Why such a low cutoff for this low-pass filter in this superregen receiver?

Thread Starter

testuserabcdef

Joined Jul 12, 2016
127
I was looking for ideas for super-regenerative receivers and I came across this website showing a circuit similar to mine:

http://jap.hu/electronic/sregrcvr.html

I attached the circuit for reference.


I accidently forgot to include a low-pass filter in my circuit, but this circuit has one (via R3 and C6). I used the calculator at http://sim.okawa-denshi.jp/en/CRtool.php and it states the cut-off frequency is about 1Khz.

Why is 1Khz chosen? If I wanted to receive audio on this thing, wouldn't 1Khz cut most of the wanted sounds instead of the super low deep voices? I don't get it? and if I wanted to receive data, wouldn't 1Khz cut-off affect data transmission speed?
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

While you are at it, what are the other values?
C7 and C8 and R4 especially, but knowing all the values would be even better.
 

Thread Starter

testuserabcdef

Joined Jul 12, 2016
127
Here are the values taken from the site

R1 220k
R2 10k
R3 33k
R4 1M
R5 56k
R6 1M
R7 33k
R8 1k
C1 22p
C2 4.7p
C3 22p

C4 2.2n
C5 10u
C6 4.7n
C7 22n
C8 4.7n
C9 470n
C10 470p
C11 47u
L2 100u
T1 BF198
T2 BC182B
T3 BC182B
 

MrAl

Joined Jun 17, 2014
11,389
Hi,

I did a quick calculation and found that the cutoff moves up somewhat from 1kHz up to around 2.3kHz but that still sounds low, but oddly enough that's around the right value for speech, but not for music. Maybe this was meant for speech as in short wave radio communications or something.
What increases the cutoff somewhat is the fact that there is a high pass filter right after the low pass filter, but then there is another cap too and so it may not go up my much.

We could probably do a simulation of that part of the circuit and see what we find. It's just three caps, two resistors, a transistor, etc.
Not too much to do.
 
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