Friday, August 22, 2014

A board SFDR

After some more thinking about it, things weren't adding up with regard to the harmonic levels seen by the ADC on the A board.  To better investigate the harmonics at the ADC,  I decided to create an external 2x cascaded 10.7MHz ceramic filter to use with the signal sources I have.  The following is a spectrum of a cleaned up 10.720 MHz source.
0 to 50MHz capture of 10.720 MHz filtered source.
This signal is -34dBm with no harmonics or noise above -90dBm.  It is based on an Si570 from older projects and has a different spectrum footprint than the ADF4351.  It was applied to unit #2 A board (lower IF gains).  Using the raw ADC capture application within the workbench tools in google code, the following ADC spectrum was obtained.
10.5MHz to 11.0MHz capture of 10.720 MHz filtered source.
The sampling rate is 1MSPS and undersampling is used to obtain a capture of the 10.7MHz signal.

A 10.720MHz input will alias to 11.00 – 10.720 MHz = 280kHz and is shown at marker 1 at -35.9dBref (recall that 10.5MHz is on the right, while 11.0 MHz is on the left).  The second harmonic of the input at 21.440MHz will alias to 21.000 – 21.440 = 440kHz which can be seen at marker 2 at a level of -76.8dBref  (Again, 21.0 MHz is on the left, while 21.5MHz is on the right).  This indicates a level difference between the fundamental and second harmonic of 76.8 – 35.9 = 40.9dB.

This is consistent with older harmonic measurements.  I had previously attributed them to the high harmonic content from the mixer and only 40dB of stop band rejection in the anti-aliasing filter, however, given the source used here that is clearly not the problem.  A quick review of the ADC datasheet showed -80dBc SFDR (unaliased, and its not clear to me how to translate this to an undersampling application).   Focus them turned to the OpAmp's used for the ADC buffer/amplifier.

The following is a graph from the ADC interfacing OpAmp using a +5V supply.
The part is used with this gain and closer to a RL=100 ohm.  While the outputs in the measured data above are not 2Vpp, I am using the part at 3.3V.  The same graph for the first OpAmp stops at 5MHz.  In short, I could have done a better job on OpAmp selection for the application.

My first intention was to rework one of the existing boards with a better part.  Unfortunately I painted myself into a corner.  The foot print is a standard SOIC8.  When you couple this footprint with 3V uni-supply operation and the desired SFDR you quickly reduce the options.  In addition, it gets pretty pricey for having two stages of programmable gains provided.

Given all of this and the fact that there are several things I have been hoping to revisit on the ADC board (simpler clocking structure, higher speed parts, higher resolution parts, ...)  I'll hold off on rework and focus my time and energy on an updated board.


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