rat-race coupler / 180 degree phase shifter - RF Cafe Forums
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Post subject: rat-race coupler / 180 degree phase shifter Posted: Wed Sep 07, 2005 7:45 am
I need to take a single ended signal at 8GHz and convert it to a balanced signal. i.e. two outputs are 180 degrees out of phase. This is in order to drive a PLL differentially.
I've simulated a microstrip rat-race coupler in order to achieve this. It seems to work ok.
Another option is to use a microstrip wilkinson power divider, then after the divider, have a meandering line on one leg which basically delays the signal to give the 180 degree phase shift necessary.
Any opinions on which is better? From my sims, the isolation with the wilkinson power divider is good, whereas the S11 is about -9dB - only ok. With the rat-race coupler, the isolation is poor, but the S11 is pretty good.
Post subject: Posted: Wed Sep 07, 2005 7:48 am
I guess the real question is - how important is isolation between the two lines? They're carrying the same signal, 180 degrees out of phase....
Post subject: Posted: Wed Sep 07, 2005 12:01 pm
I would say use the rat-race. No, actually use the Wilkinson.
Post subject: Wilkinson vs. rat-race for differential conversionPosted: Wed Sep 07, 2005 12:34 pm
If I "read between the lines" accurately, the single-ended signal is coming from a VCO.
The important factor here is going to be minimizing the effects of the following circuitry on the VCO (noise, detuning, pulling, etc). So either a very uniform S11 over the band of interest, or a very low S11, would be important.
If the S11 is very low, it can't vary very much and stay very low. That would tend to point to the rat-race coupler - IF the input impedance of the driven stage is constant. But, again, "reading between the lines", the driven stage might be a divider. Depending on its input structure, "digital" circuits (like dividers) don't always present a constant S11 themselves... which would tip the stage toward the Wilkinson. Enough? good question.
So the "devil is in the details" - as always.
Post subject: reg: coupler 180 degPosted: Thu Sep 08, 2005 5:43 pm
Joined: Wed Jul 06, 2005 5:54 pm
if ur familiar with genesys then this tool provides a synthesis option for 180 coupler. mebbe u can optimise to get the required S11 and then mebbe introduce non-ideality into the circuit.
all the best