Re: SDR transceiver having independent mixer and ADC sampling clocks

[Brian Padalino]

On Mon, Feb 24, 2020 at 10:39 AM Heikki Laamanen <address@hidden> wrote:

Hi experts in SDR RF transceivers,

 

I am working on a timing and synchronization system which generates a high-precision timing signal from a terrestrial broadcast signal.  I am looking for a suitable SDR HW-board providing independent clocks for the RF mixers and the ADC.

 

It seems that many SDR boards use transceiver chips having mixer and ADC clocks which are derived from a single clock. Thus, it is not feasible to control the ADC sampling clock frequency without affecting the mixer frequency. For example, some USRP products have Analog Devices AD9361 transceiver chips providing no option to have fully independent mixer and ADC clocks.

Is this assertion true?  According to this document:

  https://ez.analog.com/wide-band-rf-transceivers/design-support/w/documents/10064/ad936x-local-oscillator-lo

There exists 3 VCO's in the AD9361.  One for TX, one for RX, and one for the baseband.  Why do you believe the clocks are not independent?  I agree the sampling clock might not be able to achieve the resolution you want, but I don't see why they are not fully independent already.

 

 

Why would I like to have independent mixer and ADC sampling clock?

 

The target is to generate a physical clock signal having small jitter and accurate phase. Hence, I need to control the frequency of some VCO based on the information the receiver signal processing gives. The most straightforward way to do this is to control the frequency of a VCO wherefrom the ADC sampling clock is derived.

You need to be more specific regarding your jitter and phase requirements.

Also remember that time is all completely relative.  Lastly, what does accurate phase mean to you?

Brian