# AD8302: What is the maximum allowable difference in signal amplitudes in order to obtain a phase measurement?

Question asked by gr8andy on Jun 20, 2014
Latest reply on Jun 30, 2014 by jdobler

Hi all,

I have a couple of simple questions regarding the spec of the AD8302 RF/IF gain and phase detector.

I know that in order to obtain a gain measurement, the maximum allowable difference between the 2 input signal amplitudes is 30dB (assuming you are working within the -60dBm to 0dBm range).

1. What is the output voltage VMAG if the difference between the 2 input signals is greater than +/-30dB?

What is not explicit in the datasheet is that if this the same for getting a phase measurement.

2. Another words, in order to obtain a phase measurement, What is the maximum allowable difference between the 2 input signal amplitudes? ie will I get a phase voltage if one input is 0dBm and the other one is -60dBm?

3. If I can get a phase voltage up to a difference of 60dB within the working range of  of the 2 log amps (-60dBm to 0dBm), What kind of error will I get with the phase voltage VPHS?

4. When I look at the TPC curves say TPC 42 on page 12, it shows 3 curves and says

Phase Output voltage vs. Input Phase Difference, PINPA =PINPB, PINPA = PINPB + 15 dB, PINPA = PINPB – 15 dB, Frequency 900 MHz.

What is confusing me is that the 3 curves were pointed with an arrow indicating Pinpa=-15dBm, Pinpa=-30dBm, Pinpa=-45dBm.

So for example, which one of the 3 curves correspond to PINPA =PINPB? Can it be all 3? because you can have Pinpa=Pinpb=-15dBm, Pinpa=Pinpb=-30dBm, Pinpa=Pinpb=-45dBm. Or another example, which one of the 3 curves correspond to PINPA = PINPB + 15 dB? Again, can it be all 3? If so does that mean the curve for Pinpa=-15dBm = Pinpb (-30dBm) + 15dBm is identical to Pinpa=-30dBm = Pinpb (-45dBm) + 15dBm and also identical to Pinpa=-45dBm = Pinpb (-60dBm) + 15dBm? If so, does that mean the Phase Output voltage vs. Input Phase Difference is a function of the differences in amplitudes between the 2 inputs rather than the input levels (ie, Pinpa+Pinpb)?

5. If I am correct in that the Phase Output voltage vs. Input Phase Difference is a function of the differences in amplitudes between the 2 inputs, then what kind of errors will I get if one input is 0dBm and the other one is -60dBm?

Thank you so much.

Andy