The X arm whitening filters of the beatbox were modified.
Now we have about 10 times better floor level above 100Hz and ~3 better at 1Hz.
- The previous whitening was zero@1Hz, pole@10Hz, and the DC gain of the unity.
When the Marconi signal (~30MHz -25dBm) was given to the beatbox (via ZFL-1000LN),
the DC output of the beatbox was only 140mV (lame). This corresponded to 220 counts in
the CDS. (BTW the signals were calibrated by giving frequency deviation of 1kHz is applied at 125Hz.)
- If you compare the analog measurement of the beatbox output and what we see in the I phase signal,
you can see that we were completely dominated by the ADC noise (attachment 2, blue and red).
- The new whitening is email@example.comHz, pole@159Hz, and the DC gain of 10.
- This improved the sensing noise by a factor of ten above 100Hz.
- We are stil llimited by the digitizing noise between 3Hz to 100Hz.
We need steeper whitening like 2nd order from 1Hz to 100Hz. (and probably at DC too).
Now the DC amplitude is about 1.4V (and 2200 counts in the CDS).
So, it is interesting to see how the sensing limit changes by increasing
the overall gain by a factor of 3, and have (zeros@1Hz & poles@10Hz)^2.
This can be implemented on a proto-daughter board.
- By the way, the performance below 2Hz is now better than the analog one with the previous whitening.
This improvement might have come from the replacement of the thick film resistors by thin-film resistors.
(See the circuit diagram)
About the nominal power of the beatbox input.
- Marconi (-20dBm 30MHz) was directly connected to the beatbox. The RF output of -15dBm was observed at the delayline output.
- According to the beatbox schematic, the mixer LO and RF inputs were expected to be -9dBm and -19dBm.
- The nominal mixer LO level is supposed to be 7dBm. Therefore the nominal beatbox input should be -4dBm.
- Assuming 23dB gain of the preamp, the PD output is expected to be -27dBm.
- When the PD out is -27dBm, the RF mon is expected to be -5dBm. This is the level of the RF power expected to be seen in the control room.
- The output of the beatbox was measured as the function of the input to the preamp (before the beatbox input).
With the nominal gain, we should have observed amplitude of ~170. And it is now 1700 because of the whitening modification.