Operating instructions

LOCAL OPERATION
3-44
Pre-emphasis filter
The pre-emphasis filter can be switched in or out of circuit after pressing the [Mod Gen] key.
Pressing the [Pre-emph] key will toggle the pre-emphasis filter into and out of circuit. The time
constant of the filter is 750 µs, giving an increase with frequency of 6 dB/octave.
The legend
Pre-emph ON is displayed when the filtering is in circuit. No indication is given when
the filtering is out of circuit.
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
Service Monitor, with DC coupling.
[AC Ext] Selects the ‘EXT MOD IN’ connector on the rear of the
Service Monitor, 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
EXT LEVEL: 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 generator 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
EXT LEVEL 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.