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Re: SI4432 AGC #tinysa

 

So perhaps this is a naive perspective, but I would think that having AGC enabled in a spectrum analyzer would always be sub-optimal during measurement sweeps - because the changing gain/attenuation adds yet more variables to the calibration between samples within a single sweep.? Having the ability to do a "pre-sweep" of sorts to define the maximum/optimum gain settings for a given input could be valuable (along the lines of an "Auto" button for the analyzer, to pre-set gain balance), but throughout a single sweep I would expect the most consistent results to come from a fixed gain setting.

Obviously a fixed gain setting throughout the sweep may result in a lower overall dynamic range, but that seems like a tradeoff that makes sense.? Of course, all of my spectrum analyzer experience is with older units that are happy to have 70-80dB of dynamic range under ideal conditions!


Josh, KB8NYP


On Tue, Feb 18, 2020 at 6:15 PM Jerry Gaffke via Groups.Io <jgaffke=[email protected]> wrote:
I don't yet have working hardware, so can't verify your results.?

I assume the blue line represents both PGA and LNA gain.
And that the single step down and back up near the middle represents the LNA gain.

The PGA gain initially staircases down in 8 steps over a period of roughly half
a horizontal division as the four bits of PGA gain are reduced from max gain of 24dB
to min gain of 0dB.? (From page 50 of SiLabs AN440)

Your artifact is also roughly half a horizontal division, somewhat delayed from that
reduction in PGA gain.? I think it's just that the PGA gain register and the RSSI register
are not in any way synchronized.? A quirk of the Si4432 design.

I think the solution is to leave the AGC off.
Each time the RSSI register is read, check the value to see if it is getting close to
either the upper or lower limits.? If it is, then adjust the PGA and/or LNA gain manually,
wait for a millisecond or two, and then re-read the RSSI register
before advancing to the next frequency of the scan.

A couple curiosities I don't really have an answer for:

1)? Why is the 20dB of LNA gain shifting showing up as only about a third
of the scale of the 24dB of PGA gain shifting in the blue trace???

2)? If PGA gain changes are not synchronized with the RSSI register,
how is it that changing the LNA gain bit is synchronized with the RSSI register?
The LNA is positioned before the PGA in the Si4432 receiver architecture,
the RSSI is way back toward the end of the receive process.

Jerry, KE7ER?


On Tue, Feb 18, 2020 at 02:15 PM, m0wid wrote:
I have been away, and will be the rest of the week, so no time to play, however last night I did add back in the matching components for the SAW filter.? It eventually dawned on me that a highly selective SAW filter at the IF was not ideal when doing a pass with wider RBW.? Apologies for being stupid.
This is the trace now - still the artifacts from agc changes.? Is this unique to my module, perhaps I have a dodgy one?? The artifacts do not appear, or change position, with different signal levels, and do not appear if the gain value is fixed.
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