Sunday, May 7, 2017

Quick SNA Jr V3 update

With  FDIM only a couple weeks a way, I am hurrying up with the build and programming of the SNA Jr. version 3.   This uses a mixer and simple crystal filter to eliminate many of the problems with  with harmonics when using a square wave signal generator.  I have a bunch of 4.9152 Mhz. crystals that I had purchased for another project.  I use the SNA Jr version 2 and simple test fixture to match several in frequency, and having a similar response curve.
I put them in the new board and directly wired the output of the signal generator circuit directly to the crystal filter circuitry.  Then using some of the new software I am writing for the version 3 system, I checked the response curve of the filter.  I found it to have a 3db response of around 100 Hz. and an over all response of about 1.2 KHz.  Since I am running out of program space in the NANO, I also used the software to find the center frequency of the filter.  I will program this directly into the software instead of writing a calibration routine.  For not It looks like this should work OK. 

Using this signal for testing purposes, I added a pop up menu with additional functionality.  One option allows me to normalize the waveform after it is acquired, instead of having a separate normalized sweep like the version 2 software.  Another thing I added was a marker to display difference in frequency and amplitude between the marker and cursor.  This picture shows the response of the filter and the new software features.
I had added pads on the circuit board for several attenuators, but found that I am gong to be a little short on gain even with out them. I also found that with the single sided board I could not get the low level noise floor that I had with other power meter circuits I had built.  I am going to quickly make a couple changes to the board and etch up a double sided version.  Hopefully I can shield the AD8307 and get another 10 dB dynamic range in the power meter part of the circuit itself.

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