8
reasonable to set the calibration value to within ±0.05ppm. This is typical of the variation over
normal room temperature changes during the course of a day.
Finally, press and hold down the
Width
key for a few seconds to store the calibration value in
EEPROM.
Hold the key down until the C annunciator stops flashing and stays off.
Note
: To exit calibration without saving the new value, simply press
Operate
to switch off.
Press
Operate
to switch the unit off, wait a few seconds and then press it again to turn the unit
back on.
Select 50
Ω
Impedance
(and if necessary 5:1
attenuator
). Set the
Measurement Time
to 10s.
Check that the reading is the same (within 1 count) to prove that the new calibration value has
been stored.
If desired, set the
Measurement Time
to 100s, and (after Measure stops flashing) record the
final reading.
C Input Threshold Adjustment
Introduction
The prescaler used in the C input will self-oscillate if no signal is applied. To avoid distracting the
user a signal level detector circuit senses the absence of a useable input signal and disables the
divider to show 0.0 on the display. There is a preset adjustment in this circuit, which might
possibly drift with time. If the threshold is too low, then the unit will show a random count around
7GHz whenever the C input is selected without a signal. If the threshold is set too high, then the
usable sensitivity of the input is impaired.
Equipment required
2 to 6GHz signal generator with reasonably accurate calibrated level around 5 to 15mV.
A microwave grade N-type to N-type coaxial lead. (Do not use a BNC type with adapters).
Procedure
To adjust the setting, first dismantle the unit to the stage of removing the top cover. Identify the C
input PCB, the variable adjustment, a two pin link LK1 and a surface mount LED near the link.
With no signal applied, switch the unit on and select the C input. If 0.0 is displayed, short
together the two pins of LK1. The unstable count should appear; if not, then there is a fault
present.
Apply a CW signal at 3GHz and a level of 6mV. Using a suitable trimming tool, turn the variable
adjustment clockwise until the LED lights (and a count appears on the display). Then turn the
variable adjustment
slowly
anti-clockwise until the LED just extinguishes.
Increase the signal generator level until the LED lights and a count shows; this should be before
12mV. The count should be correct by 15mV. Decrease the signal generator level to 4mV and
check that the LED extinguishes and the display shows 0.0. Check at other frequencies across
the range 2 to 6GHz that the LED remains off at a signal level of 4mV.
Note
This is a general purpose setting which will achieve rated sensitivity, but at some frequencies the
level detector will permit a count with a signal level below that needed to produce a correct count.
Some customers may prefer a higher threshold setting, so that a count does not show until it is
correct. This setting can be performed at the customer’s preferred frequency, but this may
possibly mean that the unit does not meet its sensitivity specification at other frequencies.