A couple of weeks ago I ran a post about an old 73 Magazine article that extolled the virtues of putting baluns at the RF input and IF output of diode ring mixers. Dedicated experimenter that he is, Pete Juliano gave it a try:
Hi Bill,
I did the modification and used 20 bifilar turns of #26 on a FT-50-43 core for each transformer.
The most important observation is that I did not see any degradation of performance and in fact I think has helped an overloading situation I was seeing on some extremely strong signals.
The photo is of a DBM that is behind a bilateral stage that on Rx is the RF amplifier and on Tx is the Tx pre-driver. I do have another single transistor RF amp ahead of this. In normal operation the next stage would be the Band Pass Filter.
The back end is an IF amplifier, AGC, Product Detector, balanced modulator and mic amp stage all on a board that came out of a Hallicrafters FPM-300 SSB/CW transceiver. There is a you tube video of this prototype project.
73’s
Pete N6QW
Thanks Pete!
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Hi Guys,
Just finished building the PD/BM that will be used in the final circuit of the LBS Part I. I will also upload a video of the build.
The W1REX MeSquares really work very well and greatly facilitate the build. So the newbie builder -- will have one leg up by using the squares method.
73’s
Pete N6QW
Pete's video on this circuit appears below -- just scroll down a few.
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Bill, I remember in your Soldersmoke book that you had difficulty for a long time trying to understand how a mixer created the sum and difference frequencies, and how this was accomplished in a diode ring mixer. I know that you've got it all sorted out now, but I thought this was a good topic for a video anyway.
Here's my video on the subject:
73,
Alan
Alan: Thanks. Great stuff. Yea, I've been looking at the innards of mixers for a long time. In my book, I try to explain how I have come to understand the physics of the mixing action -- how the use of a non-linear element causes two signals to "multiply" and how this "multiplication" results in sum and difference frequencies. I tried to go beyond the trig functions because for me the trig didn't really explain anything.
In the book I was looking at the classic two diode mixer (beloved of Doug DeMaw!). A few years later, on the blog, I was looking at the action of the diode ring. I concluded that there is a big difference between how the diode ring works and how the two diode mixer works. RSGB provided a great diagram:
73 Bill
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I guess one way of describing the difference between a two diode mixer and a diode ring would be to say that the more simple mixer multiplies the signal by 0 and 1 (if it is operating in "switching mode), while the diode ring multiplies by 1 and -1.
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Opposite "sense" winding and resulting phase shift
My second attempt at building a Herring Aid 5 (the first was 1976) continues.
I thought I had discovered an error in the schematic that (I hoped) explained my failure to get this simple receiver running (scroll down for details). But Dex, ZL2DEX, in New Zealand spotted something that got QST off the hook and put all the blame back on me:
I had failed to check the rotational sense of the windings. The schematic called for 4 turns over the Radio Shack choke. So I just went ahead and wound them. I didn't pay any attention to the direction of the winding. I then hooked it up in accordance with the phasing dots in the diagram. And it didn't work. So I switched the coil connections around. And it worked. Aha! I thought! QST messed up! It wasn't my fault.
Dex brought me back to reality. He noted that I probably wound the coils with the wrong rotational sense. I confirmed this. I rewound the coil following the rotational sense of the choke. I hooked it up following the phasing dots of the schematic. This time the oscillator started right up. So the problem wasn't an incorrect drawing of the phasing dots. Instead it was my failure to remember that phasing is more than just the top or the bottom of a transformer's winding. Rotational sense is also important. That's why "phasing dots" are sometimes referred to as "sense dots."
This doesn't come up very often, because most of the toroidal transformers we make are bifilar or trifilar -- the windings are always in the correct sense because we twist the wires together before putting them on the coil. When we look at those phasing dots, we are focused on getting the proper tops of coils connected to the appropriate bottoms of other windings. We don't pay any attention to the sense of the windings. Thanks to Dex for bringing me back to my senses :-o
Grob's Basic Electronics has this definition for those phasing dots:
| " | Used on transformer windings to identify those leads having the same instantaneous polarity." |
This morning I did a little experiment to confirm all this: I took a toroidal core and wound a little transformer. Using a dual trace scope, I looked at the input and output wave forms. Sure enough, when the windings are in the same rotational sense, there is no phase shift. But when that secondary is wound in the opposite sense, you get a 180 degree phase shift. I know this is very basic, but it was fun to re-learn it and to confirm is.
But I still don't have the little receiver running. I think there are a few problems. That single BJT mixer stage needs a lot of RF (2.5 volts p-p) from the oscillator. Also, I think the 10 uH chokes that I am using are not the same as the chokes used in the original Herring Aid design. So when I build transformers on these chokes, they don't work very well.
But I will keep at it. It has been 38 years... I can wait another week or two.
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