I rethinked the problem after reading several papers.
OK. We have an if-strip. To avoid spectral regrowth in later stages
because of saturation, the input must be <momentary> amplitude
controlled. Pulses and bursts at the input will saturate somewhere in
the stages. An common AGC is always design-limited by solving for speed
AND compression at the same time.
Now, looking at the spectrum of a click - the same as a pulse. For
example cw transmitted by amateur radio morse. How to avoid the annoying
band splitter? One possibility is to shape the amplitude response in
time for the pulse by tanh function. This will 'compress' the spectrum.
A differential transistor stage realizes a tanh function.
A CMOS inverter is in effect the same. One of the transistors of the
differential stage is 'inverted'.
So I think a CMOS inverter already realizes an realistic and simple
pulse shaper for pulses in an if-strip.
I tested different ideas with schottky-diodes somewhere in the if-strip.
In the end, I must say that leaving such out of the design is simply the
best.
Any comments?
- Henry
ehydra schrieb:
Hi Magnus -
Well. I made homework for sure. Endless viewed websites what others had
done.
We all seek for the singularity of beauty in our art. At the moment this
is a CD4007 or similar without AGC. Cheap, effective. Not oversized. I
came from TCA440. One of the not many receiver chips where the IF is at
one output reachable. Then I found I must increase carrier frequency
beyound 30MHz. So the integrated mixer entered useless. I used an
external device. Next I found that the integrated IF-strip is not one of
the quiets... Remembering several statements about using CMOS inverters
in s.e.d. for analog reasons, the way was clear...
Implementing an AGC makes the design much more complicated. Can
oscillate etc.
Going without AGC seems a reasonable way if the modulation is one of the
phase modulation systems where amplitude is not very important.
Then look how big is the dynamic range of a simple CMOS inverter.
Meanwhile I found two examples made by others. So the way looks not to
idiotic ;-)
- Henry
Magnus Danielson schrieb:
I find it more and more curious a working concept not to find in the
I-net. I'm looking several weeks. Maybe wrong keywords.
Well, have you considered what an AGC might do for you?
It has been the traditional way of reducing the effect of signal level
on phase-detector gain and hence on the loop gain. The hard-limiter
text is to be seen in this context where the SNR steers the
compression factor, i.e. change of loop gain.
Cheers,
Magnus
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