Diode mixer to moduate a 3MHz IF signal

Diode mixer

I have built a diode mixer to moduate a 3MHz IF signal with audio from a microphone. It's hard to tell with the old oscillosscope that I have, but it looks like the signal out of the mixer is just the IF with no modulation. The primary of the output transformer of the mixer has what looks like the IF with the modulation on it on both terminals, but it looks like just the IF (unmodulated) is getting through. I tried making the load a higher impedance and adding a coupling capacitor to the output but it didn't help. The original resistance in the mixer circuit for matching was 75 ohms but I increased it by about a factor of three to keep from loading down the mic amp too much. It wasn't well matched with the circuit it was driving prior to that anyway (IF input). I'm not sure what would make it pass the IF but not modulated. Anybody have any suggestions?

George
KB1PDK

Re: Diode mixer

Are you trying to modulate AM, FM or Phase? Driving a microphone signal into a diode mixer isn't the normal approach. A schematic might help. If you want to cancel the carrier until modulation is applied, you will have to use a balanced mixer.

Brian (GW6BWX)

Diode mixer

It is AM. Thank you for your reply. A schematic is attached. It should give you a better idea what I am doing. It is the same circuit, but the one shown is being used in another part of the transceiver. The one for the MIc and the IF came out on two pages when printed it so I scanned this one instead, but it is the same configuration. Thank you.

George
KB1PDK

Re: Diode mixer

In fact using a mixer as a modulator you can get only an AM modulation. In your case you can get an AM signal with suppressed carrier or a DSB (Double Side Band).

You can visualize the DSB IF output of your modulator using a standard analog oscilloscope. The waveform should look like in the attached picture.

The diodes in your schematic are wrong connected, because they are not connected in a "ring", and will not work as a modulator (see the picture for a ring modulator).

73's

Re: Diode mixer

I agree with Vfone. I must confess to digging in my text books to read up on diode ring mixers again, it's many years since I used one.

If your schematic is labeled with real component types it wouldn't work even with the diodes facing the right way. You should use matched signal diodes, preferably ones with a low forward voltage drop and low junction capacitance. 1N4004 is a definite no-go type !!

Brian.

Actually, I used 1N4148's. I just didn't have a part file for 1N4148's in the library of the CAD program I used, so I substituted 1n4004. But I actually used 1N4148's. I reversed two of the diodes and it seems to be working better. With my scope, which is about 55 years old and only goes up to 450KHz, it is hard to tell. Thank you both for the advise. It seems to be working better.

1N4148 is a great improvement but if you can find Shottky diodes, germanium diodes or 'hot-carrier' diodes it will work even better. The diode ring works like a voltage controlled attenuator in which the modulation is the control voltage. If you can use diodes with a lower forward voltage drop it will reduce the non-linearity at low microphone levels.

Brian.

I used a n audio amp based on a 741 op amp to boost the signal from the mic and I drive a transformer (the input transformer to the mixer for the mic) with a large enough inductance so as not to load down the op amp too much. I had increased the resisters in the mixer to almost 300 ohms. Would it work better if I reduced it back down?

George
KB1PDK

300 ohms resistors should be fine if the level from the Local Oscillator is high enough to open the diodes, which for this type of mixer shall be approximately +7dBm (or 1V pk/pk on 50 ohms).

Actually a DSB diode modulator can use only two diodes, the other two giving just better LO to AF isolation.

I noticed on the schematic above that the hot lead of the primary or secondary is marked with a dot. Does phase matter in the case of the mixer that I built, and if so, how would I hook it up? Which of the three transformers does it matter for. I guess it wouldn't matter for the one that inputs the audio, since that is non-periodic and unpredictable. But the other two might matter. I'm not sure.

George
KB1PDK