# Mixing and cancelling?

Discussion in 'General Electronics Chat' started by stygian, Apr 17, 2010.

1. ### stygian Thread Starter New Member

May 8, 2009
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0
Assume you have a 12khz signal mixed with a 1.8mhz signal. Actual values not important, this is just for arguments sake. Your mixer output would then contain 12khz, 1.8mhz, 1.812mhz (the desired output) and 1.788mhz. Now correct me if I'm mistaken, but the inputs should also be 180 degree phase shifted (inverted). So then can I, by mixing the output with the input signals again, remove the 12khz and 1.8mhz? Furthermore, and this is the main question of this post, how do we get rid of the 1.788mhz signal?

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
The point of a mixer is to produce an output that does not contain the input frequencies. The mixer outputs are the sum and difference of the mixed frequencies.

http://michaelgellis.tripod.com/mixersin.html

3. ### PRS Well-Known Member

Aug 24, 2008
989
36
Hello stygian, I seem to be about at the same level of understanding as you are. Let me just suggest that you build a filter to get rid of the unwanted frequencies. Seems to me that shifting the phase of a particular frequency will not affect its performance in respect to a filter. Your idea of cancelation by 180 degree phase shift is new to me.

4. ### Wendy Moderator

Mar 24, 2008
20,993
2,731
I've never heard of the phase shift idea, I don't think it is valid. There may special cases where it works.

A balanced mixer can suppress the two fundamental frequencies, but it is not a given. Nowdays it can be done with a MC1496 chip, but back in the day it required 4 matched diodes.

5. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
Phase shifting is a standard way of suppressing one of the mixer outputs.

It's done by using two mixers and two 90 degree phase shift circuits on the appropriate inputs, then summing the mixer outputs.

It's common to use simple filtering to remove one of the widely separated outputs of a mixer (like between audio & RF) but for eg. producing single sideband modulation where you need to separate one of two products close together, the phase system is a good way to do it. The only other common way is using narrow band crystal filters such as these:
http://www.hy-q.com.au/pdf/Filter Guide.PDF

Last edited: Apr 18, 2010