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Generating phase shift between 2 DDS outputs

Question asked by michael_jefferson on Jun 21, 2013
Latest reply on Jun 26, 2013 by DSB

I have a need to generate two RF signals at the same frequency with an arbitrary and controllable static phase shift between the two. The frequency range is not fixed yet, but probably will encompass the range from 100kHz to 100 MHz. One method which has been suggested is to synchronize two DDS chips, so that they generate the exact same frequency, but use the phase registers to phase shift them relative to each other. This is not a dynamic phase shift, as in BPSK, but rather a user adjustable phase shift of 0-360 degrees to allow compensation for time (and thus phase) shifts in cables and active elements. I must provide a manual control of the phase which will remain set even if power is removed and returned to the circuit. One potential method that I was thinking about was a panel mounted pot producing a voltage proportional to desired phase shift which is read by an ADC. The digital output of the ADC might drive the phase shift register of one of the DDS chips. Crude, but maybe workable. Other ideas are welcome. This is a "set and forget" until it needs to be set again sort of circuit that must endure power outages.

 

I am looking for thoughts as to the best way to implement this. I am interested in the best spectral purity that can be gotten, so more DAC bits and higher clock rates seems a good thing. We will probably have a small PIC microcontroller involved on the board, so using this to do part of the task is feasible. Which DDS chip(s) I should consider, and how to adjust the relative phase of one to the other are also open questions. I am happy to hear any ideas others might have.

 

Thanks,

Michael

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