2011/10/27 Marcus D. Leech
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The attached two_tone flow-graph shows that close-in intermod products
are sensitive to overall
signal magnitude settings. Keep the digitla signal magnitudes lower,
and the intermod products are
quite well suppressed. The flow-graph is setup for a WBX for the
antenna settings.
Well, that sounds like the lazy solution, intermodulation products are bad, so just throwing the transmitter power away is not what I'd prefer.
Keep in mind that the CORDIC is used only when the desired target
frequency is not a multiple of
the resolution of the PLL synthesizer on whatever daughtercard you're
using, otherwise the CORDIC
NCO doesn't do anything to the signal.
Is there any way of finding out what the resolution is? We haven't been able to track it down for the RFX2400 board,
but this sounds like a nice way to test if it _is_ the CORDIC.
Connect the TX/RX port to the RX2 port through a 60dB attenuator, so you
can use the RX side to
monitor the spectrum of the TX side. The RX-side bandwidth is set to
50Khz total, which gives you
a good close-in view of the spectrum around the +/- 1Khz tones. Vary
the digital gain control, and
observe intermod peaks around the fundamental tones, and observe that
at digital gains below 0.250,
the intermod peaks become well suppressed (about 45dB down from the
fundamental tones).
Only problem there is that there is a 55 dB loop back between the in and output of the RFX2400 board, so two different radios are needed.
We have observed this as well, but as described before we do not find this to be the correct solution.
About the disabling of the CORDIC, I do not currently have a Xilinx ISE licence, but have instigated measures to get one.
When I (hopefully) do, I will try out both cutting it out and using an optimized one, written in VHDL.
Best Paul