# FAQ: Measuring the Compression Point (OP1dB) of IQ Modulators

Document created by enash on Nov 19, 2010Last modified by AndyR on Jan 31, 2012
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Q.

Could you please tell me how we test the P1dB of an IQ Modulator like the ADL5375 or ADL5370/1/2/3/4? Since the P1dB is so high, how do you apply a baseband signal of such a high high level?

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A.

We have to over drive the base band inputs which takes the baseband voltages below ground. Let's consider the example of the ADL5375 and do some rough calculations. At 450 MHz, the spec page says that 1 V pp on the differential IQ inputs will give an output power of 0.87 dBm (let's call that +1dBm). If you double the baseband drive voltage to 2V pp, then the output power increases by 6 dB to +7 dBm.

Now, the baseband inputs are differential and should be biased to 0.5 V. So a drive level of 2Vpp differential (which delivers +7dBm out) will have each input swinging from 0V to 1V (i.e. 500 mV +/-500mV).  However, the compression point is specified to be 9.4 dBm! So in order to drive the output into compression we have to increase the baseband drive beyond 2Vpp which will result in the baseband voltages swinging below ground.  We have found that it is safe to do this in the lab but have not verified that it is a good thing to do long term. As a result, the Absolute Maximum Ratings conservatively suggest that the baseband voltages should not go below ground.  Since these IQ Modulators are generally driven by baseband DACs which are configured to deliver between 1Vpp and 2Vpp differential  (it's generally closer to 1Vpp max and there is generally so additional backoff),  continuous operation in compression is not really practical.

This is a characteristic of almost all of our newer IQ Modulators which have high output compression points.