Varian UNITY INOVA NMR, Hardware Installation

Page: 46 / 65

6.1 Tuning the High-Power (Cavity) Amplifier
01-999044-00 C0501 UNITY INOVA Solids Hardware Installation
45
WARNING:
This procedure exposes the operator to high voltages. Take all
appropriate safety precautions! Exercise extreme care!
2. Turn on the low voltage power supply to energize the tube filament.
CAUTION:
Do not apply rf input or high voltage supply until at least 3 minutes
after turning on the low voltage power supply.
3. After 3 minutes , energize the high voltage supply.
4. Enter
tn='C13' dn='H1' dm='n' su
to switch to CW unblanking.
5. Apply 37 dBm (5 W) of rf drive CW by setting
dpwr
to an appropriate value and
entering
dm='y' su
.
You can check the upper limits of
dpwr
by entering
dpwr=63
and pressing Return.
Check in
dg
for the value of
dpwr
. If
dpwr=49
appears, the upper limit on the
dpwr
parameter can be changed using
config
(refer to the VNMR and Solaris
Software Installation Manual).
Adjust the input tuning and matching (slide coupling with hose clamp, see Figure
19 ) for minimum reflected power on the thru-line wattmeter. This is an repetitive
procedure, much like probe tuning.
6. Adjust the TUNE and LOAD tuning controls repetitively for maximum output
power. Check that the grid current is well within the limits of ±160 mA.
CAUTION:
To avoid damaging the internal sense antenna, do not exceed the 3-
o’clock position shown in
Figure 19 in a counter-clockwise direction.
The antenna is damaged when it contacts the moving load wall of the
cavity. Also, do not over-loosen the locking ring because the antenna
will become loose, move freely, and be damaged.
7. Rotate the output coupling lever to achieve maximum power output and then tighten
the locking disk to hold this position.
8. Again adjust the TUNE and LOAD tuning controls repetitively for maximum output
power. Check that the grid current is well within the limits of ±160 mA.
9. Enter dm='n' su .
Fine Tuning the Cavity Amplifier (Pulsed Mode)
1. Apply 46 dBm of power in 1 ms pulses with 10% duty cycle to the amplifier by
setting the following parameters:
seqfil='s2pul' pw=1e3 tn='H1'
sw=1e5 at=.01 d1=0 nt=1e6
If the amplifier trips off with an OVERDR indication, lower the input power until the
protection circuit does not activate.
2. Monitor the grid current at the test point on the status panel with an oscilloscope. The
scope shows 1 mV for each mA grid current. If necessary, raise the current set point
of the protection circuit (R83 on the cavity control board 990556), as described in
“Adjusting the Overdrive Interlock” on page 46 , to allow for 46-dBm pulses.
3. Attempt to maximize the output power while minimizing the grid current. Rotate the
output coupling to maximize the output—typically the 6 o'clock position. Adjust
both TUNE and LOAD to maximize the output. The gain should be 14 dB at 46 dBm
input.