Take a look at the simple little mixer above. I think I first saw it in SPRAT. Thinking that it was really just a simplified version of the two diode Doug DeMaw mixer that I had been using for years, I couple of years ago I built it into a little Direct Conversion receiver. It worked great. But later, I began to have doubts about it. In the words of young James Clerk Maxwell, I started to wonder about "the particular go of it."
You see, the way the DeMaw mixer is set up, both of the diodes are simultaneously on and off. This has the effect of "chopping up" the incoming RF at a rate set by the VFO frequency. Boom. Fournier. Mixing. Great.
But look at the mixer at the top of this post. Here the VFO signal is coming in on the wiper of the 1k pot. The same signal is hitting both diodes at the same time. The diodes are not being fed differentially. So D1 and D2 are NOT both simultaneously tuning on and off. Instead, when the wiper goes positive, D2 turns on while D1 is off. On negative swings of the voltage at the wiper, D1 turns on while D2 is off. For me, this made it a "mystery mixer."
This reminded me of the sub-harmonic DC receiver I built earlier in the year: The VFO runs at half the operating frequency, but the diodes are set up to switch on and sample the RF TWICE each VFO cycle. This is the equivalent of having the VFO at the operating frequency.
Could it be that this was just a sub-harmonic mixer with the VFO at the operating frequency? (I should note that Doug DeMaw published a design that actually made this mistake. See: https://soldersmoke.blogspot.com/2011/07/doug-demay-and-polyakov.html ) I knew that this would sort of work, but it would not work very well. And the mystery mixer seemed to work very well. Hmmm.
I was loaning the DC receiver with the mystery mixer in it to a local high school. I worried that I was loaning them something that I didn't really understand. I remembered that I'd been trying to figure out this mixer since early 2021: https://soldersmoke.blogspot.com/2021/02/some-thoughts-on-singly-balanced-mixers.html
Our beloved book, Solid State Design for the Radio Amateur (SSDRA) has an explanation of this circuit on page 74. But this explanation didn't seen to work for me. Check it out. YMMV.
Bottom line: I still couldn't figure this circuit out, so left it alone for while.
The other day I woke up and looked at it with fresh eyes. Suddenly it hit me. Although the VFO was hitting the diodes in the same non-differential way as is done in the sub-harmonic mixer, the RF (signal) is entering the mixer in a differential way. This means that the two diodes are taking turns sampling the upper side of L2, then bottom side of L2, via L1 and L2. This results in a complex repeating waveform that is similar to that of diode ring mixer. Within that complex repeating waveform, there are sum and difference frequencies. I did some noodling on this:
The key difference between this mixer and the sub-harmonic mixer is the way L2 is positioned: In the sub-harmonic mixer, there is no differential feed of the RF. Both diodes get the same polarity of RF. The VFO switches on D1, then D2. The RF is sampled at twice the VFO frequency. But in the mystery mixer that had me scratching my head, the RF is fed to the diodes in differential form. So while the diodes here are -- as in the sub-harmonic mixer -- being switched on and off sequentially, they are taking turns sampling the top and the bottom of L2. That provides the complex repeating waveform that we need to get the sum and difference frequencies. In a DC receiver the difference frequency is audio.
What do you guys think? Do I have this right? How would you characterize this mixer: Is it multiplying by 1 and 0? Or is it multiplying by 1 and -1?
This would be good mixer for a school project. It is simpler than a mixer with a tri-filar toroid.
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