Hi all!

As part of a group project I have to design a circuit that involves a frequency mixer in cadence allegro.

I have to do this to exploit the fact that cos(A)*cos(B) = 1/2(cos(A+B) + cos(A-B)).

Anyway, for the moment I just put in an analogue behavioral model mixer, which does the job perfectly https://en.wikipedia.org/wiki/Frequency_mixer . What I have to do next is replace that component with a circuit.

I have looked for circuits, but I don't know what is really suitable for the project. I have opamps and p and n type mosfets available to use (don't really know what the type is or whether it matters).

Sadly, there is also a current limit on the design so that's something I have to factor in too. Can anyone help? I see that a 'gilbert cell' does the job, but I can't find a mosfet implementation. Why is a voltage controlled amplifer not ok?

If anyone could help or pointers (or even feedback on my question phrasing), I would be extremely appreciative (I know this doesn't translate to much, but there you go)

Jo

 

The circuit you want is called a mixer. It does multiplication and produces sidebands at A+B and at A-B. Were you expecting some kind of magic?
Minicircuits and others make such circuit components.

 

Hi, thanks for your reply.

Yes, but the idea isn't that I purchase a mixer, it is that I design one using op-amps and/or possibly MOSFETs

 

Those components are not likely to get you there. How about transformers and diodes? Will those work for you? See for example:

http://www.radio-electronics.com/info/rf-technology-design/mixers/double-balanced-mixer-tutorial.php

 

Diodes and transformers... I suppose I will have to ask the tutor. I did see that a few designs used diodes and transformers though.

Thank you very much for the link! On this page: http://www.radio-electronics.com/info/rf-technology-design/mixers/fet-mixers.php

There is a diagram of applying both the signals to a dual gate MOSFET... I can't synthesize one out of the MOSFET I have can I? And I can only use MOSFETS and FETs interchangeably if they are only used for switching?

That isn't a circuit I can build, is it?

 

Hi,

I got in contact with the tutor and he suggested using a 'switched gain demodulator'... But when I google it I don't get any results. What is that?

I think I know what the principle is: you multiply your signal by 1 or -1 based on the frequency of modulation you want, and that is like mixing. Only question is, how to do?

 

Have you considered an NMOS or BJT balanced modulator? Google LM1496.

 

Thank you very much my good man! This appears to be exactly the sort of thing I was looking for.