[GreenKeys] ARRL Handbook TU Limiter Circuit wanted

[Ralph Mowery]

You have to remember that only one tone is sent at a time.  So it is not
comparing the amplitude of the mark to the space,but rather the tone to the
noise.  So if the tone is just slightly out of the noise it will be boosted
well above just the noise of the other tone frequency noise at that time.

Ralph ku4pt




-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Harold Hallikainen
Sent: Wednesday, January 20, 2021 3:49 PM
To: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] ARRL Handbook TU Limiter Circuit wanted


> Hi folks,
>
> Please can someone find me a scan of the the input Limiter circuit used
> in a TU in an ARRL handbook - probably around 1955 to1960.
>
> It uses a double triode (6SN7 or 12AU7) with its cathodes connected
> together and the output is taken via a transformer.
>
> I have an article that claims to use that circuit but the there are
> errors in the drawing such as the grid of the right hand half isnt
> connected to anything and the transformer secondary feeds mark and space
> filters via pot (presumably for balancing) but there are no terminating
> resistors so the pot wont do anything.
>
> 73,
> Alf
> G3WSD

Looking around, I find this:
https://ia903106.us.archive.org/13/items/arrl-1970-radio-amateur-handbook/ar
rl-1970-radio-amateur-handbook.pdf#page=294
, from 1970. It is a vacuum tube circuit (1974 uses op amps), but the
12AX7 appears to just be a two stage amplifier. There is a diode clipper
before the 12AX7.

I wonder about the use of limiters in the use of two filter envelope
detectors. Since we are comparing the amplitudes of the mark and space
signals, what does clipping the signal before the filters do?

Harold