Hello Hans
On Mon, 2025-04-21 at 12:45:03 +0300, Hans Summers via groups.io wrote:
Suppose we want to listen to a CW signal transmitted at 7020 kHz. We
set the VFO to 7020-12 = 7008 kHz. The mixed signals contain the sum
and difference of these frequencies: 14028 and 12 kHz. Only the 12
kHz components pass through the lowpass filters in front of the ADC,
and appear digitally as cosine and sine of a 12 kHz signal.
What about a signal occurring at 6996 kHx? This is the "superhet
image", it also mixes with the 7008 VFO to produce a 12 kHz IF signal.
In your scheme I don't think you are handling this image signal?
The 6996 kHz signal mixed with 7008 kHz will appear as -12 kHz (note the
sign) and 14004 kHz (removed in analog LP filter) at the ADC. Mixing it
with the 12 kHz IF results in -24 kHz, which is stopped in the digital
LP filter. There will be some remaining signal due to imperfections in
the Tayloe mixer, LP and ADC but this problem is also present in other
implementations.
The I-Q signals differ between +12 and -12 kHz. To be explicit, at the
output of the ADC, the 7020 kHz signal will appear as I = cos(om*t) and
Q = sin(om*t) (+12 kHz), while the 6996 kHz signal becomes I = cos(om*t)
and Q = -sin(om*t) (-12 kHz).
After the IF mixer: I = 1 and Q = 0 for the 7020 kHz signal, and
I = cos(2*om*t) and Q = -sin(2*om*t) (-24 kHz) for the 6996 kHz signal.
73 Anders SM5KAE
