ELOG - PLL loop for frequency noise measurement

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Wed May 15 20:08:01 2019, Anjali, Update, 2micronLasers, PLL loop for frequency noise measurement 7x

Thu May 16 13:40:13 2019, anchal, Update, 2micronLasers, PLL loop for frequency noise measurement

Sun May 19 22:08:09 2019, Anjali, Update, 2micronLasers, PLL loop for frequency noise measurement

FM_noise_AFG_Actuation_30kHzV_diff_gain.pdf

FM_noise_AFG_gain_10_different_actuation_slope.pdf

Message ID: 2343 Entry time: Wed May 15 20:08:01 2019 Reply to this: 2344 2347

Author:	Anjali
Type:	Update
Category:	2micronLasers
Subject:	PLL loop for frequency noise measurement

We are trying to setup the phase locked loop (PLL) for the frequency noise measurement of 2 micron laser. We started with a sample experiment in which we were trying to lock an arbitrary function generator (AFG) with the Marconi using PLL. Attachment #1 shows the schematic of the PLL setup. We are using a level 7 mixer (ZFM-3-S+). The RF port is connected to AFG and LO port to Marconi. Output of mixer is passing through a low pass filter with cut off frequency at 1.9 MHz (SLP-1.9 +). The output of LPF is fed into input A of SR 560. SR560 is set with 1 MHz low pass. Initially, we set a gain value of 10 dB and actuation slope of 100 kHz/V in SR 560 and Marconi respectively. The 50 Ohm output of SR 560 was connected to Marconi and the 600 Ohm output was connected to an oscilloscope to check the performance of PLL. The carrier frequency from AWG and Marconi were set close to each other (~ 11 MHz). We observed a dc output at about 60 mV on the oscilloscope. This ensures that PLL is working.
We then attempted to measure the band width. To do that, the source output from SR785 was fed into input B of SR560. Part of the source output was fed into channel 1 of SR 785, through T connector, for the transfer function measurement. We also used a T connector at the input A port of SR 560 and one of the ports of this T connector was fed into channel B of SR785. I still must interpret most of the results that we got.
Attachment # 2: Closed loop transfer function (a) Magnitude (b) Phase, at different gain values in SR 560 when the Marconi actuation slope is 10 kHz/V.
Attachment # 3: Closed loop transfer function at different actuation slope value in Marconi when the gain is 7 dB. The increase in noise at lower frequency in phase plot (b) may indicate that the phase/frequency noise of the Marconi increases if the actuation slope value is increased.
Attachment # 4: Closed loop transfer function at different actuation slope value set in Marconi when the gain is 10 dB. The transfer function measured for the case of gain = 10 dB and actuation slope = 100 kHz/V (that is the product of gain and actuation slope is larger) shows significantly different characteristics.
Using the SSUserFn option in SR785, we tried to get the open loop transfer function as well from SR 785. The functional form was $\frac{x}{1-x}$
Attachment # 5: Open loop transfer function at different gain values set in SR 560 when the Marconi actuation slope is 10 kHz/V. The unity gain band width are 0.9 kHz, 2.8 kHz and 5.8 kHz respectively when the gain values are 3 dB, 7 dB and 10 dB
Attachment # 6 : Closed loop transfer function at different actuation slope value set in Marconi when the gain is 7 dB. The unity gain band width are 3 kHz, 9 kHz and 30 kHz respectively when the actuation slope values are 10 kHz/V, 30 kHz/V, and 100 kHz/V.
We also tried to estimate the open loop transfer function from the closed loop transfer function using the equation $G_{ol}=\frac{G_{cl}}{1-G_{cl}}$
Attachment # 7 : Comparison of Open loop transfer function that is measured from SR 785 and that is estimated from the closed loop transfer function using the above expression. These two values are significantly different. Kindly correct me.