Saturday, May 16, 2015

Prj137 HD2

Continuing with the ADC and having data on the noise floor and amplifier gain, the next step is to revisit the harmonic distortion of the board.  Previous efforts on earlier boards left me with questions regarding how much of the second harmonic (HD2) response observed was from the device versus the source and filters. When measuring a strong signal it is difficult to assess the harmonic content since even good spectrum analyzers will have a harmonic response themselves larger than I am trying to investigate with a 12 bit ADC.  The basic test filter I use is a 2x cascaded 10.7 MHz ceramic with a 3dB pad between them and a LC match from 50 ohms to 330 ohms.  The vendor does not provide a broadband filter response so I have always been left wondering what the actual response is at the harmonics (best case 50dB of rejection, worst case ...).  To address this I decided to construct a notch filter and focus only on HD2.

Below is the filter schematic and spice simulation.  I used the Elsie software package and selected L/C values I had on hand for a 50 ohm termination and notch response at HD2 for a 10.7MHz signal.
Notch Filter for Approximately 21.4Hz (HD2 of 10.7MHz signal)
The filter was constructed on some scrap PCB using an Xacto knife to cut the pads.  It is a bit tedious and care must be taken to do this away from other boards to avoid copper shavings landing where you don't want them but for small things it is faster than waiting for a board.
Constructed HD2 Notch Filter
The response of the filter was measured with a spectrum analyzer set at an input level of -15dBm. The response was scanned in large increments to save time hence the steps in the response.
Measured Response of Filter
The rejection in the range of interest is -50 to -70 dB and I trust the results in measuring the notch response more than than the harmonic response since the total input signal level is low causing less uncertainty regarding the measurement device behavior.

Using the above notch filter and a 10.7MHz BPF and ADC Unit#1 the following spectra was obtained with a synthesizer/mixer source (non-DDS).
ADC measurement of 10.640 MHz input through notch filter and BPF.
Unfortunately, HD2 is only ~65dBc.  This is significantly less than expected.  In addition there are several spur clusters besides HD2 and HD3 (markers 3,4,5,6). In fact, this is worse than the 74dBc from an earlier version of the board at lower levels.  With this measurement, there is no question regarding the input level of HD2; the notch should provide in excess of 50dB of HD2 rejection, the source HD2 is measured at least -40dBc, and the ceramic filter should provide another 25-50dB of HD2 reduction.

My first estimate was that the distortion was occuring in the fully differential amplifier (FDA). The datasheet shows -80dBc to -90dBc for HD2 and HD3 under various feedback configurations and loads at 10MHz using a 2Vpp output and a single ended input. There is a figure showing 10dB of variation in this with different common mode voltages (near the range I am operating using the ADC common mode voltage output).

In an effort to probe the FDA contribution, I changed the gain of the FDA down and up (ensuring all resistors were 1%).  In no case did this change HD2, HD3 or the spur clusters levels.

The last aspect of this which I can think of is that my input resistor is small compared to general applications referenced in the datasheet.  The exact impacts of this on distortion are not clear to me (if any).

In the update of the board, a BPF filter was added between the ADC and FDA just for this concern. The next step is to leverage the filter and monitor response changes in HD2/HD3 and the spur clusters with the idea being if they are attenuated the FDA is the source, if not, the ADC is the prime contributor.

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