HP 400D - Page 20

TM 11-6625-1514-15
Section III
Paragraphs 3-22 to 3-25
+ 2 (meter indication)
+30
(RANGE switch position)
+32 (sum)
+8
(correction factor from the Impedance
+40 dbm
Correction Graph)
Example 2
For the same conditions as given above, except that
the measurement is made across an impedance of 60,000
ohms, the level in dbm is obtained as follows:
+ 2 (meter indication)
+30
(RANGE switch position)
+32 (sum)
-20
(Correction factor from the Impedance
+12 dbm
Correction Graph)
3-22. USE OF VOLTMETER AMPLlFIER.
3-23. The amplifier in the voltmeter may be used for
amplifying weak signals. With full-scale meter deflec-
tion, the open-circuit output of the amplifier is approxi-
mately 0.15 volt rms regardless of the RANGE switch
position. The impedance looking into the OUTPUT
terminals is approximately 50 ohms. The frequency
response and calibration of the voltmeter may be
affected by the impedance of a load applied to the
OUTPUT terminals. To check the effect of the applied
load: observe the meter reading obtained with no load
connected to the OUTPUT terminals and then note any
shift of reading when the external circuit is connected
to the OUTPUT terminals. If the shift is negligible,
the measurement is not being affected appreciably by
the load. Whenever the input signal is changed, i.e., a
different frequency or band of frequencies is applied,
repeat the quick check described above.
3-24. Maximum gain from the amplifier is obtainable
only on the lowest (.001 volts) range, since output level
is the same for all bands. This is due to the 10-db
amplification loss per step inserted by the RANGE
switch as it is turned clockwise. Amplification may
also be obtained on the .003, .01, .03, and 1 volt ranges.
3-25. When the voltmeter is used as an amplifier,
select a range which gives a meter deflection near
full scale. Off-scale signals more than twice the value
of the position of the RANGE switch will cause severe
distortion.
3-4
00102-2