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11
has a flat frequency characteristic, and that it provides at least one volt,
peak-to-peak, at 100% carrier modulation (±75kHz deviation).
Correcting for
Response
Errors
Subcarrier injection measurement accuracy is a function of mod-
monitor output flatness. If mod-monitor demodulation response
exhibits HF roll-off as it approaches 100kHz, use the manufacturer’s
published correction chart (or make one of your own!) to assure
accuracy of subcarrier injection readings.
The input impedance of the Model 540 is 100k-ohms, essentially an
open circuit. To obviate any need to address impedance matching or
source termination, keep the interconnecting cable as short as possible.
Input Level
Adjustment
The 540 Monitor is most accurately calibrated by applying a steady-
state input signal equivalent to 100% carrier modulation; that is, a
carrier deviation of ±75kHz. If the mod-monitor has a built-in
calibrator which drives its own indicator to this figure, and if that same
test signal is delivered to the mod-monitor’s composite output
connection, calibration of the 540 is very easy. Simply adjust the 540’s
rear-panel
INPUT GAIN
control until the bargraph display reaches the
CALIBRATE
mark on the scale. (This is directly above the “10%”
designation and corresponds to 100% carrier modulation.)
If the mod-monitor does
not
have a built-in calibration source, an
equally accurate adjustment may be made by driving the exciter to full
modulation with a test oscillator signal, probably best done during a
maintenance period. Set
INPUT GAIN
as described above.
Another calibration method allows you to use normal program material
for setup. The
INPUT GAIN
control may be set with acceptable accuracy
by setting program peaks displayed in the
CALIBRATE
mode to the same
total-peak-modulation values indicated by the mod-monitor. In
CALIBRATE
, multiply the Model 540’s scale markings by 10 to show
peak excursions of 10% to 130% modulation (10kHz to 100kHz carrier
deviation). Since 540 metering is peak-responding and incorporates a
peak-hold feature, it should indicate the same peak levels indicated by
the mod-monitor. However, if mod-monitor metering is
not
a true,
peak-responding display, defer instead to calibrated
peak flasher
set
points.
