The first stage is an R board providing attenuation of 0 - 30dB or +20dB of LNA gain. The mixer stages are B boards with a mixer and synthesizer. The first two filters are cascaded SAWs. The last filter, ADC and IF amplifiers (not shown) are on an A board. High side injection is used throughout, that is the first LO is set 1575MHz above the frequency of interest while the second stage is fixed at 1575+315MHz and the third stage at 315+10.75MHz. The block diagram is shown below followed by a picture of the bench setup.
|Three Stage Spectrum Analyzer Configuration|
|3 Stage Conversion Using Beagle Bone Black and BREC boards and filters.|
A rough 3 stage calibration was conduct (so subsequent results are indeed in dBm) and as a quick wide band test, a 240MHz input was used with the first 1GHz scanned using Si.
The spurs are a different story and are going to take some more effort to understand. I always knew that the synthesizer would generate fractional spurs and could generate integer spurs and reference spurs. I have been surprised by the quality of the spectra and have not taken the time to investigate the fine structure to this point.