Tnx Roelof and Jim for help and comments!
Jim, some questions:
Am 18.07.2011 19:17, schrieb James Moritz:
Dear Stefan, LF Group,
So what is missed in my circuit is the allpass network that makes a 90
deg delay of the input signal.
...Unless the software generates I/Q outputs from the sound card L/R
stereo outputs.
Why? The design Andy shows has a mono audio input. This is what i am
searching for :-)
Scale the values of either R or C proportional to the inverse of the
centre frequency (or both - it is 1/RC time constant you need to
scale). Yo might find faster op-amps neccessary at high audio
frequencies.
OK, i did that for a center frequency of 12 kHz but the result was poor.
Then is systematically played with the values and found two pairs of
1n/5k8 and 15n/2k27 which showed at least 47 dB image rejection in the
LF band, peaking at -76 dB at 12.55 kHz, which will be a theoretical
value of course. I'm going to try these values in a practical circuit
and see what comes out.
A spectrum analyzer is available at work so i can report about the
results, hopefully this week.
The propagation delay adds an error to the phase shift, causing
imperfect cancellation of the unwanted sideband, so it helps if the
propagation delay is small
4000 series CMOS is a bit slow really - I have used a 74HC4052 in an
I/Q receiver, which works quite well at 136k.
OK, understood. So i will use a 74HC4052, a 74HC74 and LM358 which all
are available at a local store here, just 1km away (VLF near field ;-) )...
Tnx agn es 73, Stefan/DK7FC
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