[TenTec] guide to adjusting Orion receiver; filters and performance

[Sinisa Hristov]

Eric Scace K3NA wrote:

> According to KF6DX's description of the Orion AGC systems, once a signal has crossed
> the AGC threshold, it may be either (a) amplified up towards the target AF output level;
> or (b) attenuated down towards the target AF output level.  In other words, the AGC
> system is not limited to just reducing gain -- it may increase gain!

Internal mechanism is immaterial. Overall gain (whatever it may be) is constant
for signals below the threshold, and decreases above it to prevent increase in
audio volume. This is easily verified.


>    According to my understanding, this is the key to the "weak signal reception" technique that W4ZV and others have described.  If:
>    -- a weak signal is hovering just a little bit above the ambient QRN level, and
>    -- one sets the AGC threshold to a value between the ambient QRN and the desired signal's level, and
>    -- keeps the AF gain relatively low
> then the AGC will add gain while the signal is present (and for a period of time determined by hang), but not add gain while the
> signal is absent.  This draws the signal up out of the noise.

By no means.
The first, and minor point is that there is no such thing in Orion's AGC.
The second and important point is that such an action could not separate
signal from noise because the noise would be amplified as much as the signal,
therefore the signal to noise ratio could not be altered that way.

DSP noise reduction acts similar to your description, but it is based on
selective and adaptive filtering, not on simple gain change. And it works
only with signals strong enough to be distinguished from noise in short time.


>    Of course, the operator needs to ride the AGC threshold pretty attentively to maximize > this effect.  (Hang should be minimized and decay should be rapid.)

"Rapid decay" is actually a rapid increase of gain, the worst enemy
of weak signal reception because it (in conjuction with even more rapid
decrease of gain) causes noise and signal to modulate each other
and reduce readability.

It should be stressed that AGC does not and cannot help in weak
signal reception - it can only make thing worse by introducing
rapid gain changes. All the fuss is about preventing AGC to
interfere with weak signal reception.


73,

Sinisa  YT1NT, VA3TTN