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.