Saturday, September 12, 2015

Browser controlled synthesizer (Part4)

The fourth step in constructing a simple webpage controlled RF synthesizer is the mechanical assembly and packaging. (The previous steps are described in  part3part2, and part1)

Originally I had visions of constructing shields on the boards in RF sensitive areas and packaging the entire setup in a nice professional small case.  Let’s just say I’m learning…  Rather than getting hung up on the difficulties and errors in this regard I decided to forge ahead.  All I really needed was a dust cover.  The real concern is just protecting the electronics from having a tool fall on them or more likely a SMA connector brush across them as the cable flips over unexpectedly while reconfiguring a setup. To this end I decided to use some scrap plywood and acrylic.  The plywood provides enough mass that the instrument doesn’t get dragged around on the bench with cable tension and is sturdy enough that you can stack things on top of it.  Granted, it provides no shielding and I’m not entirely sure of the ESD properties.  Same for the acrylic face plate.  The face plate is thin enough that you can get the connectors through it and you can at least see the LEDs to know if it is powered on and should be working.  The following are a couple of pictures of the assembly. 
BBB Browser Controlled RF Synthesizer on Mounting Plate

Synthesizer with dust cover
The example below captures the control panel during a sweep along with the measured spectrum.
Browser based control panel.  Sweeps from 37MHz to 67MHz (1MHz steps) and from 0dBm to -30 dBm (1dBm steps).  At the time of capture the output at that steps was at 50MHz/-13dBm.

Spectrum of output.  There is a 30dB pad between the generator and the analyzer.  You can see the 3rd harmonic of the low frequency portion of the scan on the far right near 74MHz.
Similar sweeps and single frequency outputs are achievable through 2GHz.  Beyond this point the amplifiers used start to lose gain and the level drops and level control is reduced.  The following captures a sweep across 1.4GHz - 1.5GHz in 5MHz steps.
Constant level sweep from 1400MHz - 1500MHz in 5MHz steps.  Output at -5dBm with 30dB pad between analyzer and synthesizer.

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