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Q&A

Location of AGC in a superheterodyne RX chain

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RF novice here. If using a superheterodyne receiver for the purpose of FM/FSK modulated signals, I'm wondering if the location of an optional automatic gain control (AGC) circuit is always placed on the IF amplifier in such receivers, or if there are other options?

I've been reading Superhet Radio AGC - Automatic Gain Control and similar articles and they often refer to AGC as a means to adjust volume control in AM radio broadcast receivers and similar. The article says "the most widely used sources of the AGC voltage are from the demodulator".

However, I'm working with FM short range devices using single chip RFIC solutions, which come with an optional AGC feature. The circuit designers seem to assume that I already know exactly where the AGC is located and what it does. The datasheet of the RFIC does unhelpfully not show any schematic picture of the RX chain, but explains it with words:

The internal low-noise amplifier (LNA) is designed to be a wide-band LNA that can be matched with three or four external discrete components to cover any common range of frequencies in the sub-GHz band. The LNA has extremely low noise to suppress the noise of the following stages and achieve optimal sensitivity; so, no external gain or front-end modules are necessary. The LNA has gain control, which is controlled by the internal automatic gain control (AGC) algorithm. The LNA is followed by an I-Q mixer, filter, programmable gain amplifier (PGA), and ADC. The I-Q mixers downconvert the signal to an intermediate frequency. The PGA then boosts the gain to be within dynamic range of the ADC. The ADC rejects out-of-band blockers and converts the signal to the digital domain where filtering, demodulation, and processing is performed. Peak detectors are integrated at the output of the LNA and PGA for use in the AGC algorithm.

This definitely sounds as if they refer to the first amplifier before the mixer, rather than the IF amplifier.

When using similar parts in the past, I was pretty certain that the AGC was located on the IF amp and was used to compensate for variations in the IF, by checking the output of the demodulator like in the linked article above. For example enabling AGC on those other parts would screw up image rejection calibrations, which in turn suggests it was located on the IF amp(?). In my current case, the mixer is provided with a high accuracy local TCXO, so perhaps AGC of the IF amp isn't necessary?

What's your experience about AGC when it comes to FSK receivers, is there a "standard" location where it is placed? Could there be multiple AGC?

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The LNA appears to be the one that is adjusted and, that makes a lot of sense because that's where yo... (3 comments)

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