There are two main parts to the electronics, which are all analog. The first part is the feedback electronics, which filter, condition, and amplify the control signals, and then drive the mirror actuators. The second part is the RF electronics, which are used for generating the input spectrum of light, as well as providing local oscillators (LO) for the demodulation process. This section is meant to be a general description.
Schematics are given in Appendix D.
Feedback Electronics
A general outline of the feedback electronics is shown in Figure 4.4. There are some
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SR560 PZT
Compensation Fast
HV driver Fast PZT
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Slow monitor Slow PZT HV driver
Figure 4.4: Block diagram of the feedback electronics. The elements in dashed boxes are on the optical table. The "-B" input of the SR560's is used for calibrated offset adjusts, as well as some transfer function measurements. The SR560's also have a second buffered output used for monitoring signals. The second inputs of the HV drivers also are convenient places for making transfer function measurements, when above unity gain.
modifications to this outline in some cases. The ¢Mz does not use a SR560 in its feedback. The tj;_ feedback also doesn't use a SR560, and furthermore the PZT compensation module is a unique set of electronics, the output of which goes to two slow HV drivers differentially, driving 8M2 and 8M3. As mentioned before, the feedback to <I>+ and <I>_ is done as a combination of the actuators in both arms. Feedback for <I>+ is done by driving both ETMs as well as SM4 and SM5 with the same sign. Feedback for <I>_ is done by driving ETMl and SM4 with an opposite sign
from ETM2 and SM5. A box of electronics, called the "CM/DM driver," was built which splits the output of the <I>+ and <I>_ compensation modules into signals for each of the four actuators in both arms, and then appropriately sums and differences the signals. The output is then directed to the high voltage drivers for ETM1, ETM2, SM4, and SM5. The CM/DM driver also includes test inputs for the <I>+ and <I>_
modes.
The SR560 amplifiers function not only as amplifiers and filters, but also as con- venient points for summing offsets, signal monitoring, and transfer function measure- ments. They also do a nice job of breaking ground loops.
The PZT compensation modules contain a variable gain and a feedback sign switch. The electronics for splitting the feedback into fast and slow paths is done in these modules. They also contain a switchable boost stage, which adds roughly 500 gain at low frequency. The boost is turned off for acquisition, and turned on once all loops are locked. There is also a switchable integrator in the slow path, which is also left off until lock is acquired.
The high voltage (HV) drivers have x 100 gain. The fast HV driver is capable of driving ±200 V, while the slow HV driver can output 0 to +1000 V. The slow HV drivers also contain a bias knob, which allows the user to vary the voltage to the slow piezos by hand. All of these HV drivers include test inputs as well as ...;-100 monitors.
The loop gains for all paths except for
<P-
have a 10 Hz pole with no boost, and two 10 Hz poles plus a 5 kHz zero with the boost on. When the integrator is turned on, one of these poles moves to DC. Most of these poles and zeros are formed in the PZT compensation electronics. In the exceptions, a 10 Hz pole is removed from the PZT compensation and is implemented in the HV drivers for the fast and slow actuators. Additionally, low pass filtering was done in the SR560's at high frequencies (2:: 30kHz). The<P-
compensation has a single 10Hz pole, which moves to DC when its integrator was turned on. Additionally, a 2 kHz pole is used to low pass the slow PZT resonances as much as possible. There is no boost stage in the<P-
feedback.RF Electronics
RF electronics serve two purposes. First, they drive the Pockels cell and AOM which generate the RF sidebands on the laser light. Second, they supply the local oscillators for the mixers to demodulate all of the signals, as well as a phase shifter for each LO to control the demodulation phase.
The RF frequencies needed are f, 3f, and 3 f - f. It's not an absolute necessity that f and 3f are exact multiples of each other. It is necessary that the third frequency be exactly 3 f - f, and furthermore the phase of 3 f - f needs to be stable relative to the f and 3f frequencies. This is most easily done by generating all three frequencies from a single oscillator, which is done on a single board. A HP8656B synthesized signal generator is used to generate 27 MHz (f). This is split three ways by a power splitter. One path is amplified and sent to the AOM. The second path is frequency doubled using a Mini-Circuits SYK-2, followed by tuned resonant filters to reject the fundamental as well as higher harmonics. The third path uses a circuit which creates a square wave from the 27 MHz input, using a Motorola MC10115 quad line receiver chip. The square wave has odd harmonic components, the first being 81 MHz. Several resonant filters are used then to pass the 81 MHz component, while suppressing all other harmonics. Figure 4.5 shows a block diagram of this board.
27 MHz out
Square wave generator
Figure 4.5: Block diagram of the RF frequency generation board.
+19 dBm
+6dBm
-2 dBm
There are three 54 MHz LO's, three 81 MHz LO's, plus the Pockels cell 81 MHz.
A board was designed which splits the input four ways. All four paths are amplified to
output roughly