Part Number: LMX2572
I'm using the cofiguration LMX2572, buffer, I/Q mixer, PA, all stages located in extra compartments. Nevertheless I have incedental FM on the LO from the modulation envelope. By searching the leakage I was touching pin by pin of the LMX with a tweezer and looking to the phase noise of the LO signal. To my surprise almost no change when touching the LO output (pin22). When touching pin 24 (chip select for SPI) the phase noise increases dramatically (the pin is connected to a controller output via a LC filter). Is this a known effect? Any explanation?
If you see spur at the SPI write frequency (especially with VCO4), then it might be related to this. If you can increase the write speed, it will reduce this spur, if this is the issue.
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In reply to Dean Banerjee:
thank you for the reply. What I see not not a discrete spur. The SPI is inactive at the time (i..e. the CSB input is fix on logical 1). It is rather so that the total noise level of the LO signal rises by touching the pin. I seems to be extremely sensitive to even small AC signals. I will check this behaviour later this week with the EVB to see if it is a general issue of the chip or of my design.
In reply to Holger Eckardt:
OK, thank you for the response. We don't see much issues with this, but typically we put a voltage divider, which has a pull-down resistor. I wonder if putting a pull-down resistor would reduce this sensitivity to noise.
the pull-down resistor helped a bit but did not eliminate the effect. Meanwhile I checked with the eval-board and found it even stronger since there is less filtering on the PCB. However I am meanwhile pretty sure that this issue is not the origin of my orignal problem (IFM on the LO from modulation envelope). So I consider it not really as solved but for now as insignificant.
Thank you for your help.
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