I Unser setup and calibration
a) Adjusting offset and calibration of Unser.
This procedure should be carried out if the Unser's offset is more than 1 V, but there is no other problems with the Unser. This can happen, for instance, if the magnetic field of the environment is to high. The zero adjustment displaces the working point of the parametric magnetic amplifier and may cause the Unser drift for a few hours depending on amplitude of adjustment. Each time after changing the offset, the Unser calibration must be performed again.
1. Set the range switch on "Range A"
2. Set Calibration switch on "Calibration OFF"
3. Set the Test switch "OFF"
=1.5pc =1 4. In Hall A find a M3 screw located on the top of the front-end electronics box (that is small box beside the BCM monitor).
=1.5pc =1 5. Insert a screw driver in the hole and rotate the 20-turn potentiometer to adjust the offset until the voltage on Unser DVM in Hall A control room will be less than 1 V.
=1.5pc =1 6. After the Unser is stabilized (if there was any drift caused by zero adjustment), perform the Unser Calibration. In the control room turn on the current source, and change the current from 10 to 100 A in steps off 10 A, Read the corresponding Unser output on DVM. This completes the calibration of Unser.
b) Functional Unser Test
This procedure should be carried out only, if you have valid reasons to be suspicious that the Unser is not working properly.
1. Observe that LED "Range A" is lit, and read value from DVM.
2. Set the calibration switch to "calibration".
3. LEDs "CAL.ON" and "Range A" should lit.
4. Observe that the reading on DVM increases by +8 V.
=1.5pc =1 5. Return "Calibration" switch to OFF, and observe that the LED "CALL.ON" is off.
6. Set the Calibration switch to "Calibration Neg."
7. Observe that the LEDs "CAL.ON" and "Range A" are lit.
8. Observe that the reading changes by -8 V.
If in step 4 or step 8 the reading on DVM exceeds either +10.5 or -10.5 V, the sensor may saturate and require demagnetization (to be explained later) and zero adjustment.
=1.5pc =1 9. Return to "Calibration OFF" (The range should still be set to "Range A"
10. Set the Test switch to "Test ON"
11. Observe that two LEDS: "Test" and "Range B" are lit.
=1.5pc =1 12. Observe that the reading does not change more than 0.5 V. If this functional test completes successfully, it means that the basic functioning of the Unser is correct.
c) Setup of Unser - Demagnetization
This procedure should be performed if power was off or if the Unser is saturated, or if it has a constant drift for some reason.
=1.5pc =1 1. Check the line voltage on the rear of the back-end chassis in Hall A.
=1.5pc =1 2. Connect the back-end chassis to the mains. This will start a low frequency high current demagnetization, that should last up to 60 seconds. During the demagnetization, it is important that there be no interruption. If power is interrupted during this sequence, the demagnetization process is incomplete and must be restarted. To restart the demagnetization process one must wait at least 15 minutes. II Cavities setup and calibration As mentioned earlier, the cavities are already tuned (their resonant frequency, Q factor and coupling are adjusted) and they are also calibrated. However, it is recommended that absolute calibration of cavities be performed for each time shift and that coupling and resonant frequency be checked at least every 6 months.
=1.5pc =1 a) Relative and absolute calibration of cavities
The calibration procedure of the Hall A cavities has two steps, relative and absolute calibration.
In relative calibration one needs to determine the slope of the cavities responds to the current going through it, ie. the curve output signal versus injected current. This can be performed by putting the known current through cavities using a 1497 MHz current supplier. By knowing this slope, the relative current calibration can be done. The absolute calibration, is obtained by determining the absolute current value at one point on the current-output slope using the Unser monitor.
Since the Unser has better accuracy at higher current, the calibration should be performed at the highest current possible. Accurate absolute measurement at lower current is then possible, because cavities have good linearity and extrapolation to lower current will not introduce additional errors.
The cavities calibration procedure is as follows:
=1.5pc =1 1. On the input of a cavity connect the high 1497 MHz frequency current source (one is available at the accelerator division group).
=1.5pc =1 2. Adjust the frequency of the source to 1497 MHz, and its output signal power to +2 dBm (that corresponds to 100 A CEBAF).
=1.5pc =1 3. Decrease power of signal to, -4 dBm (50 A), -12 dBm (20 A), -18 dBm (10 A), -24 dBm (5 A), -32 dBm (2 A) and -38 dBm (1 A), and read the output on corresponding digital multimeters at Hall A counting house. With this the relative calibration is accomplished.
=1.5pc =1 4. Ask the operator to deliver the highest current beam possible to Hall A.
=1.5pc =1 5. Read output from the cavities and the Unser for that current. With this the absolute calibration of cavities is completed for the entire region covered by the cavities.
b) Setup of cavities
When installed, cavities are checked and adjusted. The intrinsic Q0 factor of cavities is checked to be 3000, and they are tuned so as to keep the loaded Q at 1500. The geometric shunt impedance, R/Q of the cavity is adjusted approximately to 180 and resonant frequency is tuned to be 1497 MHz.
The cavities should be working properly for long periods of time, i.e. there are no reason to readjust them. However, we recommend that they be checked at least at intervals of 6 months. The following is the procedure. It involves a calibrated network analyzer (VNA) and therefore it assumes that one knows how to use it. In addition one should as well to have moderate experience with microwave measurements.
=1.5pc =1 1. Calibrate the VNA to a frequency of 1.497 GHz with 20 MHz span and single 1 port calibration with 801 points using s-parameter set.
=1.5pc =1 2. Attach the cable to the probe of the BCM setting the VNA to the Smith chart function. Loosen the nuts around the conflate flange until the probe can moved somewhat freely, then adjust the probe until the tuner marks approximately 30o of the horizontal axis.
=1.5pc =1 3. Looking at the VNA and hitting the scale button and then the phase offset button in the menu adjust the phase offset until the circle swept out on the screen lies on the right hand side of the Smith chart. Adjust the probe until the circle swept out along the Smith chart overlaps the 50 circle of the Smith chart. If the circle is not symmetrically divided by the real axis set a marker on 1497 MHz and adjust the tuner until this marker lies on the real axis. You may want to align the phase offset again after this point. This is an iterative process and may take you a couple of times to get it right, but it is the best way to critically couple the probes.
=1.5pc =1 4. Once you achieved critical coupling, look at the SWR function on VNA. It should be very close to 1.1:1. With the cables in place lightly tighten the conflate flange nuts, the coupling should not change. Make a swap plot of VSWR, S11, for the record. Once this is done the flange can be fully tighten down.