Image freqs & IQ demods?????
 

Basic dumb question:

I read this in passing and am not sure I understood correctly. It
seemed to say that you can do image rejection with a quadrature
demodulator (by summing the outputs).

For example, feed 11MHz to an IQ demod running at 1MHz. The I & Q
outputs will each have 10MHz & 12MHz components in them. Sum the
outputs. One of the frequencies will phase-cancel, the other will add
constructively. Is this correct?

thx
Michael
mdunn **at** cantares.on.ca

Yep. It's called an image-reject mixer, and is well-covered in all the
usual references.

Why they are not more widely used in ham radio remains a mystery.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte

Yep. It's called an image-reject mixer, and is well-covered in all the
usual references.

Why they are not more widely used in ham radio remains a mystery.

Laura Halliday VE7LDH

Usually has something to do with providing quadrature LO without going to 4X
frequency.

W4ZCB

Yes, that is correct.

However, the choice of frequencies in your demod example is likely to give
you other problems not obvious from the basic theory. Harmonics of the 1 MHz
are likely to show up in the 10 (or 12 Mhz) output. There are also likely to
be many mixing products of all of the harmonics of 1 MHz and any other
frequency that can mix with 1 Mhz to produce a 10 (or 12) Mhz output. In
general, it's problematic to make a frequency conversion (mixer) process
where the LO is substantially lower than the output frequency, and
especially so if the LO is an exact sub-multiple of the output.

Joe
W3JDR


"Michael Dunn" wrote in message
.. .
Basic dumb question:

I read this in passing and am not sure I understood correctly. It
seemed to say that you can do image rejection with a quadrature
demodulator (by summing the outputs).

For example, feed 11MHz to an IQ demod running at 1MHz. The I & Q
outputs will each have 10MHz & 12MHz components in them. Sum the
outputs. One of the frequencies will phase-cancel, the other will add
constructively. Is this correct?

thx
Michael
mdunn **at** cantares.on.ca

In article .com,
laura halliday wrote:

Yep. It's called an image-reject mixer, and is well-covered in all the
usual references.

Why they are not more widely used in ham radio remains a mystery.

Maybe now that monolithic implementations are a dime-a-dozen they
will be...?

laura halliday wrote:
Yep. It's called an image-reject mixer, and is well-covered in all the
usual references.

Why they are not more widely used in ham radio remains a mystery.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte

The fact that it's difficult to get image rejection better than 40dB
perhaps? Or am I behind the times?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com