Re: Three (and a Half) AGC schemes #ubitx-help

Jerry Gaffke
 

One reason to have an AGC system is to prevent a sudden high level signal from being painfully loud.
A pair of back-to-back diodes in the appropriate spot might be sufficient
The diodes will cause the loud signal to distort until the gain knob is twiddled,
but no need to rip off the headphones first.  That may be all we need here.

These radios are very good at receiving sensitive signals, as good as some of the
relatively expensive commercial gear.  But tuning in a strong signal can then
overload it.  One solution to this is an RF gain control, perhaps just a pot in line with the 
antenna signal prior to the first RF amp.  Don Cantrell has described an even easier
alternative on the Bitx40, where he cuts power to the first RF amp:
http://bitxhacks.blogspot.com/2017/01/nd6ts-rf-gain-control-with-tombstoning.html
May not work so well on the uBitx, as the BiDi amps are quite different.

As Henning notes in post https://groups.io/g/BITX20/message/32043
it is the AF preamp (Q16 on the Bitx40, Q70 on the uBitx) with its gain of 46dB that
limits the dynamic range of the receiver. 
So reducing gain in the presence of strong signals should occur before the AF preamp.
Most AGC schemes presented in this forum attenuate after the AF preamp.

The really good AGC schemes detect the signal level back in the RF part of the rig
so they can respond very quickly, before the signal is felt in the audio sections.
This RF detection would have to occur after the 12mhz filter on the Bitx rigs, otherwise
signals you couldn't hear anyway would cause the gain to be reduced.  But if you detect
after the 12mhz filter without careful shielding, you will mostly be detecting the BFO.
For this reason any simple to implement scheme on the Bitx rigs should probably 
be detecting audio and not attempt to detect RF.

So, an adequate solution for many might be back-to back diodes to prevent ear damage,
plus some sort of manual RF gain control to throttle back any strong signals we wish to hear.
Many users might prefer this, as you hear the band as it really is rather than the artificial
compression introduced by AGC.

If AGC is implemented, I think a good compromise on the Bitx rigs would be to detect
after the audio preamp stage, then use that result to attenuate somewhere prior to the audio preamp. 
 
Jerry, KE7ER


On Sun, Feb 25, 2018 at 09:33 pm, K9HZ wrote:

Well I guess the question to ask is what is your goal?  Do you want to maintain a constant audio level with varying signal strength, or do you want to maintain a constant RF/IF/Audio gain in your radio.  They are much different… have much different impact on the radio.  Limiting and compression can occur at any stage.  So far you have demonstrated an audio gain level control.  If you want t TRUR and traditional AGC, you need to start at the front end of the radio to prevent it from over-loading with strong signals.

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