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.
