Thursday, July 31, 2014

Spectrum Analyzer Configuration Experiments

Now that I have some compact filters with decent stop band rejection, I decided to try setting up a classic three stage IF conversion architecture.

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.
The R board, first B, and A board are on a single Beagle Bone Black(BBB) at lower left.  The second and third down conversions are B boards on their own BBB in the center and right of image.  The test input source (a Prj114 USB PIC controlled ADF4351) can be seen between the first and second BBB.

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 idea with 240MHz is that is is not a multiple of 25MHz (the PFD reference in any of the synthesizers).  The 25MHz spurious responses at markers 5,6, and 7 were not investigated at this time. Next a 7L12 calibration signal was used as the input.  This is a 50MHz signal with harmonics using a non compensated crystal.  The idea being that this is a non-ADF4351 based source and provides the lowest possible phase noise and spurs I can generate.  The wide band scan of that input is shown below.
The following is a 500kHz span about 50MHz of the same input signal.

Notice the spurs at the markers.  These were not expected.  A close up view of the input using a 50kHz span is captured in the next signal with envelope history on.
Basically, the filters and conversion are working as expected.  During calibration the anticipated gains were not as initially expected.  This is due to how I am using the LT5560 mixer.  I am using the device essentially single ended and over a wide range of frequencies in the first stage.  The input and output termination change over the frequency spectrum.  This is well documented in the part datasheet.  While it is a wide band part it was not intended to work simultaneously across its full range, rather, to be matched and balanced over a specific portion of its operating region.  This was a trade off I elected to make for power, cost and flexibility. The device has worked well and as stated - its just that I'm beginning to see the effects of those tradeoffs.

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.

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