[Mep-dev] Re: Tuesday Challenge for 9 December 2008 (Michelle)

[Grant Hodgson]

> 2. What local oscillator scheme makes the most sense for a 3.4GHz
  5.6-8GHz duplex system?

Thursday response :-

Treat Tx and Rx as being completely separate (KISS).   For each of Rx 
and Tx, use a single loop, fractional-N PLL operating at the Rx/Tx 
frequency**.  Use an external VCO; this allows for the optimum choice of 
VCO - either integrated on silicon or in a 0.5" square package, 
dependent on system needs.  No messing about with IFs at 144MHz or any 
other frequency for that matter.  (And then use an I/Q modulator to 
generate the Tx signal and an I/Q demodulator for the receiver.)

** In practice, the IF on receive will probably be something like 10kHz 
rather than 0Hz, but a Frac-N PLL can cope with this.  Tx IF could be 0Hz.

I've already got phase noise plots of such a system working at 3420MHz; 
at some point soon I hope to have phase noise plots for 5.7GHz.

regards

Grant  G8UBN