TROUBLESHOOTING AND REPAIR
INTERNAL MODULATION OSCILLATOR
The wave data is stored in two EPROMS (U2, U3). Three control lines from U1
control the wave-form selection, allowing up to eight waveforms to be selected via a
front panel special function. When a wave other than the sine wave is used,
consideration for the low-pass filter cutoff frequency (200 kHz) should be made. There
will be a progressive deterioration of the fidelity of waves with increased frequency,
depending on the wave shape selected. This relates to the higher frequency components
of the waves other than sine wave.
Pulse Generator 6F-4.
The pulse generation mode is selected via front panel or IEEE Special Function
commands. The frequency of the pulse generator can be set from 10 Hz to 200 kHz.
Frequency and pulse width are determined by numeric values written to the oscillator.
The built-in pulse generator can be used as a modulation source for the internal AM,
FM, ΓΈM and pulse.
The pulse generator is based on the custom IC (U1), which contains a programmable
period and pulse width sections. Both the period and the width of the pulse can be set in
increments of 100 ns. Internally, the pulse frequency is rounded and set to the nearest
100 ns period increment of the entered modulation frequency.
U4, U5, Q1, Q2, and associated components provide conditioning for the external and
internal pulse signals. When in the pulse generation mode of operation, the internal
pulse at INT MOD is preset to 1V pk via U5. U4 buffers the pulse output of the custom
IC (U1) to provide MOD OUT, which is fixed at 4.5V CMOS/TTL logic level. Both,
the frequency and the pulse width are set numerically through front panel entry or via
IEEE commands. In pulse generation mode, MOD OUT is terminated with a nominal
180 ohms.
To avoid the ambiguity of the pulse output from being set to a DC value, the software
limits the pulse width to a value that is no wider than the set period minus 100 ns, and it
prevents it from being set narrower than 100 ns.
See the Special Function list in Appendix B for selections modulation oscillator modes
of operations. The appropriate Special Functions allow the selection of direct digital
synthesis, output waveform, pulse generator and pulse width setting. In addition, by
selecting the appropriate special function code, it is possible to enable MOD OUT to
be continuously on (default) or to be turned on only during the selection of internal
modulation.
Signal Routing 6F-5.
The modulation oscillator is set up to select the active outputs by means of six analog
switches. Signals from U1 control the various switch functions to route the pulse
generator and direct digital dynthesizer output signals to the two outputs (INT MOD,
MOD OUT) of the Modulation Oscillator PCA.
The two switches associated with U9A, S1, S2, U6A, U6B, facilitate the connection of
the direct digital synthesizer to the internal modulation source (INT MOD). The two
switches associated with U11B, S3, S4, U6C, U6D, facilitate the connection of the
direct digital dynthesizer to the modulation output (MOD OUT). The two switches
associated with U4 and U5, S5, S6, U6B, and U6D facilitate the connection of the pulse
generator to both the modulation output (MOD OUT) and to the internal modulation
source (INT MOD).
6F-3
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