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ADRF6806 and Ultra Low Frequency (<1 Hz) Baseband

Question asked by Indra on Oct 8, 2016
Latest reply on Oct 17, 2016 by Indra

Hi,

 

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.

Register settings:

Input reference x2

PFD Freq: 20 MHz

VCO Freq: 3360 MHz

Divider: 84

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.

 

Thank you,

 

Indrajit.

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