271
Users Manual
8-2
Introduction
Two modes of operation are available from the AM menu:
1. Amplitude modulation using the internal trigger generator as the modulation source
in which the modulation depth is expressed as a percentage and constant modulation
depth is maintained as the main generator (carrier) amplitude is varied.
2. VCA (voltage controlled amplitude) mode, in which the main generator amplitude is
directly proportional to the external modulating signal voltage applied to the rear
panel
VCA IN
socket. Suppressed carrier modulation (SCM) is achievable in this
mode.
AM mode is turned on and off with alternate presses of the
AM
key; the lamp beside the
key lights when
AM
mode is on. The AM parameters are all set from the AM edit menu
which is selected by pressing the
EDIT
key followed by the
AM
key. When AM edit is
selected the lamp beside the
AM
key flashes to show edit mode regardless of whether
AM mode is currently selected to be on or off.
SOURCE=EXT VCA
TGEN=1.00ms 1.000kHz
INT MOD DEPTH=030%
INT MOD=SQUARE
Amplitude Modulation (Internal)
With the edit cursor in the
SOURCE
field of the AM edit menu the
DIGIT
keys or rotary
control can be used to toggle the source between
EXT VCA
and
TGEN
(Trigger
GENerator) i.e. between external VCA mode and internal AM mode.
Modulation Frequency
Select
TGEN
in the
SOURCE
field and move the cursor to the
TGEN
field to set the
period of the internal trigger generator, the modulation source for internal AM. The
internal trigger generator produces a square wave with a period that can be set from
0.02 ms (50 kHz) to 200 s (0.005 Hz). Period entries that cannot be exactly set are
accepted and rounded up to the nearest available value, for example 0.109 ms is rounded
to 0.12 ms. The generator output is available as a TTL level signal at the rear panel
TRIG/SWEEP OUT
socket.
Beside the generator period value the equivalent frequency is shown; this is for
information only and is not an editable field.
Because the internal trigger generator can be used by the trigger, gate, FSK and AM
functions, and can be set from their respective edit menus, an information field is
displayed in brackets beside
TGEN
when this is selected as the source. This field will
show
[FREE]
when
TGEN
is not used elsewhere, or one of the four letters
G, F
,
A
or
T
to indicate that the generator is currently set as the source on the
GATE
,
FSK
,
AM
, or
TRIG
menus respectively, in addition to the menu currently displayed.
Modulation Depth
Move the edit cursor to the
INT MOD DEPTH
field to set the modulation depth
between 1 % and 100 % in 1 % increments. The maximum output (20 V p-p into an open
circuit) cannot be exceeded and clipping will occur if modulation attempts to drive the
1.888.475.5235
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