2023-03-14
Check of cabling around the FAB monitor. Removing Al foil on top of the FAB increases the signal.
Without the amplifier, the noise level of the FAB is +/-2 mV.
With the amplifier, the noise level of the FAB is +/- 40 mV.
The source of noise is the RF. Without RF, this noise does not appear. This is consistent with the observation last week.
Schottky measurement does not give the similar signal as Friday. TORA has a decent signal.
Use another amplifier. It does not help.
Measure the response of two amplifier: #1 and #2 written on the surface.
With 1 mV input, the amplification is around 100.
With 2 to 16 mV, the amplification is around 160-170.
Above 17 mV, the amplification is decreasing ~140. Start showing deformation of the sinusoidal waveform.
The detailed data is available by David PDB.
Something seems wrong with FAB. The plan is
Looking at TORA (with scope downstairs) and try Schottky scan with TORA.
After lunch, decide if we open the FAB.
Amplifier Verification
We measured the performance of the amplifier with a signal generator using the experimental set-up shown in the adjacent figure. The first set-up was to verify that the signal generator was outputting the expected signal, and the second set-up was to measure the performance of the amplifier.
The amplifier is an NF-SA-220F5. Spec sheet is available on archive.org.
The nominal gain is 46 dB (~200x), and its maximum output is 2 V (pk-pk), so we expect the maximum usable input voltage should be 10mV. We used a signal generator capable of outputting only 1 mV (pk-pk).
This measurement was performed quickly, and only rough values were recorded. We did not store waveforms for all settings. Peak-to-peak voltages were either measured by eye, or using scope measurements.
Signal Generator Verification
To verify that the signal generator could output a 1 mV (pk-pk) signal, we plugged it directly into the scope. The signal generator output expects a high-impedance termination, so the scopes input impedance was set to 1 MOhm. A 10 MHz signal was measured, but noise made it difficult to discern its amplitude. We reduced the scope bandwidth to 20 MHz and turned on averaging (16 samples) to make the measurement. We were then able to measure a 1 mV (pk-pk) (0.5 mV amplitude) signal. The signal amplitude was then varied (e.g. up to around 10 mV) and we consistently observed a signal with the set amplitude. We took this to mean that the signal generator was operating as expected.
Amplifier Verification
Note: peak-to-peak voltage was measured either by eye or from the scope's measurement function. This was done without regard to the signal shape, so for higher input voltages the output was distorted but this did not necessarily affect the "gain" measurement.
10 MHz Input Signal
Input Voltage (mV pk-pk) Output Voltage (mV pk-pk) gain (out/in)
1 100 100
2 300 150
3 440 147
4 620 155
5 810 162
6 910 152
7 1200 171
8 1400 175
9 1600 178
10 1770 178
11 1940 176
12 2100 175
13 2290 176
14 2470 176
15 2640 176
16 2790 174
17 2910 171
18 3000 167
20 3190 160
30 4050 135
2 MHz Input Signal
Input Voltage (mV pk-pk) Output Voltage (mV pk-pk) gain (out/in)
1 100 100
10 2070 207
20 3600 180
Conclusion
This measurement was not performed in ideal conditions, so results should be considered only as an indication of amplifier performance. With this in mind, the amplifier gain was not 200x over the whole input range, but was the expected order of magnitude. We did not see signal distortion until the upper end of the input voltages. The gain was not identical for 2 MHz and 10 MHz, but it was comparable.
FAB Capacitance Measurement
To see whether we could identify a problem with the FAB without breaking the vacuum, we measured the capacitance via the feedthrough with an LCR meter.
A long BNC cable was used, but the LCR meter was not properly zero'd, so the difference in capacitance was measured with the cable connected and disconnected. The measurement speed was set to slow, and the frequency was set to 100 kHz.
Disconnected: 0.58 uF
Connected: 0.71 uF
This gives a FAB(+feedthrough cable etc) capacitance of (130 +- 20) pF. This uncertainty is just estimated from the flicker on the screen, and no detailed analysis was performed.
Conclusion
The measured FAB capacitance is less than the published value of 300 pF, but this difference was not considered out of the range of possibility due to the change in location of the FAB. This test has not provided any conclusive evidence either way.