Tuesday, September 21, 2010

S-meter for the MC3361

Sometimes when you want something...it's better to do it yourself!

I searched and tested some s-meter circuits (in fact it were 2) for the MC3361 who doesn't has RSSI output but in one circuit I didn't had the transistor, so placed an equivalent without success, the other it didn't worked... so decided to design my own "signal meter" to connect the MC3361.
After some testing and rough calculations here's the double 455Khz amplifier and rectifier.

It is nor perfect or optimized and if you take the transistor parameter calculation, you should see the bias resistor are a big on the high side, but never mind, it works.
Other thing, I think the meter I used is a 250uA (it isn't marked) and I didn't care to verify but if you have to much displacement on yours , just place a serial resistor.

Keep the wire connecting to pin 5 as short as possible or you might start receiving some broadcast stations or hum.

The germanium diodes weren't marked and I forgot the reference (I have them for some time), maybe some OA90 or something 119 (can't recall if it isn't a 1n34). Just ask a glass diode one with a green stripe :). Any crystal radio detector diode should work and even 1n4148 might do the job, just have fun and test!

The circuit was made using a technic similar to island boards used extensively by PY2OHH, you can see some examples here: http://py2ohh.w2c.com.br/trx/bitx/bitx.htm

....ah...this is just an indicator s-meter, any similarity with a real calibrated one is purely fictional...


Julian said...

I use the S-meter in my head. It works with any radio and is just as accurate as yours. :)

Ricardo - CT2GQV said...

I would say, even more accurate than mine!
Anyhow this was done just because the meter is in place on the front panel and it looks cool :)