Maker Pro
Maker Pro

Pre-cancelling mixer products (DAC sample selection)

H

Hobbyist

Jan 1, 1970
0
I'm working on a hobby project where I need to get a "phase locked"
pair of sinewaves (25Mhz and 27Mhz) from point A to point B. I've
considered several approaches and I've concluded that the most
appropriate way is going to be "analog modulating" an LED with a
signal generated by a DAC. For a first test I'm thinking 1350 samples
(i.e. 25 * 27 * 2) fed to a 54 MSPS converter. I'm picturing a modest
bandpass filter (26Mhz +/- 1Mhz) after the DAC but basicly the DAC
output will directly modulate the current flow through the transmitter
LED.

My problem comes in generating the sample values. I generated a plot
of a simple sin25MHz( t ) + sin27Mhz( t ) for each sample point. The
resulting "mixer products" were obvious. My hobby level books
(magazines mostly) say they are (f1 + f2), (f1 - f2), (f1 + 2f2), etc.
My first question is: Are these "mixer products" new information which
can be removed from my DAC output or are they inherently required if
I'm going to want to recover the individual source signals at the
receiver?

If I can remove the "mixer products", then I was thinking it would be
a lot easier if I didn't generate them in the first place. I thought
an easy way to do this (for the first few "influential" products at
least) would be to add additional "compliment" waves to the initial
suming process so that each product would be "cancelled" out of the
sample stream. Does this make sense? Could anyone provide (or direct
me to) a simple description of how to determine the correct phase and
amplitude of the "compliment" wave for each "mixer product"? (It's not
at all clear from my initial plot)

Thanks for any and all comments or suggestions,
Hobbyist
 
J

John Larkin

Jan 1, 1970
0
My problem comes in generating the sample values. I generated a plot
of a simple sin25MHz( t ) + sin27Mhz( t ) for each sample point. The
resulting "mixer products" were obvious.

Obvious, but nonexistant. If you plot the sum indicated, it may *look*
like there are additional frequencies, but there really aren't, at
least not unless you then pass the sum through a nonlinear process. A
DAC is nonlinear, a bit, and the LED will be too, but these shouldn't
be too serious.
My hobby level books
(magazines mostly) say they are (f1 + f2), (f1 - f2), (f1 + 2f2), etc.

Again, you need a mixer (a nonlinear device) to create the additional
frequencies. Linear summing (as, say, with resistors) is not 'mixing'
in this sense. Audio folks consider linear summing to be 'mixing', but
RF jocks know that a true mixer is a multiplier.

John
 
B

Bob Masta

Jan 1, 1970
0
Obvious, but nonexistant. If you plot the sum indicated, it may *look*
like there are additional frequencies, but there really aren't, at
least not unless you then pass the sum through a nonlinear process. A
DAC is nonlinear, a bit, and the LED will be too, but these shouldn't
be too serious.


Again, you need a mixer (a nonlinear device) to create the additional
frequencies. Linear summing (as, say, with resistors) is not 'mixing'
in this sense. Audio folks consider linear summing to be 'mixing', but
RF jocks know that a true mixer is a multiplier.

John

As a curious aside to this, note that the same effect
occurs when you linearly add two audio sine waves
(as in an audio-type "mixer"). If you look at the
sum on a scope, you can see the non-existant
difference frequency as peaks in the overall waveform.
The curious part is that you can hear this "beat
frequency", even though it isn't really threre.
Yep, you do need a nonlinearity for this to happen,
but it's in the auditory system inside your head!


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
 
F

Fred Abse

Jan 1, 1970
0
If I can remove the "mixer products", then I was thinking it would be
a lot easier if I didn't generate them in the first place. I thought
an easy way to do this (for the first few "influential" products at
least) would be to add additional "compliment" waves to the initial
suming process so that each product would be "cancelled" out of the
sample stream. Does this make sense? Could anyone provide (or direct
me to) a simple description of how to determine the correct phase and
amplitude of the "compliment" wave for each "mixer product"? (It's not
at all clear from my initial plot)

Picking a mixer device with as near a square law characteristic as
possible will reduce the unwanted products. JFETS are quite good in this
respect.
 
Top