Monday, August 11, 2014

Single Stage Conversion Spurious Responses

In the three stage conversion there were multiple spurious response noted.  Rather than trying to understand them with 3 mixers and LO's I decided to back up to a single stage, pull out the Analog Devices PLL simulator and account for what I was seeing.

The basic test configuration uses a Prj114 source set at 500MHz.  This is an integer multiple of the PFD frequency of the PLL so it should generate no fractional spurs.  I have not been able to observe integer or reference spurs in any of my measurements so I was not concerned with them.  In addition, the Analog Devices ADIsimPLL output indicates their level should be negligible with my particular configuration. The single stage conversion is set with a LO of 510.75MHz nominally.  This is a fractional setting and should generate fractional spurs.

Just to get oriented, a 100MHz span was taken at the mixer output using an SA0314 to see what was going into the A board.  That spectra is below.
While the RBW is large, the IF harmonics are quite clear and quite large.  Using this signal, the ADC application was used to capture the ADC results.  These are not in calibrated units but are in dB relative to a full scale ADC value.
After a fair amount investigation, the discrete spurs here appear to be IF harmonics.  The ADC filter only provides 40dB of out of band attenuation.  The harmonics are high due to using the mixer in a single ended configuration.  Even in a 3 stage conversion where the last LO is fixed, the input signal can have energy within the pass band IF that when a harmonic is applied, the result alias and end up near the non harmonic. Basically, a better mixer design and ADC filter should address these.

So we put aside the IF harmonics and focus on the near in phase noise of the IF (and consequently fractionally generated LO).  The ADIsimPLL results for this synthesizer setting are shown below.
ADIsimPLL results for 510.75MHz synthesis, low-noise (dither off), divider outside PLL loop, 35kHz loop bandwidth
Based on this I would expect a smearing due to spurs about 60dBc out to at least 30kHz.  Using the ADC results (same data as in previous ADC figure)  and looking only at 30kHz about the carrier we see:
While the spurs are not discrete, the lobe off the carrier does rise to -60dBc and roll off from there and is in the range of expected spurs.  I am actually quite happy with these results and being able to account for them.  There are several options to reduce the close in phase noise including using the dither option on the ADF4351 or using integer synthesizer configurations in the second and third stage down conversions.  However, compared to the spurious responses due to the mixer IF harmonics and ADC filter the close in phase noise is a topic for a different day.

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