J. Freed, 

Update on Double Mixer Progress 81593, Proceded through step 2a of the double mixer test plan T2400327. Initial results are sugesting a possible noise improvemnt compaired to the other options in the area of interest for SPI.

DM_PN1.pdf Shows the phase noise test run in step 2a of the Double Mixer Test plan. While not a true 1-to-1 comparison of the phase noise performance of the double mixer compered to other options (step 2b is for that), it shows that adding the double mixer into this system (except for the peaks) improves phase noise performance by a factor of 2-5 from 100 Hz to 20 kHz. Of note, there is a large peak are centered around the 4096 Hz that is of interest to SPI. As there was only a cursory attept to properly phase match the signals in the internals of the double mixer for this initial test, the 4096 Hz sideband was not properly removed. 

A possible cause of this phase miss match is our phase delayer inside the double mixer (ZMSCQ-2-90B) causes a phase delay of about 89.82 degrees at 80MHz and not the 90 degrees we are expecting.

Possible fixes

• Try noodling the digital phases of the 4096 Hz Sin and Cos to see if that makes a difference
• In analog, replace 90 deg splitter with normal splitter and try to create the 90 phase shift in a different way
  • A 90 deg phase delay component tuned specifically for 80 MHz
  • A high-pass filter / low-pass / band-pass filter that has 90 deg phase at 80 MHz.
  • Create a tunable phase shifter.

The very low frequency (<0.05Hz) contains DC noise caused by the external phase mismatch of the double mixer and the reference source for the phase noise measurments. It is not an indication of double mixer drift and there has not been yet investigations in drift.