Newbie needing help designing a mixer

[Amish_Fighter_Pilot]

Hello! I am new here, and fairly new to electronics. I have no formal education, but I have a lot of projects so I am learning what I can here. Currently, I am making a meter that allows me to take RF interference and listen to it as sound. I need to build a mixer for this though.

My transmitter operates from very low frequencies(sub 100Hz) to 500KHz, and I will not be transmitting a modulated signal. I can transmit in almost any waveform, but will probably use sine the most. I will be creating the audio via the mixer by clipping off the top of the signal and turning it into audio. So I transmit at 5KHz for example and then I set my local oscillator on the receiver to either 5.5KHz or 4.5KHz to create a 500Hz tone as the difference product of the mixer. Then I will be able to hear frequency, phase, and amplitude variations as audio if they are near the target frequency. I will also need to make an adjustable low pass filter of some kind to kill anything above the output band. The audio ranges would probably go to a headphone amp daughterboard after that.

So my big problem here is that I do not know electronics well enough to pick the values for this mixer. I can't find an existing product that works in these frequencies. I have been leaning toward a diode ring as a double balanced mixer, but I have also been looking at Gilbert Cells. I'm trying to keep this as basic as possible given my inexperience, but I'm on a mission here and really need to put something together. Can anyone give me some guidance on selecting specific diodes for this? Will I need to amplify the raw antenna signal before mixing?

Also, does anyone know of a diode mixer circuit that doesn't use transformers? I am trying to keep inductors out of this design as much as possible. What is my best design for this?

I have KiCad and sort of know how to use it. Does anyone know if the Spice simulator it has can properly simulate a signal mixer? I'm a little lost on how to do something that advanced with it. I know this is a lot of odd questions sorry! Thanks in advance for any help!

[jwet]

If you really want a diode mixer, my first stop would be Mini Circuits.  They make many- as you've noticed, your low frequencies make this difficult, they have transformers internally and have a lower F limit.  Look at their site- they are a great resource.

If I was trying to do what you wanted and wanted to use hardware. I would use the SA612, these are gilbert cell mixers that can operate down to DC and are very easy to use.  Ignore their internal oscillator at your frequencies.  The SA612 has been revised several times but mostly stayed the same.  Old timers still call it the NE602 which was the original part and you'll find thousands of applications using it.

Another very low frequency mixer option is an MC1495- technically, this is a multiplier but if you go through the theory, that's what mixers are.  This is very vintage stuff.

SPICE can simulator mixers and there are models around for the  SA612 or MC1495.  LTSPICE is especially good and free and has a lot of tutorial type circuits.

At these kind of low frequencies, you also can do this kind of mixing in the software domain in a PC, DSP or ARM CPU.  The Teensy 4 is a pretty hot arm chip that can do this kind of work.  Its about $20.  The supplier Paul Stoffgren makes an audio board and has an amazing audio library that would be great for a beginner.  It use sort of patch cords and drag and drop blocks that then go into the Arduino compiler. 

You can also experiment usuing a PC and a soundcard- there is DSP or SDR type software that can be used to hack around and even here what you create.  Check out Spectrum lab- its pretty amazing for a free software.

Have Fun

[TimFox]

Normal operation (with transformers) of diode-ring modulators is really switching:  the high-power "LO" input switches the diodes that pass the low-power "RF" input to the "IF" output.
At low frequencies, one could use four CMOS analog switches driven by appropriate logic gates to do the same thing without transformers.
For example,  https://www.ti.com/lit/ds/symlink/cd4066b.pdf?ts=1732951924405&ref_url=https%253A%252F%252Fwww.google.com%252F

[TimFox]

Some CMOS-switch modulators from the guitar-effects crowd:
https://www.charlielamm.com/gear/balanced-modulator/index.php

[Amish_Fighter_Pilot]

Thanks for the great replies! I have been super busy lately and I haven't had much time to get on here. Sorry I am slow to respond.

I designed a mixer circuit that I am having some trouble getting just right. It's an entirely new type of mixer circuit so far as I can tell, but I need to filter the output more. It uses entirely passive components, and basically only produces the difference between two reference frequencies(in theory at least) as amplitude modulation of the average of the two input frequencies. Kind of weird and I am still trying to figure out how to demodulate the output.

Here is a picture showing the mixed output and the original two signals. Sorry I couldn't get the scale quite the same because I don't know LTSpice very well yet. V3 is sine wave at 1KHz, and the V4 is sine wave of 1.2KHz. The output has a peak-to-peak of 200 on the clusters(not sure the term for that), but it is obviously made up of a single higher frequency wave that is amplitude modulated. I just want the 200Hz out of that, but I am struggling to figure out how to get it. Any thoughts on how to accomplish that would be greatly appreciated!