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Broadband performance of transmit control loop using ADL5330 and AD8318

Question asked by kenbaby on Aug 10, 2011
Latest reply on Aug 11, 2011 by Jim

I have a working circuit using the standard implementation of the ALC loop for these two chips. I have  a 20 watt PA in the loop and suitable attenuation in the coupled feedback path from a 20 dB coupler.


The loop locks and works fine, but I am disappointed with the swept frequency performance between 30 and 512 MHz. Ideally I would like to see a variation in power at room temperature of +/- 0.5 dB over the frequency band for an input range to the VGA of 10 dB. Am I being over optimistic?


The detector is working in the input voltage range -35 to -15 dBm Vin. In fact the variation worsened at these settings compared to my  initial range of -6 to -26 dBm. The very best I have seen is around +/- 1 dB variation across the frequency band. I also note that there is no long term trend across the range. Output power rises and falls, with some odd discontinuities thrown in. At no point do I hit the 1.4V  limit in terms of control voltage to the VGA.


Incidentally, how much effect will harmonics have at a level varying from -20 to -60 dBc depending on where I am looking in the band?