This requires tinkering. He needs to try altering the internal processes of
the box, say by temporarily changing the number of active MIMO antennas or
the clock frequencies used for the logic. When he discovers what affects the
4.3 KHz tone he can pursue that to the physical cause. This sort of
frequency might be related to the execution speed of a software control
loop. I presume there is a microcontroller or FPGA he can tinker with. He
should also probe the power buses to look for 4.3 KHz current draw and look
for the associated device. He might need to break traces to add loops for a
current probe or small series resistors. If need be, he can use the FFT
feature in his scope to separate the tone from other noise. You didn't say
whether the noise is additive or multiplicative (AM), but that's a powerful
clue. He can look at the RF passing through the signal processing chain to
see where it first appears. If it's right out of the LNA, try inserting a
POL regulator to see if that cleans it up.
Orin
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Tom Comberiate
Sent: Monday, April 11, 2016 4:12 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] EMI question
Hi all,
A friend of mine was telling me about some anechoic chamber testing he's
been doing on a MIMO array at around 10 GHz where he's seeing 4.3 kHz
amplitude-modulated tone onto his signal of interest. He still sees it when
he switches to a different type of transmitter with a different power source
so I'm thinking it's coming from his receive chain somewhere, maybe from a
power supply on the receive board he fabricated. He moved his array to a
different building in order to see if the facility we were in had some quirk
that was causing it but the issue remains.
Is anyone on the list willing and able to provide some guidance on how to go
about debugging and resolving this issue? Any resources you can provide
would be much appreciated as he and I are both beginners when it comes to
EMI testing (if that's the appropriate name for it).
Many thanks,
Tom
--
Thomas M. Comberiate, Ph.D.
tcomberiate@xxxxxxxxx
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