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(b)
The catch on the terminating load.
(c)
The plug and socket of the cable joining thedetector unit and the monitor.
2.4 MEASUREMENT PRECAUTIONS
When malting a measurement, the following points should be borne in mind:
1)
Ideally, the impedance presented by the power meter to a transmitter under
test should be purely resistive.
In practice, the power meter impedance will,
inevitably, include a small reactive component which will be reflected back into the
transmitter tuned output circuit, partially detuning it.
In order to obtain a true
measure of the power output capabilities of a transmitter, its output stage should
always be tuned to produce maximum indication on the power meter.
2)
In circumstances where the power meter is required for use merely as a
terminating device, it is better to make connection directly to the terminating load
as this presents a particularly high quality resistive termination having a v. s. w. r.
of about 1. 04.
To remove the load for this purpose see Sect. 2. 6.
3)
Because of the time lag in thermocouple heating, and the low resistance of the
meter circuit, wait a few seconds before reading the power.
4)
The peak voltage that can be applied to the power meter is limited by the in-
line detector insulation; this is rated to withstand 60 V peak, which exceeds the
peak voltage of a 100% sine wave amplitude modulated signal of 10 W mean power.
5)
In the event of long-term storage in conditions of extreme humidity, check 50 Hz
calibration before use (refer to Sect. 5. 4).
If
necessary, dry out by placing the power
meter in an oven (+50 °C to +70 °C) for at least one hour.
2.5
MAKING POWER MEASUREMENTS
To make a power measurement proceed as follows:
1)
Set the Range switch to suit the expected power level of the source.
If this is
unknown, use the 10 W position as a precaution.
2)
Turn the INSERT PROBE and Sensitivity controls fully counter-clockwise.
3)
Connect the source under test to the Input socket and note the reading on the
DETECTOR meter.
Bring this reading to about half scale by turning up the Sensi-
tivity control.
If
this is insufficient, turn the INSERT PROBE control clockwise.
NOTE:
If the probe is inserted too far, the v. s. w. r. of the instrument may
increase.
Do not insert it until the Sensitivity control is at maximum.
8
2502 (1a)
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