Automatic Gain Control (AGC)
The mcHF has two AGCs:
this AGC has been built by Clint KA7OEI a few years ago and it works very well.
this AGC has been built by Warren Pratt and is part of the WDSP library. It was implemented for the mcHF because it has some more features that might potentially be useful in very specific RX situations. You can choose between the two AGCs with the menu entry AGC.
The Standard AGC is very easy to use and adjust and does a real good job. If you are satisfied with this AGC in most situations, just use this AGC and do not expect a much better result with the WDSP AGC unless you are willing to spend quite a bit of time for adjustment and use of the WDSP AGC. The standard AGC lacks double AGC detectors and a hang AGC. The WDSP AGC is much more complex to use and adjust and should only be used if the user is ready to spend some time and patience to search for the best adjustment parameters and continues to learn and test this AGC with different parameter settings. This AGC has double AGC detectors (one with short and one with long time constants, thus potentially better for a suppression of short noise impulses) and a hang AGC with adjustable hang time.
Figure 1: Relationship between input and output signal level WITH (A-->D-->F) and WITHOUT (A-->B-->C) an automatic gain control (AGC) with an indication of some of the WDSP AGC parameters that can be controlled by the user. Inspired by a paper by Phil Harman: “A discussion on the AGC requirements of the SDR1000” (I added the relevant AGC parameters for better understanding).
If you want to use the WDSP AGC, please read this paper first and additionally use figure 1 above to understand how the AGC works: Phil Harman (VK6APH): A discussion on the Automatic Gain Control (AGC) requirements of the SDR1000. - http://www.flexradio.com/downloads/vk6aph-agc-discussion-powersdr-pdf/
This paper is excellent, fun to read, instructive and essential for the understanding of the functioning and the adjustment for the WDSP AGC.
Because Phil has explained the AGC so well in his paper, I will now restrict myself to the explanation of the menu adjustments of the parameters implemented in the mcHF firmware and assume that the reader is well informed about how these parameters affect the AGC functioning.
choose between “Standard AGC” and “WDSP AGC”: please note that the S-Meter in old school style does not work when switched to “WDSP AGC”: use the S-Meter based on dBm instead.
choose between predefined parameter sets for the AGC. Decay means the time constant until the AGC recovers from a strong signal to achieve higher gain for a low signal.
Indicates the output signal level difference between very low input signals and very high input signals. If set to 0dB, every signal has the same volume and the RX sounds flat. A good sounding effect can be adjusted with settings between 3 and 10dB, but that is very dependent on the hearing habits of the user. Experiment with this, until you achieve a pleasant and non-tiring result.
Enable the Hang AGC which works like this: Hang is enabled when the signal level exceeds the AGC hang time threshold. When the level decreases (no more speech, for example), the hang counter is enabled and holds the AGC gain constant for the hang time, and after the hang time has expired, the gain increases quite fast. Nice for medium to strong SSB signals.
Hang AGC: hang threshold and hang time fully adjustable, but will not be saved.
set AGC Hang enable to ON
set AGC Hang time to your prefered time period (in ms)
set AGC Hang threshold: while you are tuned to the desired signal,
decrease the AGC hang threshold in the menu, until the AGC box changes its color to ORANGE,
decrease a little little bit further --> ready!
AGC-box in a nut-shell:
AGC-box is blue without text: AGC has nothing to do (the input signal is lower than the AGC threshold)
AGC box is blue with text "AGC": AGC is working on the signal (the input signal is higher than the AGC threshold)
AGC box is orange with text "AGC": AGC is working, HANG AGC is enabled and the input signal is larger than the AGC threshold AND larger than the AGC HANG threshold.
If HANG AGC is not enabled, the box will always be blue.
###AGC Hang threshold for signals exceeding this level, the hang AGC is activated. For lower signals, nothing happens
###AGC threshold this is the most important WDSP AGC parameter. It determines the input signal level where the AGC knee is situated (see figure 1). AGC action takes place for input signal levels exceeding this level. It is given as a gain figure in dB. If this threshold is too low, AGC action takes place with band noise leading to annoying hiss for RX. If it is set too high, AGC action starts only with very large signals and this can potentially lead to distortion in the ADC with large signals. There is an AGC indicator, a blue box right of the load indicator. If it says "AGC" the input signal level has exceeded the AGC threshold, which means AGC action is taking place. So it is very easy to adjust the AGC threshold:
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Set AGC WDSP to FAST (for faster reaction of the AGC indicator box)
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Set mcHF to a frequency without a signal (only band noise)
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Adjust AGC threshold to the point where the blue AGC box display changes between ON/OFF or a little bit lower
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Set AGC WDSP to FAST/MED/SLOW/LONG/very LONG again after your preference
###Fixed gain if the AGC WDSP Mode is “OFF”, the user-adjusted figure for “AGC threshold” is taken as the “Fixed gain” in dB. Be very careful, with large signals without AGC, it can be very very loud!