Understanding double balanced mixer

I was playing around with a double balanced mixer the yesterday. In an attempt to understand how the thing worked, (I could find no good explanations on the web.) I made a little spice file. (To be attached later to thread.) This worked fine. I then replaced the two transformers with opposite polarity voltage sources at one half the amplitude, and kept the series inductance. This doesn=92t work nearly as well! The transformers are doing something that I don=92t understand. (I find transformers a bit mysterious in general.)

Any help would be appreciated,

George H.

LTspice file without transformers below.

Version 4 SHEET 1 880 680 WIRE 320 -64 160 -64 WIRE 416 -64 320 -64 WIRE 528 -64 464 -64 WIRE 160 -48 160 -64 WIRE 528 -48 528 -64 WIRE 320 -32 320 -64 WIRE 416 -32 416 -64 WIRE 320 48 320 32 WIRE 320 48 256 48 WIRE 416 48 416 32 WIRE 464 48 464 -64 WIRE 464 48 416 48 WIRE 528 64 528 32 WIRE 160 80 160 32 WIRE 528 160 528 144 WIRE 768 160 528 160 WIRE 160 192 160 160 WIRE 160 192 -96 192 WIRE 528 192 528 160 WIRE 768 192 768 160 WIRE -96 208 -96 192 WIRE 160 208 160 192 WIRE 768 288 768 272 WIRE 528 304 528 272 WIRE -96 320 -96 288 WIRE 160 320 160 288 WIRE 320 320 320 48 WIRE 416 320 416 48 WIRE 320 400 320 384 WIRE 320 400 160 400 WIRE 416 400 416 384 WIRE 416 400 320 400 WIRE 256 432 256 48 WIRE 528 432 528 384 WIRE 528 432 256 432 FLAG -96 320 0 FLAG 768 288 0 SYMBOL ind2 176 -64 M0 WINDOW 0 84 20 Right 2 WINDOW 3 102 61 Right 2 SYMATTR InstName L2 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL voltage 160 64 R0 WINDOW 123 0 0 Left 2 WINDOW 39 -157 61 Left 2 WINDOW 3 -195 24 Left 2 SYMATTR SpiceLine Rser=3D10 SYMATTR InstName V1 SYMATTR Value SINE(0 1 100k) SYMBOL ind2 176 304 M0 SYMATTR InstName L3 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL diode 400 -32 R0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 400 320 R0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 336 384 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL diode 336 32 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMATTR Value 1N4148 SYMBOL ind2 512 400 M180 WINDOW 0 61 71 Left 2 WINDOW 3 57 35 Left 2 SYMATTR InstName L5 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL ind2 512 48 M180 WINDOW 0 40 108 Left 2 WINDOW 3 41 73 Left 2 SYMATTR InstName L6 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL voltage 528 48 R0 WINDOW 123 0 0 Left 2 WINDOW 39 92 72 Left 2 WINDOW 0 79 13 Left 2 WINDOW 3 77 47 Left 2 SYMATTR SpiceLine Rser=3D10 SYMATTR InstName V2 SYMATTR Value SINE(0 .5 10k) SYMBOL res -80 304 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL res 784 288 R180 WINDOW 0 -39 78 Left 2 WINDOW 3 -47 33 Left 2 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL voltage 160 304 R180 WINDOW 123 0 0 Left 2 WINDOW 39 59 24 Left 2 WINDOW 0 94 80 Left 2 WINDOW 3 45 50 Left 2 SYMATTR SpiceLine Rser=3D10 SYMATTR InstName V3 SYMATTR Value SINE(0 1 100k) SYMBOL voltage 528 288 R180 WINDOW 123 0 0 Left 2 WINDOW 39 -157 15 Left 2 WINDOW 0 -87 81 Left 2 WINDOW 3 -192 47 Left 2 SYMATTR SpiceLine Rser=3D10 SYMATTR InstName V4 SYMATTR Value SINE(0 .5 10k) TEXT -320 416 Left 2 !.tran 400u

Reply to
George Herold
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Here's the file with transformers if you are interested.

George H.

Version 4 SHEET 1 880 680 WIRE -96 16 -304 16 WIRE 608 16 -16 16 WIRE -304 32 -304 16 WIRE -304 128 -304 112 WIRE -128 176 -304 176 WIRE 112 176 -48 176 WIRE 320 176 160 176 WIRE 416 176 320 176 WIRE 480 176 480 128 WIRE 480 176 464 176 WIRE 528 176 480 176 WIRE -304 192 -304 176 WIRE 112 192 112 176 WIRE 160 192 160 176 WIRE 528 192 528 176 WIRE 320 208 320 176 WIRE 416 208 416 176 WIRE 608 256 608 16 WIRE 112 272 48 272 WIRE -304 288 -304 272 WIRE 160 288 160 272 WIRE 208 288 160 288 WIRE 320 288 320 272 WIRE 320 288 256 288 WIRE 416 288 416 272 WIRE 464 288 464 176 WIRE 464 288 416 288 WIRE 528 288 528 272 WIRE 576 288 528 288 WIRE 48 304 48 272 WIRE 528 304 528 288 WIRE 160 320 160 288 WIRE 320 320 320 288 WIRE 416 320 416 288 WIRE 608 368 608 336 WIRE 320 400 320 384 WIRE 320 400 160 400 WIRE 416 400 416 384 WIRE 416 400 320 400 WIRE 256 432 256 288 WIRE 352 432 256 432 WIRE 528 432 528 384 WIRE 528 432 352 432 WIRE 208 448 208 288 WIRE 352 496 352 432 WIRE 576 544 576 288 WIRE 576 544 464 544 WIRE 208 560 208 528 WIRE 576 640 576 624 FLAG -304 288 0 FLAG 48 304 0 FLAG 464 544 Vout FLAG 608 368 0 FLAG -304 128 0 FLAG 480 128 Vpos FLAG 352 496 Vneg FLAG 208 560 0 FLAG 576 640 0 SYMBOL ind2 96 288 M180 WINDOW 0 -7 76 Right 2 WINDOW 3 -7 42 Right 2 SYMATTR InstName L1 SYMATTR Value 200=B5 SYMATTR Type ind SYMBOL ind2 176 176 M0 WINDOW 0 -7 42 Right 2 WINDOW 3 -5 73 Right 2 SYMATTR InstName L2 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL voltage -304 176 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value SINE(0 2 100k) SYMBOL res -32 160 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 10 SYMBOL ind2 176 304 M0 SYMATTR InstName L3 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL diode 400 208 R0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode 400 320 R0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL diode 336 384 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D3 SYMATTR Value 1N4148 SYMBOL diode 336 272 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMATTR Value 1N4148 SYMBOL ind2 624 240 M0 WINDOW 0 -64 56 Left 2 WINDOW 3 -71 8 Left 2 SYMATTR InstName L4 SYMATTR Value 200=B5 SYMATTR Type ind SYMBOL ind2 512 400 M180 WINDOW 0 36 90 Left 2 WINDOW 3 9 -39 Left 2 SYMATTR InstName L5 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL ind2 512 288 M180 WINDOW 0 40 108 Left 2 WINDOW 3 41 73 Left 2 SYMATTR InstName L6 SYMATTR Value 100=B5 SYMATTR Type ind SYMBOL res 0 0 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 10 SYMBOL voltage -304 16 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 1 10k) SYMBOL res 224 544 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL res 592 640 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R4 SYMATTR Value 1k TEXT 136 176 Bottom 2 !K1 L1 L2 L3 1 TEXT -320 416 Left 2 !.tran 400u TEXT 704 144 Top 2 !K2 L4 L5 L6 1

Reply to
George Herold

(LTSpice file snipped)

The polarities are wrong. The transformers are arranged as below (note the dots), which leads to the noted voltage source arrangement.

.---------- | .--------- /+\ ( o ( ) -. ( \-/ )|( | )||--------- < == > o---------- )|( o | -' ( /+\ ( ( ) '--------- \-/ | '---------- (created by AACircuit v1.28.6 beta 04/19/05

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www.wescottdesign.com
Reply to
Tim Wescott

Oh -- I'm not sure about your voltage levels, but your input and output impedances are way high for the usual diode DBM. And some source impedance should be assumed in the LO: you're really interested in driving the correct current through the diodes; drive a fixed voltage through them and the current (which goes exponentially to voltage) will likely be either too high or too low.

The standard rule of thumb is 7dBm at 50 ohms. That's something like

1.4V p-p (or maybe 2.8 -- it's been a while. Do the math). But you need that source impedance! Put resistors in series with your inductors on the drive side, at least.
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www.wescottdesign.com
Reply to
Tim Wescott

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Ahh Duh! Thanks Tim, amazing how 'stupid', I can be sometimes. I flipped my voltage sources and everything is fine.

George H.

Reply to
George Herold

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Yeah, I wanted to make one that works below 100kHz. So I ordered some Audio transformers. 600 ohm impedance.. so the 1 k ohm is not too far off.

The audio transformers only claim to work up to 15kHz. But here's a 'scope shot of the DBM at 100kHz LO and 1 kHz RF

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Things get a bit 'squirrelly' above 300kHz.

The same drives but 10kHz on the LO.

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It's great fun, I've got this SRS spectrum analyzer so I can play with the drive levels and watch all the harmonics move around.

George H.

Reply to
George Herold

o----------

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Have you checked the MiniCircuits catalog? I seem to recall that they have mixers that work down to 50kHz or even lower. They're on the web, at the obvious address.

Things will work at your 600 ohm impedance level, but the distortion-free power levels will suck. If I recall what you're trying to do that shouldn't be a big deal -- just don't try to use them for receiving LORAN.

Also check the amateur radio literature. Winding a pair of balanced transformers for 100kHz service shouldn't be too tedious if you start with the right core material -- audio transformers need all the windings so they'll work down at 300Hz (or 30Hz, or whatever); those windings (and core losses) just get in the way at higher frequencies.

On most of those mixers there's at least one port that works all the way down to DC, and if there's no bells and whistles there's nothing to keep the mixer from operating in "reverse". So you do need to inject your high-level drive into the LO at the rated frequency, and you do need to deal with signals above the rated frequency at the "RF" port, but there's nothing to say that the "IF" port (the one that goes down to DC, usually) has to be an outie -- you can inject your low-level low frequency signal there and extract your high-frequency signal at the "RF" port.

--
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Reply to
Tim Wescott

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Yup not only the catalog but I've got an SBL-3+ from minicircuits lashed up in front of me. Good for 25 kHz to 200 MHz. (well my some what sloppy soldering job may not get to 200 Meg.)

.

Well, for like $3 I bought a few from digikey. I'l have to measure it's freqeucny response.

Yeah I saw that on some audio site. (they call 'em diode ring modulators) Putting the one signal into the 'IF' port and taking the output from the RF. I'll try that with the mini-circuits DBM. It would be nice if I could run the LO at ~50kHz and then modulate at ~1kHz.

Well it works... but the output is not so clean. I'll play around with spice at home tonight.

George H.

Reply to
George Herold

Look up LM1496

Jamie

Reply to
Jamie

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