10
External Sweep
TG330: When EXT SWEEP is selected the SWEEP IN/OUT socket becomes
an input.
TG320/TG315/TG310: The SWEEP IN socket is the external sweep control input.
The generator frequency can be swept, DC programmed or modulated by a suitable control
voltage applied to this input. The instrument sums the SWEEP IN voltage with the internal control
voltage derived from the FREQUENCY vernier to determine the operating frequency; the display
shows the resultant frequency.
A positive voltage increases the frequency; for frequency control with positive-going DC inputs
the vernier should therefore be set to the lower frequency limit of the range to be swept. For
example, a 0V to +3V signal will sweep the generator 3 decades up from range minimum, set by
the vernier, to range maximum.
Similarly, a negative voltage decreases the frequency and for negative-going DC inputs the
vernier should be set to the upper frequency limit of the range to be swept. For example, a 0V to
3V signal will sweep the generator 3 decades down from range maximum, set by the vernier, to
range minimum.
To use a sweep signal which is symmetrical about ground, the vernier should be set to give a
frequency at approximately the centre of the band to be swept.
In external sweep mode the LIN/LOG selection is inoperative, i.e. the FREQUENCY vernier is
always operating linearly, as marked.
Note: Non-linear operation may result when the sweep input voltage is excessive; that is, when
the attempted generator frequency exceeds the range limits.
Amplitude Modulation (TG330 only)
Depressing the AM ON/OFF button selects AM. The depth of modulation can be adjusted over a
0% to 100% range using the modulation DEPTH control. When AM is selected the output
amplitude will drop to 50% at 0% modulation.
With the AM INT/EXT button in the INT (internal) position, the modulation source is an internal
400Hz sinewave oscillator. The INT-AM annunciator is displayed.
With the AM INT/EXT button in the EXT (external) position, and the EXT COUNTER ON/OFF
button in the OFF EXT AM position, external AM modulation is possible via the AM/COUNT IN
socket; the AM-EXT annunciator is displayed. The modulating signal applied should have no DC
offset or should be AC coupled. A 2V peak-to-peak signal gives approximately 100% modulation
with the modulation control at maximum. Modulating the generator with a squarewave gives step
changes in the output amplitude which are suitable for testing signal compressors and automatic
gain controlled circuits.
Applying a DC offset of approximately 1V gives suppressed carrier modulation. The DC offset
should first be adjusted to suppress the carrier, and the modulating signal then applied.
Applying a DC offset greater than 1V will invert the MAIN OUT outputs with respect to AUX
OUT.
The generator can also be asynchronously gated on and off using the AM facility. Select EXT AM
and apply a suitably offset squarewave to the AM/COUNT IN input. The squarewave V
low
should
be approximately 1V to give carrier suppression and the V
high
should be +3V to +4V to give
normal full output.
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