Vco Adjustment - Wavetek 39A Maintenance Manual

VCO Adjustment

This should not normally be necessary and L6 is sealed at the factory. However if a problem is
suspected or components in this circuit have been changed carry out the following test first.
Set the output to 10MHz squarewave and check that the voltage at TP200.3 is −9.5V to −10.5V.
Check LED 200 is off.
Only if the voltage is outside these limits should L200 be adjusted to −10V ±0.2V. L6 core must
then be resealed again to reduce phase noise caused by mechanical vibration. Use only non-
corrosive silicon rubber.
VR200 Adjustment
Not normally necessary. Must only be adjusted with the default calibration values loaded or
CAL07 set to 0128. At CAL07 adjust VR200 for 0Vdc ± 5mV.
Amplitude Flatness
This should not normally be necessary. Set to 20Vpk-pk and use a 50Ohm terminator, frequency
to 100kHz sinewave. Adjust oscilloscope to show exactly 6 divisions. Frequency to 10.00000MHz
and adjust L208 for exactly 6 divisions. Frequency to 10.1MHz and adjust L204 for exactly 6
divisions. These two adjustments should only be done using a high quality oscilloscope with a
bandwidth of at least 100MHz.
Remote Calibration
Calibration of the instrument may be performed over the RS232 or GPIB interface. To completely
automate the process the multimeter and frequency meter will also need to be remote controlled
and the controller will need to run a calibration program unique to this instrument.
The remote calibration commands allow a simplified version of manual calibration to be
performed by issuing commands from the controller. The controller must send the CALADJ
command repeatedly and read the dmm or frequency meter until the required result for the
selected calibration step is achieved. The CALSTEP command is then issued to accept the new
value and move to the next step.
While in remote calibration mode very little error checking is performed and it is the controllers
responsibility to ensure that everything progresses in an orderly way. Only the following
commands should be used during calibration.
WARNING: Using any other commands while in calibration mode may give unpredictable results
and could cause the instrument to lock up, requiring the power to be cycled to regain control.
CALIBRATION <cpd> [,nrf]
START
SAVE
ABORT
CALADJ <nrf>
CALSTEP
For general information on remote operation and remote command formats, refer to the Instrument
instruction manual remote operation sections.
20
The calibration control command. <cpd> can be one of three
sub−commands:−
Enter calibration mode; this command must be issued before any other
calibration commands will be recognised.
Finish calibration, save the new values and exit calibration mode.
Finish calibration, do not save the new values and exit calibration mode.
<nrf> represents the calibration password. The password is only required
with CALIBRATION START and then only if a non−zero password has
been set from the instrument's keyboard. The password will be ignored,
and will give no errors, at all other times.
It is not possible to set or change the password using remote commands.
Adjust the selected calibration value by <nrf>. The value must be in the
range −100 to +100. Once an adjustment has been completed and the
new value is as required the CALSTEP command must be issued for the
new value to be accepted.
Step to the next calibration point.