Martin Houde, 8-April-1997
The control box for the chopping secondary is located in the sidecab, high on the equipment rack; you will probably need the step ladder to access its front panel. It is connected to a HP digital scope located on the floor near the equipment rack, utilisation of this scope is fairly straightforward as it is menu driven; but the observer might want to take a few minutes to get familiar with it.
A) FOR HETERODYNE OBSERVING
1- The chopper status is displayed at the bottom center of the antenna computer monitor display in the control room.
2- Make sure that the locking switch on the left of the control box is set to "ANALOG".
3- To start chopping, issue the following command at the UIP prompt on the VAX :
SEC xxx yyy zzz
where xxx is the chop throw on the sky in arcseconds, yyy is the chop frequency in Hertz and zzz is the tolerance on the beam position on the sky in arcseconds. If zzz is omitted, 1/4 of the beamwidth is selected by default.
4- Make sure the green LED (GO) is lit on the front panel of the control box. If it isn't, the red LED (STOP) should be lit; push the GO button and the green LED will come on.
5- On the chart located on the wall in the sidecab on the left of the control box, choose the P, I, D and G settings that correspond to the values of chop throw and frequency that are closest to the ones you chose in step 3. Dial those numbers on the corresponding knobs on the control box.
6- Looking at the HP digital scope on the floor, adjust the waveshape of the chop throw, using the P, I, D and G knobs, to get as close to a square wave as possible. P and D affect the damping, I the slope on the flat parts of the wave and G the speed of the switching. Make sure the scope is triggering on chanel 3 and in manual mode.
7- When tuned properly, the HI and LO LED's should be blinking in a complementary way and at least one of them should always be lit, except for the transition periods (from HI-to-LO and LO-to-HI).
8- The chopper can be stopped manually at any moment by pushing the stop button on the control box.
9- Always remember to stop chopping at the end of the night by entering the following command at the UIP prompt:
SEC/STOP
B) FOR SHARC OBSERVING
1- Make sure that the locking switch on the left of the control box is set to "TTL".
2- Make sure the scope is triggering on chanel 1 and in manual mode.
3- On the chart located on the wall in the sidecab on the left of the control box, choose the P, I, D and G settings that correspond to the values of chop throw and frequency that are closest to the ones you want. Dial those numbers on the corresponding knobs on the control box.
4- To start chopping, issue the following command at the UIP prompt on the VAX :
SEC xxx yyy zzz
where xxx is the chop throw on the sky in arcseconds, yyy is the chop frequency in Hertz and zzz is the tolerance on the beam position on the sky in arcseconds, it can be omitted and has no effect with SHARC.
The VAX will answer by giving you an offset voltage and a peak-to-peak voltage (Vpp).
5- Make sure the green LED (GO) is lit on the front panel of the control box. If it isn't, the red LED (STOP) should be lit; push the GO button and the green LED should come on. If it doesn't, try reducing the TTL GAIN (second knob from the left on the control box) and try pushing the GO button again ...
6- You now have to adjust the amplitude of the throw by using the HP digital scope located on the floor. You first have to determine the levels at which the maximum (V2) and minimum (V1) of the chopping waveform have to be set as seen on the scope :
V1 = 412.5 mV - Vpp/2
V2 = 412.5 mV + Vpp/2
where Vpp is in mVolts and defined in step 4.
Using the "Dv Dt" menu button on the scope, set the levels and adjust the chop throw with the TTL OFFSET and GAIN knobs located on the front panel of the control box. Make sure that V1 and V2 on the scope menu are set to channel 1.
7- Still looking at the HP digital scope on the floor, adjust the waveshape of the chop throw, using the P, I, D and G knobs, to get as close to a square wave as possible. P and D affect the damping, I the slope on the flat parts of the wave and G the speed of the switching.
8- When tuned properly, the HI and LO LED's should be blinking in a complementary way and at least one of them should always be lit, except for the transition periods (from HI-to-LO and LO-to-HI).
8- The chopper can be stopped manually at any moment by pushing the stop button on the control box.
9- Always remember to stop chopping at the end of the night by entering the following command at the UIP prompt:
SEC/STOP
10- at the end of the night, put the locking switch on the front panel of the control box to "ANALOG".
SETTINGS FOR THE CHOPPER CONTROL BOX
loop parameters
throw error risetime freq. P I D G
(") (") (msec) (Hz)
15 --- --- 4.1 4.32 3.09 5.48 10.00
30 --- --- 4.1 4.00 4.24 6.98 10.00
30 2 40 1.123 4.12 2.48 5.05 11.00
45 2.6 55 1.123 3.28 2.20 5.48 11.00
45 3.1 50 1.123 3.44 2.16 5.42 11.00
45 3.6 45 1.123 3.73 2.34 5.82 8.18
60 4.6 40 1.123 4.64 2.03 5.80 8.18
60 --- --- 4.1 3.82 3.10 5.02 8.20
90 3.6 50 1.123 6.06 2.81 6.80 6.85
90 --- --- 4.1 2.38 3.00 3.10 9.47
120 4.0 65 1.123 7.60 3.02 9.09 3.86
180 5.4 56 1.123 9.5 3.04 10.00 3.04
180 5.4 ~65 3.0 8.48 2.36 9.52 5.14
240 5.1 ~70 1.123 9.3 2.98 9.70 4.03
240 --- --- 3.0 8.66 2.16 9.66 4.56
300 7.7 ~65 1.123 10.00 2.65 10.00 3.88
300 4.6 ~65 3.0 8.58 2.56 10.00 4.56
360 8.0 ~70 1.123 10.00 2.72 10.00 3.77
360 ~5.0 ~65 3.0 8.85 2.71 10.00 4.15
420 5.6 ~80 1.123 10.00 3.07 8.84 3.46
420 5.6 ~90 3.0 8.46 3.94 10.00 4.06
Notes:
- These numbers are not absolute and are known to change with
temperature ... but they are a good starting point.
- 1.123 Hz is just a weird frequency that I use to do tests, there's
nothing special about it.
- the settings are frequency dependant, if you use 3.1 Hz instead of
3.0 Hz you will need to do some further adjustments.
- the conversion factor on the scope is ~12.2 mVolts/".
- please let me know if you find better values for P, I, D and G.
M. Houde. houde@ulu.submm.caltech.edu
04.08.97