt the same satisfactory layout to maximize isolation of the necessary circuit sections from each other.

    Great little receiver. I am really enjoying experimenting with it!

  3. I just noticed 2 typos in the post I made but cannot go back and edit it. I will just have to learn to live with the imperfection, despite my OCD’s attempts to get the better of me.

    4:48am. Holy cow. Time to sleep. Thanks for the VRX-1 again!

  4. @Dave Richards AA7EE
    Sorry about the problems with the CAPTCHA! I’ve turned it off for now since I’ve got Akismet turned on. It seems to be doing a good job of catching spam and the CAPTCHA doesn’t seem to be too much of a speed bump anyway.

  5. @Dave Richards AA7EE
    Thanks for all of the excellent feedback. I certainly will keep all of it in mind in future receiver development. Wish I had something more witty or profound, but I’m running on a few hours of sleep, so this comment will have to do!

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