Revisiting the LT5517

Some time ago (just before Linear Tech & Analog Devices merged), I requested a few samples of LT's 5517 quadrature demodulator and 5598 quadrature modulator.   They happily supplied a few samples of each, but the local LT rep also called me at work and asked if I'd also like to have a dc678a evaluation board (https://www.analog.com/en/resources/evaluation-hardware-and-software/evaluation-boards-kits/dc678a.html#eb-overview).  Of course, I said, "Hell yes!" and it appeared in the mail soon after (no charge).  But then it sat unloved and unused for years until I recently decided to see how well it worked at HF.

The 5517 board is the green one in the upper left corner.  A 2-channel baseband preamp is the Manhattan-style board next to it.  The board below the 5517 board is a split supply boost converter that powers a 100x gain, 2-channel baseband preamp using THAT1510 instrumentation amps.  These devices are very similar to the widely used INA217.  

Although the 5517 is spec'd for use from 40 - 900 MHz, it's been reported to work as low as 3.5 MHz with ~0dbm LO(x2) drive.  For a quick test, i used a TinySA as a LO tuned to twice the 20m FT-8 ("testing beacon") frequency.  The I/Q output from the preamp was fed to the phase shift/audio amp stages of my analog phasing receiver.  (That beast was built modularly for such testing purposes.)


There was a small, but noticeable, increase in background noise when the antenna was connected at 14.1 MHz   The cyclic drone of FT-8 was immediately evident (and loud) when I tuned to 14.074 MHz.  I didn't experimentally evaluate the board's ability to handle very strong signals and still have to quantitatively assess opposite sideband suppression.  By ear, opposite sideband suppression was at least as good as my other phasing receivers (approx 40 db).  As far as sensitivity goes, I was able to hear signals at low as -115dbm, but not -118dbm. 

Anyway, the 5517 worked for me at 20m and higher frequencies.  Eventually, I'll build a clean LO and a simple Teensy-based DSP back-end for it and end up with a useful, experimental receiver.  A few other hams (notably PA0RWE) have experimented with this device, but it hasn't been widely adopted.  It's a small package (4mm square) with 16 pins, but not impossibly small for home assembly if you're experienced with small SMT parts.  A few hams have also sniffed around the LTC5598 (similarly small package).  AC0ZJ described his experiments with the 5598 in QEX (March/April 2019).  Analog Devices still lists the 5517's status as "production", but the 5598 is marked "last time buy".  I'll have to see if there are more modern equivalents available at a reasonable price.
     



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