I continue to work on the product detector of my Lafayette HA-600A. This work has caused me to brush up on my understanding of how mixers really work.
I think one of the most interesting mixer circuits is the diode ring. With just four diodes and one or two transformers, this device manages to take an incoming signal and multiply it by either 1 or -1 depending on the polarity of the local oscillator signal. That is pretty amazing.
Alan Wolke W2AEW did an excellent video on this: https://www.youtube.com/watch?v=junuEwmQVQ8
Inspired by Alan, I took my most recent homebrew diode ring mixer (with transformers from Farhan, diodes from Jim W8NSA, and a PC board base from the CNC mill of Pete N6QW) and hooked it up to two signal generators and an oscilloscope. I had the local oscillator at 10 MHz and the signal oscillator at 7 MHz. You can see my results in the pictures (above and at the end). You can see the resulting difference frequency (3 MHz) in the broad up and down pattern. And you can see the sum frequency (17 MHz) signal in the faster oscillations. All you would need is some filtering to separate them out.
I really like the RSGB Handbook diagram (above). I think the bottom schematic with its crossed diodes really explains how the phase reversal takes place: when the LO turns on D1 and D3 (the horizontal ones), multiplication by 1 takes place. But when the LO turns on D2 and D4 (the crossed diodes), up goes to down and down to up, creating phase reversal, or, in math terms, multiplication by -1.
At a more basic level, mixing takes place whenever -- in a non-linear circuit -- one signal is controlling the gain or attenuation experienced by the other signal. A complex waveform results, a waveform that contains sum and difference products. A circuit like the diode ring, that alternately multiplies by 1 and -1, is non-linear in the extreme, and the multiplication is controlled by the LO. The results can be seen in the diagram's complex waveforms, on Alan's Tek 'scope, and on my Rigol. And in those complex waveforms you can SEE the sum and difference frequencies. That is really cool.
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Hi Bill,
ReplyDeletepretty nice experiment - now that you have it all set up - maybe try to simulate what happens in your reciever? Turn off the LO, put an AM signal on the RF and crank up it's amplitude to see if you can make it an accidental envelope detector.
Christian: Yea, that is the problem. I've done that and it did act like an accidental envelope detector. Now, there are possible explanations: perhaps I could use more shielding around the diode ring. Perhaps I need a diplexer at the output. But for the moment I have gone back to a version of the two diode, one toroid singly balanced mixer. I redesigned the circuit a bit to make it more similar to Leon's CMOS design. More on this later today or tomorrow. 73 Bill
ReplyDeleteYou have proven what too much RF power does with the DBM. In the reciever the IF (that you put as RF into the DBM) has to be strong enough to turn on diode D-2. It will be strong enough to turn on other diodes (like in the DBM) then as well?! No need to look for shielding or diplexer - the problem is right at hand imho.
ReplyDeleteWell, if things are working right, the IF signal should not be turning on any of the diodes. Only the BFO should be turning the diodes on and off. With the diode ring mixer the problem got better if I significantly backed off on the RF gain, but SSB still sounded bad. So I'm not completely sure it is just a matter of the IF level being too high. I will do some additional tests. It would be nice to get the diode ring in there.
DeleteThinking about it some more, to attenuate the IF signal into the RF port of the DBM, why not just put a series resistor 50-500 ohms? Might have to put a resistor parallel to the input as well to get the impedance there somwhere reasonable. Let's see if i can sleep now...
DeleteI'm whacking myself upside the head! Going back to my days as a NZPO Telephone Tech (1970 +/-), 'Toll' traffic was by the 'Carrier' system, and duty there was assigned to the advancing that have absorbed the (relay-)logic of the electro-mechanicals. With the traffic conveyed by co-ax (industrial-scale 'hard-line), underground and more robust in all senses than the open-wire balanced lines, it was essentially Radio, and had to be SSB for the number of 'stacked' channels. It was all phase-synchronized end-to-end. I never realized that until Digital took over and the gear was decommissioned, with little diode-ring blocks a dime a dozen. S l o w on the uptake! :) Those RSGB diagrams bring it all back.
ReplyDeleteThanks Bill and 73!