Operating instructions

OPERATING MANUAL 2945A
46882-311D 3-41
External Modulation
An external modulation source may be used to modulate the RF generator signal. Pressing the
[Ext Mod] key will display the external modulation input selection menu.
The options available are:-
[DC Ext] Selects the ‘EXT MOD IN’ connector on the rear of the
instrument, with DC coupling.
[AC Ext] Selects the ‘EXT MOD IN’ connector on the rear of the
instrument, with AC coupling .
[Mic
]
Selects Microphone (connected to the ACCESSORY socket
on the front panel) as the external modulation source.
The selection is shown on the display next to the
Ext Level Source legend. The frequency range
for each input option is specified in the Performance data section of Chapter 1.
The level of the external modulation signal should be 1.0 V rms. This retains correlation of the
modulation level indicated on the display with that applied to the RF signal.
Setting external modulation level
Press the [Ext Mod] key, which as well as displaying the menu mentioned above, will highlight
the
legend. Use the data input keys to set the required level, followed by the
appropriate [Hz], [kHz] or [%] key.
Hint
The modulation source level can be measured using the AF INPUT connector on the front panel
and AF LEVEL bar chart, or by monitoring the modulation level of the RF g enerator using the
Dx TEST mode.
Microphone input
A microphone suitable for connecting to the accessory socket, is available as an optional
accessory. See Options and accessories, in the Performance data section of Chapter 1.
The microphone input circuit contains a limiter to prevent over-modulation, and the external
modulation level setting remains effective when the microphone option is selected. This gives
versatility of test options to suit the user's requirements.
The microphone input must be selected to make use of the PTT switching facility available in the
Tx TEST mode.
The level of the modulation from the external source will add to the level of any internal
modulation and will therefore be included in the total sum of modulation sources.
The state of the external modulation source can be changed by pressing the orange [ON/OFF]
function key when the
legend is highlighted. Off is displayed next to the external
modulation parameters when the external modulation is disabled.
Note that only one type of modulation can be applied to the RF signal. For example:-
If the [%] terminator key is used to complete a modulation setting when an FM deviation
level is already set from another source, the latest selection will cause the previous
setting to be cancelled.
AF input level measurement
The measured level of signals applied to the AF INPUT connector can be displayed in volts, dBm,
dBV or mW. The selection is made from setup page 2, as described under Audio Level Measured
in: on page 3-12. When the input level is displayed in dBm or mW, the signal is assumed to be
measured across 600 Ω.
The 600 Ω interface unit allows the Service Monitor to provide a 600 Ω termination. (See Audio
Input Impedance on page 3-12.) With the input impedance set at 600 Ω, the indicated level is