[minima] Re: Impedance matching audio amp and filter
- From: "José A. Amador Fundora" <amador@xxxxxxxxxxxxxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Tue, 24 Feb 2015 09:25:36 -0500
It is not a matter of matching, it is a matter of "filter termination".
The response of an LC filter depends critically on its terminations.
PSpice is a good tool to evaluate it by means of simulation. Real life
may be a little more complicated, a model is better as it resembles
better to reality, but a complex model is harder to nearly impossible to
simulate.
I have used Active Sallen Key low pass filters using gain ~ 1 (actually,
0.999...) using transistors, which does help to combat tirednedness by
high frequency audio noise where there is no desired signal, as the
crystal filter on the input end has removed them. Any audio bandwidth in
excess of the filter bandwidth leads to that problem. I used an extra
active audio filter in my old Swan 700 using a Chebycheff LPF with three
sections of a TL084, and cutoff on 3.0 kHz. The fourth section was a
high impedance input buffer. It did alleviate the HF noise quite a bit.
All "simple" SSB radios are plagued by SSB demodulation of desired
signals, and DSB demodulation of noise, worsened by AGC. A wideband high
gain IF amplifier with a crystal filter at its input end may do it
easily. A better solution is using a "simpler" crystal filter at its
output end to stop DSB noise, or resorting to an image reject (other
sideband noise) demodulator which is fairly more complex. SNR with those
solutions may be better, which is what counts for digging out the weak
ones, which often are other QRP stations.
My advice is using an active LPF after the demodulator, is the simplest
and fairly effective way. It has no termination issues and can be quite
small. It is also quite inmune to stray fields.
73,
Jose, CO2JA
On 23/02/2015 15:26, Franz DX wrote:
Hi Minima specialists,
I am experimenting with different audio filters directly after the
product detector of the Minima (like the three pole lowpass filter in
Minima version 2 consisting of a 33mH choke and two capacitors) and
different audio amps, but I am having problems with hiss noise.
When I use no audio filter and connect the product detector directly
into the audio preamp, the hiss is not existent (except for a tiny
little bit). But when I use a filter like in Minima version2 with a
serial 33mH choke and two capacitors of 100nF connecting to ground,
the hiss is getting really loud and I am also observing a high pass
effect, although it is constructed and meant like a lowpass!
My suspicion is that it has to do with wrong impedance matching,
having a low impedance 50ohm output out of the product detector and a
high impedance input to the audio preamp.
But how can I match these? Using an LC circuit like one at the input
of Joe´s IF amp? I am convinced this is a really fundamental and easy
to solve problem, so I would really appreciate a link or a hint where
to read about a solution.
Thanks in advance!
Also, maybe the problem would be solved if I use an FST3253 as a
switching product detector like Joe planned it?
Best wishes,
Frank DD4WH
50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964
http://cujae.edu.cu
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