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Figure 5-10.
Digital Voltmeter (DVM) Block Diagram
5-73.
Switching lor the DVM input is contained on the Scope/ DVM Control module (A3). One often internal
measurement points may be selected lor measurement The switching action is controlled by the processor
and is performed as required to obtain the information on the CRT. To keep the CRT information current, each
of the required measurements are made in sequence at an approximate rate of thirty per second. The net effect
is a multiplexing of the voltage information to the processor.
5-74.
Two modulation signals (MOD CAL AUDIO and CARRIER+ MOD LVL) and a demodulated signal
(DEMOD CAL AUDIO) are made available to the peak detectors. Positive and negative peak determination of
the selected signal enables the processor to determine the level of modulation.
5-75.
A Lowpass Filter (LPFL) removes the DC component from the CARRIER+ MOD LVL signal so that the
generate RF output level can be determined. Refer to paragraph 5-30.
5-20
5-76.
The RF INPUT POWER and OVERTEMP signal lines from the RF Input module provide the processor
inputs for the internal wattmeter. (Paragraph 5-38). External wattmeter element inputs (EXT FWD PWR and
EXT RFL PWR) from the front panel jack provide the information for the external wattmeter display.
5-77.
A signal line from the DC input jack on the rear panel (BATT VOLT) is brought to the processor for
battery voltage determination. The voltage is attenuated by a factor of 10 to stay with the 10 volt maximum
input to the select switch. The processor uses the battery voltage measurement to warn the operator when the
battery is near it's discharged state.
5-78.
Sin ad determination utilizes the two remaining inputs to the select switch. For a discussion on the sinad
function see para 5-96.
5-79.
The selected internal measurement signal is then passed through a range attenuator. Signals from the
Select Switch have a 0 to+ 10 volt range while the DVM input has a 1 volt maximum input requirement. The
processor automatically determines and sets the correct range on the attenuator so that the input level to the
DVM is maintained at less than 1 volt For levels from the select switch less than 1 volt, the attenuator is ranged
to the unity gain position for maximum measurement resolution.
5-80.
A select switch following the internal range attenuator gates either the internal measurement points or
the external 'input to the DVM circuitry. External DVM inputs are applied through the front panel jack to the
Front Panel! nterface module (A 12). On the Interface module, a processor controlled switch selects between a
direct coupled or a capacitively coupled path lor DC and AC measurements respectively. A range attenuator
follows the ACIOC switch to provide processor controlled autoranging over a four decade range. Input
voltages from 1 millivolt to 300 volt can be handled through the DVM Input
5-81.
For DC measurements a lowpass filter (LPFL) removes AC signal components. The filter provides
approximately 25 dB rejection at 50 Hz so that accurate DC measurements can be made with superimposed AC
line ripple. When the AC measurement mode is selected the LPFL is reprogrammed for less than 0.5 dB
rejection at 10 kHz.
5-82.
Positive and negative DVM input levels are full-wave rectified by the Absolute Value circuit on the
Processor 1/0 module (A7). The outputs of the Absolute Value circuit provide a positive voltage level equal to
the magnitude of the input voltage and a SIGN BIT indicating the polarity of the input signal. For AC
measurements a lowpass filter is switched into the Absolute Value circuit to filter the rectified AC input lor it's
average level. The processor then multiplies by 1.11 to obtain the RMS value.
5-83.
An analog to digital converter (A/D) converts the magnitude voltage level into a 1 O-bit digital word. This
digital word when combined with the SIGN BIT is a binary representation of the input voltage level. The
peripheral interface adapter transfers the information to the processor.
5-84. Oscilloscope
5-85.
Three basic functions are provided for by the system oscilloscope. The alphanumeric and modulation
displays provide operating mode and control information lor the system. The external oscilloscope feature
augments the total system as a general purpose test instrument A block diagram of the oscilloscope function
is shown in figure 5-11.
5-86.
Drive signals lor the CRT are provided by circuits on the Scope Amplifier module (A2). Horizontal and
vertical signals are amplified by their respective amplifiers from 0.5 volt/division input levels to the levels
required, on the deflection plates. A Z-Axis Modulator circuit controls the cathode to grid bias voltage on the
CRT to effect intensity control.
5-21
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