I wanted to check the status of the ISS. The AOM driver response was measured on Friday night.
The beam path has not been disturbed yet.
- I found the AOM crystal was removed from the beam path. It was left so.
- The AOM crystal has +24V power supply in stead of specified +28V.
I wanted to check the functionality of the AOM driver.
- I've inserted a 20dB directional coupler between the driver and the crystal.
To do so, I first turned off the power supply by removing the corresponding fuse block at the side panel of the 1X1 Rack.
Then ZFDC-20-5-S+ was inserted, the coupled output was connected to a 100MHz oscilloscope with 50Ohm termination.
Then plugged in the fuse block again to energize the driver box.
Note that the oscilloscope bandwidth caused reduction the amplitude by a factor of 0.78. In the result, this has already been compensated.
- First, I checked the applied offset from a signal generator (SG) and the actual voltage at the AOM input. The SG OUT
and the AOM control input are supposed to have an impedance of 50Ohm. However, apparently the voltage seen at the
AOM in was low. It behaved as if the input impedance of the AOM driver is 25Ohm.
In any case, we want to use low output impedance source to drive the AOM driver, but we should keep this in mind.
- The first attachment shows the output RF amplitude as a function of the DC offset. The horizontal axis is the DC voltage AT THE AOM INPUT (not at the SG out).
Above 0.5V offset some non linearity is seen. I wasn't sure if this is related to the lower supply voltage or not. I'd use the nominal DC of 0.5V@AOM.
The output with the input of 1V does not reach the specified output of 2W (33dBm). I didn't touch the RF output adjustment yet. And again the suppy is not +28V but +24V.
- I decided to measure the frequency response at the offset of 0.53V@AOM, this corresponds to the DC offset of 0.8V. 0.3Vpp oscillation was given.
i.e. The SG out seen by a high-Z scope is V_SG(t) = 1.59 + 0.3 Sin(2 pi f t) [V]. The AOM drive voltage V_AOM(t) = 0.53 + 0.099 Sin(2 pi f t).
From the max and min amplitudes observed in the osciiloscope, the response was checked. (Attachment 2)
The plot shows how much is the modulation depth (0~1) when the amplitude of 1Vpk is applied at the AOM input.
The value is ~2 [1/V] at DC. This makes sense as the control amplitude is 0.5, the applied voltage swings from 0V-1V and yields 100% modulation.
At 10MHz the first sign of reduction is seen, then the response starts dropping above 10MHz. The specification says the rise time of the driver is 12nsec.
If the system has a single pole, there is a relationship between the rise time (t_rise) and the cut-off freq (fc) as fc*t_rise = 0.35 (cf Wikipedia "Rise Time").
If we beieve this, the specification of fc is 30MHz. That sounds too high compared to the measurement (fc ~15MHz).
In any case the response is pretty flat up to 3MHz.