Section 6F
Internal Modulation Oscillator
MODULATION OSCILLATOR BLOCK DIAGRAM 6F-1.
Refer to the Modulation Oscillator Block Diagram (Figure 6F-1) to identify the major
functional sections and follow the signal paths of the internal modulation oscillator.
INTERNAL MODULATION OSCILLATOR CIRCUIT DESCRIPTION 6F-2.
The modulation oscillator is configurable as either a direct digital synthesizer (DDS)
or as a pulse generator. Both functions are implemented in a custom integrated circuit
and are synthesized from the main reference frequency source of the instrument.
The Mod Oscillator PCA provides two outputs:
• An internal modulation source (INT MOD)
• A modulation output source (MOD OUT), which is available at the MODULA-
TION OUTPUT BNC connector at the front panel.
All power, data, control and clock signals are received by the Mod Oscillator PCA via
a bus connector (J1) and clock connector (J2). The Mod Oscillator PCA has two
outputs, INT MOD and MOD OUT.
Direct Digital Synthesized Wave Generator 6F-3.
The direct digital synthesizer frequency can be set from 0.1 Hz to 200 kHz with
resolution of 0.1 Hz. It is the modulation source for the internal AM, FM, øM, and
pulse functions. The amplitude of the internal modulation source (INT MOD signal) is
a leveled 1V pk which is internally routed to the Modulation Control PCA (A11). The
amplitude of the modulation output is controlled by a level DAC. The oscillator is
based on an algorithmic wave generation method, which provides a very accurate and
stable signal source of high purity and low harmonic distortion. The main function of
this system is implemented in the custom integrated circuit U1 and it uses an external
wave lookup table (U2, U3), and a 12-bit wave reconstruction DAC (U7 and U9B).
Since a discrete time sampled method is employed in generating the various waves, a
low pass antialiasing filter, R6, R7, C13, C14, L1, is required to reject the sampling
frequency, the alias signals and the out of band spurious from the output signal.
The amplitude of oscillation at the MODULATION OUTPUT connector is controlled
by a 12-bit multiplying DAC (U8, U11A). This output level can be set between 0 to 4V
pk, with 1-mV pk steps, into a 600-ohm load.
6F-1
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