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ADL5387 extended frequency range and multiple IQ demodulator switching

Question asked by dmarcosgon on Nov 5, 2014
Latest reply on Nov 5, 2014 by bsam

We are redesigning a wideband SDR direct conversion/low IF receiver and in our previous design we succesfully used the ADL5387. For the new version we need to extend the frequency range down to 20 MHz and the ADL5387 is the only broadband matched IQ demodulator we've found in the market that gets close to covering the required frequency range. Our experiments with the first design indicate that its performance doesn't degrade significantly down to 20 MHz, starting to really degrade below 15 MHz or so. Reading the datasheet we found that several parameters are specified in the graphs down to 20 MHz and it also seems that they don't degrade too much. Given that we will only make less than 20 units of this new receiver, do you think the performance of the ADL5387 will be consistently good?

 

We also want to extend the upper frequency range and for that we thought about switching between a ADL5387 and a ADL5380. Input switching before the input baluns seems easy as there are lots of good RF SPDT switches but switching the differential outputs seems hard as we haven't found any differential SPDT switches for this application. We thought about using a USB 2.0 switch or discrete high bandwidth analog switches but we're concerned about imbalances and linearity. The idea is to directly drive the dual ADC from the outputs of the demodulators, AC-coupling them through a 500 ohm terminated low pass filter and center-tapping the termination at optimum common mode voltage from the ADC (which is an AD9643). Any idea about how to switch between the two IQ demodulators?

 

Thanks in advance for the support

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