In line with industrial practices, I did two tone third order intermodulation test today on the FSS RFPDs. This test was inspired by procedure described in this technical note by MiniCircuits and this paper at IEEE.

Method

• Similar to previous measurement, two RF tones are generated using Moku.
• However, now, they are separated by an audio frequency, 10 kHz in our case. This is marked as DF in the plots.
• These tones are centered around the resonant frequency of the RFPD marked as CF in the plots.
• The two signals are combined together through ZFSC-2-1W+.pdf, but this time all ports of the combiner have an attenuator on it.
• 3 dB attenuators at the input port and 6 dB at the output port. This was mentione din MiniCircuits as a step to improve impedance matching near the combiner.
• Ideally, two low pass filters also should be put at the input port, but I couldn't find any with high enough cut-off frequency.
• The combined source signal was getting attenuated by 8.75 dB. So I just drove the input signals higher by this amount.
• The Source signal is fed to the Test IN of the RFPD and RF out is read through AG4395A.
• The power in each harmonic is calculated from the spectrum and plotted together against source power.
• Since all RF signals are the near-resonant peak of the photodiode, there would be little to no difference in TF seen by them.

Inference:

• I think because of the 100 kOhm resistor at the Test In port of RFPDs makes the source signal too feeble.
• I'm just measuring instrument noise upto source power of -10 dB. I can't go above 0 dB in current setup atleast.
• At 0 dB source power, which is equivalent to 3 uApk photocurrent from the photodiode, the third-order harmonics are 60 dB lower.
• One way of reporting this is, Spurious Free Dynamic Range (SFDR) at 36(37) MHz is 60 dBc at 0 dBm source power of SN009 (SN010).
• Another way this figure is reported in industry is by extrapolating at which point the third-order harmonic power becomes the same as fundamental.
• But I do not have enough distortion showing data points to extrapolate to calculate the intercept.
• Last plot is when I connected the cables together which go to DUT. We see SFDR of 60 dBc here itself.
• The last two data points are when input range of AG4395A are very high and hence has increased noise floor.
• So probably in my measurements also, I'm just seeing distortion due to measurement apparatus itself.
• But this at least sets an upper bound of distortion to SFDR of 60 dBc at the resonant frequencies (within the errors that I, a graduate student, can make).

Datasheet for MAX4107

• The datasheeet says that between 30-40 MHz, the SFDR is approximately 50 dBc (this is actually lower than what we measured).
• Also, the third-order intercept is mentioned at 15 dBm source power.

Data