I am trying to implement an IQ receiver for ultra low frequency baseband (between 1 mHz to 1 Hz).
I am using the ADRF6806 Evaluation board to run the experiments.
While the system demodulates the signal fine till about 2 MHz, the amplitude goes down quite rapidly after that.
I assume this is because of the Minicircuits TCM-9 impedance transformer at the outputs of the IQ channels. But I am unable to observe the DC at expected levels (1Vp-p) directly on the differential outputs. Currently I am observing 200 mVp-p for a 1 Hz baseband.
The Minicircuits baluns are still on the board and they are not disconnected. But as per the schematic, it should not matter as I am measuring directly at the differential outputs, and I am getting the correct VCOM offset voltage (1.65V).
RF Input @ 70.000 001 MHz 0 dBm.
LO 70 MHz
Reference Input 10 MHz.
Input reference x2
PFD Freq: 20 MHz
VCO Freq: 3360 MHz
LPEN: 1 - in High power mode the level is even lower
Measurement is being done with a Keysight MSO-X 2024A. RF Input and External Reference from a R&S SMF 100A Signal Generator.
My first question is:
The TCM-9 is not expected to load the ADRF6806 at DC frequencies significantly to affect the output so dramatically - so why am I not getting the expected DC levels on the IQ channels?
My second issue is related to the Reference lock.
I seem to only be able to get a lock when the VCO output is at 3360 MHz. Although the specifications say the VCO can be tuned down to 2800 MHz, I am hardly able to get it down below 3312 MHz. At the top end, I can go up to 3480 MHz. Any setting outside of this range fails.
My board is setup for integer division to have lower spurs.
I would welcome some insight from your end.