Hello. I would like to ask one question about AGC (automatic gain control).

I need to design the LNA with AGC for the FM receiver.

As I know, digital AGC require many control bits and switches which complicate the design, analog AGC has higher resolution than digital ways. Except this point, could you please let me know the pons and cons of analog and digital AGC?

And if I will choose analog AGC to achieve fast response, than what is the gain settling requirement of FM system, and how fast the AGC loop should achieved?

Thank you very much.

Thank you very much.

The gain of the digital AGC is controlled by the receive baseband through a digital-to-analog converter (DAC), whereas that of the analog AGC is self-adjusted without the control of the receive baseband. However, much implementation of the AGC function in the digital baseband requires the digital signal processor (DSP) to be awake all the time, which causes significant power consumption. The analog AGC not only saves the digital AGC from an additional DAC but allows the DSP to sleep most of the time as well, reducing power consumption.

Therefor, in the point of power consumption, I feel analog AGC is the better choice.

Also digital AGC require many control bits and switches which complicate the design.

Can I understand like this way?

Thank you very much.

The digital system of most devices can be done with selective on-off of sub sections of the devices so the power consumption issue is not valid.

Also power consumption of a small block of logic at a l;ow clock rate is negligible generally.

The gain control DAC is a few switches I expect. No siginificant area.

The analog device will have some good sized capacitors for both the functionality and the internal compensation of the op-amps.

The digital method I expect will be lower power and smaller area.

Hi,

Quote:


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

Thank you so much to aaron_do and loose-electron.

I learned a lot from you ^-^

The opinions are really helpful for my research.