Saturday, May 30, 2015

Prj137 Onboard BPF

Given the state of HD2 from the previous post with the BBB 10MSPS ADC board, I used Elsie to design a 3rd order BPF centered at 10.7MHz with a 3.5MHz bandwidth.  It takes a bit of trial and error to arrive at a synthesizable filter (i.e. one that uses standard values and is not overly sensitive to part value variations).  The intent is not to provide complete band rejection, rather to limit the noise from folded Nyquist bands and to knock down any harmonics created by the FDA.  The filter values along with a spice run are shown below.
SPICE steup of FDA with BPF prior to ADC.
The 902pF shunt capacitor is an 820pF with a 82pF stacked on top (the target was 900pF however these were unavailable at reasonable prices and shipping times).

BPF Filter Response
For comparison purpose the first spectrum below is the same measurement setup with no filter.  In this case (and all following) the gain has been reduced using an Rf=100, Rg=25 (this was one of the attempts to see if gain was exacerbating the harmonic distortion).

Av=4, Filtered DDS source at 10.640Mhz, no FDA-ADC filtering.
Rather than just populating the entire filter and seeing what happened, I decided to populate each stage and evaluate HD2/HD3 and the noise floor.  I chose to evaluate the center shunt stage first since I thought this had the best chance of producing expected results without imbalancing the differential arms.  Those results are below.
Av=4, Filtered DDS source at 10.640MHz with shunt stage of FDA-ADC BPF populate.
At first it seems worse, however, the noise floor has been reduced by the BPF limiting the folding noise across Nyquist zones.  This presents the illusion of the spurs being more prominent not because they have risen but because the floor as dropped.  HD2 is now roughly -74dBc and HD3 is not noticeable.  I did not pause to measure the gain and evaluate the filter insertion loss.  If one stage was good more must be better...  Unfortunately, this wasn't true.  Adding the first series made things slightly worse and with a higher noise floor.  Adding the second series stage and final BPF stage helped HD2 slightly but retained a slightly higher and tilted noise floor.  Addition of the final LPF (shunt C and series 5 ohm resistors) returned things to slightly better than the above.  The results from this configuration are shown below.
Av=4, Filtered DDS source at 10.640 MHz with 3 stage BPF.
HD2 is now approximately -80dBc but HD3 is similar but a little higher.  Interestingly enough, in no case did the spur clusters ever change.  These are now the dominant (and unexplained) feature.

At this point it was time to total up the bill for all of the filtering.  I knew the insertion loss was creeping up since the DDS level had to be adjust up to hit nearly full scale on the ADC.  I decided to return the FDA to a high gain previously used Rf=493 and evaluate the results.
Rg=25, Rf=493, Filtered DDS source at 10.640MHz with 3 stage BPF.
As expected the spectrum did not change with the increased gain. Using the same technique as in previous gain measurements, the gain was measured as Av=15.6(15dB) but Rg/Rf indicated it should be Av=9.9(20dB).  The FDA datasheet indicates this type of gain is readily supported by the device at this frequency so it looks like the insertion loss is 5dB for the 3rd order BPF.  This is higher than I had hoped for, however, I did not use the highest Q components made, rather the best ones in stock for these values at the time.

So in the end it appears the higher than expected HD2/HD3 response is predominantly deriving from the FDA and how it is being used.  Taking them down to a reasonable level adds components and increased gain needs on the FDA.  This seems like a losing game.  I may have been better off placing all of the desired gain off board, then filtering, and then conducting the differential conversion on the board with a transformer.

This is still usable for my purposes, however, the spur clusters merit some attention.  Originally I had attributed them to DDS artifacts, however, it became clear this was not the case when using a source comprised of a synthesizer and mixer.  In both cases the spurs are identical in level and structure and follow the fundamental in frequency and level.

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