I perceive this as a problem. Others may not.
Adjusting AGC-T "properly" results in no AGC at all.
Reason: AGC-T is really RF Gain and the AGC Threshold is fixed at such a high level that constant volume changes occur due to fading and various signal strengths.
I demonstrate this in this video.
https://www.youtube.com/watch?v=9z_c4S-LV0g
We end up "riding" the AGC-T control to compensate. It's exactly like using a receiver from the 1930's.
I would like to see AGC-T to be a true AGC Threshold setting. We should be able to move it down - left and lower the threshold so that AGC worked on weaker signals.
Threshold = Point where things start to work.
Low threshold should provide AGC at LOW signal levels.
What we have is not a threshold at all.
Turning AGC-T all the way up - to the right should result in NO AGC. Threshold raised!
The current design is faulty.
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Posted 3 years ago
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George Molnar, KF2T, Elmer
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George Molnar, KF2T, Elmer
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AGC-T is indeed RF Gain. It has nothing to do with AGC. I think I proved this.
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Next we need to look at the jumpy S meter that is impossible to read. AGC has no effect on it either.
My Drake 2B's meter works better!
George Molnar, KF2T, Elmer
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IF gain is normally regulated by AGC.
AGC decay normally controls S meter hang time.
This ain't normal.
Yes I know there's no IF. Not a problem for software though
George Molnar, KF2T, Elmer
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Of course, "a simple matter of programming" could provide legacy-like features, if desired. Sounds like an excellent suggestion for the FRS Lab. For me, I prefer the s-meter to show input signal level, and RF gain to vary signals on the scope. Perception after demodulation is important, but probably (again, for me) should be downstream.
Dang it, now you're making me want to troll eBay for a used but good 2-B. Fine receiver, it was. Loved the copper chassis, cheery glow, and overall look and feel.
Have a great weekend & thanks for the exchange.
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Of course, any such implementation isn't going to be perfect but what Flex has done with there AGC-T works better in software than the Collins Radio implementation does in hardware, at least on my radio's....
I for one really like how the AGC-T control works on the Flex. I would only like them to add a horizontal dotted line across the panadapter screen to indicate where the THRESHOLD is set, that would be helpful....
I might add, in the Flex, the AGC-T does not effect the S-meter reading like it does in all older and big box radios, which I see as a distinct advantage over other implementations.
Steve, hope I didn't miss the point of your post. If I did I apologize...
Regards, Dennis, k0eoo
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I've worked on Collins equipment for many years. The AGC in them is just typical of most any radio. The IF signal is amplified, rectified and the resulting voltage is fed back to RF and IF stages. DC bias for those stages is provided by the power supply and made adjustable by the RF gain control. There are no "threshold" adjustments but there is the usual slow, med, fast switch positions for AGC decay time.
The problem with that era of Collins receiver was slow AGC attack time which resulted in thumps on strong cw stations. Various mods were attempted over the years.
The problem with our Flex machines is there seems to be virtually no AGC at all until the signal is very strong. This results in annoying volume fluctuations as received signals change strength.
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R9 in the 75A4 and R57 in the 75S1 set the AGC Threshold just above the receiver noise floor, plus, in these receivers the RF Gain is controlled by adjusting the AGC negative bias, base line, up or down. That's why you see the S-meter in those radios move up or down indicating the input signal threshold. point.
In other radios of the time the RF Gain control adjusted the gain of the RF and IF amplifier tubes and was totally independent of the AGC system... In those radios the RF gain control was usually in the cathode circuit of the RF and IF amplifier tubes.
Steve, I would like take any further discussion on this subject off line as not to bore the community with such OLD information. My email address is on QRZ....
Best Regards, Dennis, k0eoo
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What do you think about this idea to automate the AGC-T setting? It doesn't address the issue you raised but at least reduces/eliminates the frequent adjusting that is currently required.
https://community.flexradio.com/flexradio/topics/option-to-automate-the-agc-t-setting
Regards, Al / NN4ZZ
al (at) nn4zz (dot) com
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There is another way to approach this. Use Audio Compression. With audio compression you are only limiting the audio not the RF signal. The best would be a combination of RF & AF AGC! Then you could decide do I want RF AGC, AF AGC or Both and guess what with SDR the possibilities are almost endless!
That was one of the things on my wishlist https://community.flexradio.com/flexradio/topics/kc2qma-smartsdr-wish-list-like-who-doesnt-have-one
73/KC2QMA
John
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You bring up a vary good point. Because the Flex has such great dynamic range a RF GAIN control is really NOT necessary. That may be another reason why Flex implemented a design whereby they vary the AGC threshold and not the apparent gain of the signal. I like the fact that the S-meter measurement doesn't change as you vary the AGC-T control or as you tune across a signal...
And, yes John, maybe some form of receive audio compression would be a good solution.
Regards, Dennis, k0eoo
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I also prefer the s-meter to be independent from AGC as it is now. I want to know what the signal is at the ANT Input because that’s what counts to me. I also think of my 6500 like an RF analyzer measurement device.
I think at a minimum if they can adjust the ratio of the threshold to track stronger signals better things will work better. But for me control over AGC parameters are a must!73/KC2QMA
John
Tim - W4TME, Customer Experience Manager
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https://www.youtube.com/watch?v=Hwm6rszuMis&feature=youtu.be
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Like I said the other option is to use Audio Limiting to deal with the differences in sound level.
73/KC2QMA
John
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Good video, I only operate CW and thought FAST was the proper setting so never tried SLOW but from your video it looks like it might be better. I'll give it a try. I was adjusting AGC-T a little differently but ended up close to the same settings as you. (when I start hearing the band noise, I adjust up about 20-30 more).
It sounds like (based on Tim's comments) that the request for an automated AGC-T feature might be added at some point. It would sure take a lot of the fiddling out of the equation. You could just set the threshold you prefer and it would adjust automatically. In addition to the variations due to the band in use, and the band conditions at the time, if you have a gain antenna the noise level changes depending on the direction your antenna is aimed.
Regards, Al / NN4ZZ
al (at) nn4zz (dot) com