Hi,
Quote:it's hard for me to find the advantage of analog AGC compared with digital AGC except no switching noise from digital steps. ^-^
Actually the amount of switching noise is probably negligible in your case since your main amplitude control will be done by a limiting amplifier. In fact, I think I would avoid "analog" AGC on the LNA stage in particular since you would be providing a feedback path for noise right to the front of the system.
assuming we aren't limited to FM systems, here are my "educated guesses"
Speed:
- Digital loop with look-up table could settle very quickly. The system will integrate amplitude over a certain number of periods (enough to average out the AM information) and then feed back the answer in a single pass.
- Analog loop speed depends on the amount of information in the low frequency amplitude modulation, and the stability of the system. The signal effectively has to go around the loop several times before the system settles.
Resolution:
- The digital loop uses a high-resolution ADC followed by a digital peak detection. Linearity can be high.
- The analog loop uses a logarithmic amplifier and a peak detector. Output is averaged and compared to a reference voltage. Log amp and peak detector limit the linearity.
Size:
- For digital it totally depends on the implementation. There are ways to make it small.
- For analog, it might depend on the bandwidth of your "video filter". It may not be large either.
Power consumption:
- digital can be turned OFF or use a very low clock frequency. Only dynamic current consumption.
- analog is more difficult to turn OFF and consumes DC current.
Simplicity:
- For digital, stability is a non issue. Design is easily scalable to other technologies and benefits from technology scaling.
- For analog, no system clock is needed so it might be better for fully analog systems (maybe this applies to you?). Technology scaling isn't so good for analog.
If you really don't want to do a digital approach, maybe a hybrid approach would work. For example, you could have an RSSI followed by a comparator with hysteresis and some asynchronous logic. Just a thought...
cheers,
Aaron