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Entry  Sun Mar 28 20:05:33 2010, rana, Summary, PSL, FSS Work from Sunday: AOM/VCO level set wrong Untitled.png
    Reply  Sun Mar 28 20:51:31 2010, rana, Summary, PSL, FSS Work from Sunday: Cavity Suspension is Ridiculously Undamped! Untitled.png
       Reply  Sun Mar 28 23:17:46 2010, rana, Summary, PSL, FSS Work from Sunday: noise spec in2.png
          Reply  Sun Mar 28 23:47:47 2010, rana, Summary, PSL, FSS Work from Sunday: Open Loop Gain fssloop.pngsweep2.png
             Reply  Mon Mar 29 01:11:33 2010, rana, Summary, PSL, FSS Work from Sunday: RF Out Spectrum rfout.pngTEK00000.PNGTEK00001.PNGTEK00002.PNG
             Reply  Mon Mar 29 02:07:50 2010, Koji, Summary, PSL, FSS Work from Sunday: Open Loop Gain 
Message ID: 2724     Entry time: Mon Mar 29 01:11:33 2010     In reply to: 2723
Author: rana 
Type: Summary 
Category: PSL 
Subject: FSS Work from Sunday: RF Out Spectrum 

I measured the RF spectrum coming out the FSS RFPD to look for saturations - its close to the hairy edge. This is with the 8x power increase from my AOM drive increase. I will increase the FSS's modulation frequency which will lower the Q and gain of the PD to compensate somewhat. The lower Q will also gain us phase margin in the FSS loooop.

 

I put in a bi-directional 20 dB coupler (its only rated down to 30 MHz, but its only off by ~0.3 dB at 21 MHz) between the RFPD and the FSS box. I looked at the time series on the 300 MHz scope and measured the power spectrum.

The peak signal on the scope was 40 mV; that translates to 400 mV at the RFPD output. Depending on whether the series resistor in the box is 20 or 50 Ohms, it means the MAX4107 is close to saturating.

As you can see from the spectrum, its mostly likely to hit its slew rate limit (500 V/us) first. Actually its not going to hit the limit: but even getting within a factor of 10 is bad news in terms of distortion.

Besides the multiples of the modulation frequency, you can see that most of the RMS comes from the strange large peaks at 137.9 and 181.1 MHz. Anyone know what these are from?

TEK00000.PNGTEK00001.PNGTEK00002.PNG

On the middle plot above, I have enabled the 20 MHz BW limit so you can see how much the amplitude goes down when only the 21.5 MHz SB is included. You can also see from the leftmost plot that once in awhile there is some 400mV/10ns slewing. Its within a factor of 10 of the slew rate limit.

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