AGC in FM tuners ?

[Vulpine]

Dear all,

i'm wondering why there are FM tuners with AGC in the IF chain. So far my belief was, that for FM reception you just need to amplify/limit the signal to a level that is suitable for the demodulator.

AGC ahead of the mixer would make sense to me (reduction of signal to minimize chance of overloading the mixer).

Could someone please enlighten me ?

Cheers,
Arnie

[owen]

It prevents overloading the demodulation stages from over-strong signals - the sooner you do that after the initial gain stage the better - it provides a more conistant signal level to work around....



Owen

[Vulpine]

Thanks Owen,

what would be the result of overloading the demodulation stages ? Audible audio distortion ?

I'm thinking of a PLL demodulator which (in theory) would only demodulate the strongest signal, IM-products shouldn't be that much of a problem then ?

cheers,
Arne

[owen]

Yup.

It would induce distortion - audible and not-so audible.

The biggest loser for a non-agc stage would be the weaker signals would simply disappear.....

Owen

[phase_accurate]

Since there is no information in the signal amplitude there is no need for AGC. You can use heaps of gain and build your IF strip in a way that it is clipped already with the antenna input shorted.

It is important that the proper signal is selected before any such nonlinear operation is made otherwise you will suffer from IM distortion.

The Revox A76 tuner for instance had an if filter with low group-delay distortion followed by such a limiting IF amplifier. Even when two competing signals are on the same frequency the one which is approx 3 dB higher will "win" and be listenable.

ftp://ftp.studer.ch/Public/Products/...A76_Serv_E.pdf

Regards

Charles

[timpert]

Hi,

In the Revox A76, an AGC can be found. Look on page 29 in the service manual, there you see diode D104 configured as a rectifier to increase negative bias (which decreases gain) for G1 of Q101 with increasing amplitude of the mixer (Q102) output. It's a classic way to implement AGC.

The mixer needs an unclipped signal at a certain level to perform well, and with the large range of signal levels the receiver must handle, gain control before the mixer is required. The A76 has it in the best possible place: right at the input stage, preventing any stages further down the pipe from becoming saturated by a large input signal. The deliberate limiting by clipping of the IF signal only occurs after mixing and filtering.

[Vulpine]

Hi all,

thank you for your valuable replies, i think i got the pciture.

cheers,
Arne

[phase_accurate]

The mixer needs an unclipped signal at a certain level to perform well, and with the large range of signal levels the receiver must handle, gain control before the mixer is required.

This is right but the AGC is not needed for the demodulator as it was claimed first.
The linearity problem of the mixer could also be solved by the use of a high level mixer so you would have an FM tuner without any form of AGC.

Regards

Charles