Thursday, 28 June 2012

Don't always depend on AGC in digimode working!

The effects of receiver gain, dynamics and AGC (Automatic Gain Control) are quite easy to "measure" with digimode waterfall displays.

I suspected that the AGC of my Kenwood TS-850S is not very optimal for digimode working, so I arranged a simple AGC test. The frequency was 14.076 (the JT65 section on the 20m band) and the antenna was a 20m delta loop up about 20m. The JT65 mode was chosen, because the transmission times are long enough to get accurate comparison results.  During the JT65 transmission minute (less...47 seconds?) I switched the AGC on and off a couple of times, after having adjusted the RF gain manually to an acceptable level. 

Here is how AGC affects reception of JT65 signals. Click the image to show it bigger.
The figure above shows interesting things:
  • When AGC is On, the RF gain is automatically reduced so that the strong signals don't cause clutter on the band. However, some weak signals become unreadable. Since this TS-850S has quite good receiver dynamics, it is not very critical, compared to cheaper rigs, but anyway: some weak, possible dx stations suffer besides strong signals.
  • When AGC is Off, the gain has to be adjusted manually with the RF gain knob. If gain is 100% without AGC, the strong signals clutter the band and probably make signals unreadable. When finding a level where there is a little bit background noise, there could be still some clutter, but weak signals get readable. In the figure above there is some clutter, but it does not affect the readability.
When I found out this for the first time, it felt like magic: I can find new stations by tuning the RF gain manually! And more stations with less gain...weird, eh? :) Not quite so, but the best compromise has to be found. The RF gain knob is now of the most important knobs on my rig in digimode working. Soon I worked new continents and countries on all bands. Optimally the AGC should to be tuned all the time, depending on the situation on the digimode band. 

Don't forget to set the AGC on and the RF gain to max when working CW and SSB! I have missed some opportunities because of my bad memory here... The default AGC level is most probably still ok for CW and SSB working, but e.g. this rig was created before modern digimodes did exist! In modern rigs there are often separate settings for digimode working and I suspect the AGC is also affected.

Note that simpler rigs, like my IC-718 do not have any possibility to switch off the AGC and set the RF gain manually.

Last note: Don't mix AGC with ALC (Automatic Level Control). Also ALC  is important in digimode working, at least in phase shift keying modes, but that's another story.


  1. Very good explanation. Thank you very much
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