Fluke 9640A Series, Инструкция по эксплуатации

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9640A
Instruction Manual
6-6
Frequency Modulation
Frequency modulated outputs are derived from a frequency modulated 100 MHz to
250 MHz carrier signal which is generated on the A1 Synthesizer PCA by a high
frequency DDS circuit. The modulating waveform is generated internally in the digital
domain, but a dedicated A-D converter is included to allow the instrument to accept an
external analogue modulation source.
For output frequencies greater than 125 MHz, a combination of frequency translation and
phase lock techniques are used to convert this carrier to frequencies in the range 1 GHz to
4.024GHz. This signal is then fed to the A2 RF Output Assembly where carrier
frequencies down to 125 MHz are derived by successive binary division. For output
frequencies < 125 MHz and deviation values < 0.12 % of the carrier frequency the output
is derived by further binary division of this signal.
For output frequencies < 125 MHz and deviation values > 0.12 % of the carrier frequency
the DDS signal is applied directly to the A2 RF Output PCA, where carrier frequencies
down to 9 MHz are derived by binary division.
Amplitude Modulation
For carrier frequencies > 125 MHz amplitude modulation is achieved by applying a
modulation component to the control input signals of the voltage-controlled attenuator
circuits used to control carrier amplitude. At these frequencies, feedback from an output
envelope detector is used to maintain modulation accuracy and correct for non-linearities
in the voltage controlled attenuator circuits. To achieve the required modulation index
range in this mode, the signal is also pre-modulated by an additional voltage controlled
attenuator stage in each of the three high frequency signal paths.
For carrier frequencies < 125 MHz amplitude modulation is achieved by applying the
modulating signal to an analogue multiplier which is switched into the carrier signal
forward path. In this mode the multiplier is also used to control the output signal level.
The modulating waveform is generated by a 32-bit numerically controlled oscillator
(NCO) which feeds a wave table, but a dedicated A-D converter is included to allow the
instrument to accept an external analogue modulation source.
Instrument Control
Overall control of instrument functions is provided by the A3 Digital PCA which accepts
data from the A6 Front Panel Assembly user interface. The A3 Digital PCA responds by
setting control bits on the A1 Synthesizer PCA and the A2 RF Output PCA via an 8-bit
address/write bus. Control bits for the A9 Leveling Head are also set and relayed by the
A2 RF Output PCA via a serial interface.
An 8-bit read bus allows the A3 Digital PCA to receive self-test and status data from the
A1 Synthesizer PCA and A2 RF Output PCA, as well as A9 Leveling Head-specific
calibration data stored within an EEPROM device in the A9 Leveling Head. The
A3 Digital PCA also handles external data communication via the GPIB and RS232
ports.
Operating voltages for the A3 Digital PCA are all derived from the A4 Power Supply
PCA and are ground (Earth) referenced. Optical-isolators on the A3 Digital PCA allow
A1 Synthesizer PCA and A2 RF Output PCA to exchange floating data with the
A3 Digital PCA.