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 Wed Feb 20 23:55:51 2019, gautam, Update, ALS, ALS delay line electronics Thu Mar 7 01:06:38 2019, gautam, Update, ALS, ALS delay line electronics Tue Mar 12 22:51:25 2019, gautam, Update, ALS, ALS delay line electronics Wed Mar 13 01:27:30 2019, gautam, Update, ALS, ALS delay line electronics Sun Mar 17 00:42:20 2019, gautam, Update, ALS, NF1611 cannot be shot-noise limited? Thu Mar 28 19:40:02 2019, gautam, Update, ALS, BeatMouth with NF1611s assembled Fri Mar 29 21:00:06 2019, gautam, Update, ALS, BeatMouth with NF1611s installed Sun Mar 31 15:05:53 2019, gautam, Update, ALS, Fiber beam-splitters not PM Wed Apr 3 09:04:01 2019, gautam, Update, ALS, Note about new fiber couplers Wed Apr 3 12:32:33 2019, Koji, Update, ALS, Note about new fiber couplers Fri Apr 5 00:33:58 2019, gautam, Update, ALS, Promising IR ALS noise Fri Apr 5 11:49:30 2019, gautam, Update, ALS, PSL + X green beat recovery Mon Apr 8 00:04:08 2019, gautam, Update, ALS, IR ALS noise budget Mon Apr 8 18:28:25 2019, gautam, Update, ALS, EX Green PDH checkout Mon Apr 8 23:52:09 2019, gautam, Update, ALS, EX Green PDH error monitor calibrated Tue Apr 9 00:16:22 2019, rana, Update, ALS, EX Green PDH error monitor calibrated Tue Apr 9 00:18:19 2019, gautam, Update, ALS, EX Green PDH error monitor calibrated Tue Apr 9 18:44:00 2019, gautam, Update, ALS, EX Green PDH discriminant measurement Tue Apr 9 19:07:12 2019, gautam, Update, ALS, IR ALS noise budget Thu Apr 11 01:10:05 2019, gautam, Update, ALS, Large 2kHz peak (and harmonics) in ALS X Wed Apr 17 00:50:17 2019, gautam, Update, ALS, Large 2kHz peak (and harmonics) in ALS X no more Wed Apr 17 11:01:49 2019, gautam, Update, ALS, Large 2kHz peak (and harmonics) in ALS X no more Wed May 29 18:13:25 2019, gautam, Update, ALS, Fiber beam-splitters are now PM Fri May 31 15:55:16 2019, gautam, Update, ALS, PSL + X beat restored
Message ID: 14475     Entry time: Thu Mar 7 01:06:38 2019     In reply to: 14468     Reply to this: 14477   14478
 Author: gautam Type: Update Category: ALS Subject: ALS delay line electronics

Summary:

The restoration of the delay-line electronics is complete. The chassis has not been re-installed yet, I will put it back in tomorrow. I think the calculations and measurements are in good agreement.

Details:

Apart from restoring the transimpedance of the I/F amplifier, I also had to replace the two differential-sending AD8672s in the RF Log detector circuit for both LO and RF paths in the ALS-X board. I performed the same tests as I did the last time on the electronics bench, results will be uploaded to the DCC page for the 40m version of the board. I think the board is performing as advertised, although there is some variation in the noise of the two pairs of I/Q readouts. Sticking with the notation of the HP Application Note for delay line frequency discriminators, here are some numebrs for our delay line system:

• $K_{\phi} = 3.7 \ \mathrm{V/rad}$  - measured by driving the LO/RF inputs with Fluke/Marconi at 7dBm/0dBm (which are the expected signal levels accounting for losses between the BeatMouth and the demodulator) and looking at the Vpp of the resulting I/F beat signal on a scope. This is assuming we use the differential output of the demodulator (divide by 2 if we use the single-ended output instead).
• $\tau_d = \frac{45 \ \mathrm{m}}{0.75c} \approx 0.2 \mu s$ [see measurement]
• $K_{d} = K_{\phi}2 \pi \tau_{d} \approx 4 \mu \mathrm{V/Hz}$ (to be confirmed by measurement by driving a known FM signal with the Marconi)
• Assuming 1mW of light on our beat PDs and perfect contrast, the phase noise due to shot noise is $\pi \sqrt{2\bar{P}\frac{hc}{\lambda}} / 1 \ \mathrm{mW} \approx 60 \ \mathrm{nrad /}\sqrt{\mathrm{Hz}}$which is ~ 5 orders of magnitude lower than the electronics noise in equivalent frequency noise at 100 Hz.
• The noise due to the FET mixer seems quite complicated to calculate - but as a lower bound, the Johnson current noise due to the 182 ohms at each RF input is ~ 10 pA/rtHz. With a transimpedance gain of 1 kohm, this corresponds to ~10 nV/rtHz.

In conclusion: the ALS noise is very likely limited by ADC noise (~1 Hz/rtHz frequency noise for 5uV/rtHz ADC noise). We need some whiteningWhy whiten the demodulated signal instead of directly incorporating the whitening into the I/F amplifier input stage? Because I couldn't find a design that satisfies all the following criteria (this was why my previous design was flawed):

1. The commutating part of the FET mixer must be close to ground potential always.
3. The DC gain of the I/F amplifier is low, with 20-30dB gain at 100 Hz, and then rolled off again at high frequencies for stability and sum-frequency rejection. In fact, it's not even obvious to me that we want a low DC gain - the quantity $K_{\phi}$ is directly proportional to the DC transimpedance gain, and we want that to be large for more sensitive frequency discriminating.