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5-30.
The SYNTH RF signal is amplified and leveled in the RF Input module (A 11 ). The signal level at the
output of the wideband amp is detected and compared to the AM MOD & DC REF signal from the front panel
level control.
If
there is a difference between the two signal levels, the ALC amp provides an error voltage. The
error voltage controls the attenuation of the Voltage Controlled Attenuator (VCA) in the direction that will
make the detected RF output equal to the AM MOD
&
DC REF signal. There are two possible VCA's for the
output leveling. The VCA within A 11 is used for frequencies from 1 MHz to 1000 MHz. For frequencies below 1
MHz, the VCA on A 11 is set to minimum attenuation and the VCA on the RF Synthesizer module is used for
leveling. Amplitude modulation is incorporated by suming the modulation signal with the DC reference signal
to force the leveling loop to vary the output level in proportion to the modulating signal. The signal from the RF
level detector (CARRIER+ MOD LVL) is used by the processor for the determination of f1F output level and the
percent AM. The leveled output range of the Wideband Amp is from -3 dBm to+ 13 dBm (0.16 to 1.0 Vrms)
5-31.
The leveled output from the Wideband Amplifier is applied to the Generate/Monitor (T/R) switch. For
AM, FM, and CW signals the switch connects the amplifier output to the Step Attenuator. For Double Sideband
Suppressed Carrier (DSBSC) the T/R switch is in the "R" position where the amplifier output is connected to
the local oscillator port on the receive mixer and the attenuator is connected to the RF port. The DSBSC MOD
signal is then used to drive the IF port of the mixer giving a DSBSC signal at the RF port and thus at the Step
Attenuator.
5-32.
Coarse level control in 10 dB increments is provided by the Step Attenuator. The total range of the
attenuator is from 0 dB to 130 dB attenuation. For the basic R2001 B the Step Attenuator is controlled directly
by a shaft to the front panel knob. With the IEEE control option the Step Attenuator is electrically
programmable and controlled by the processor. The front panel knob in this case is connected only to a rotary
switch which directs the processor in setting the attenuation level. Under IEEE control, commands via the IEEE
bus determine the attenuator setting. (See section 21.)
5-33.
The RF signal from the Step Attenuator passes through the input protection circuitry to the RF Output
jack. A level detector on the RF Output jack monitors the power level at the jack. If power in excess of200 mW is
applied to the Output jack, the protection circuit will activate and switch the RF Output jack to the internal 50
ohm load. This action protects the Wideband Amp and Step Attenuator against burnout. A signal line from the
protection network signals the processor that the system is in the protected mode. The processor in turn
activates the CRT and alarm warnings.
5-34.
Power Meter
5-35.
Input power measurements are made with the RF Input terminated into an internal 50 ohm load. This
termination is the same one used for the protect mode when in the generate or monitor functions. A block
diagram of the power meter is shown in figure 5-5.
5-36.
For the power meter mode the processor sets the WATT METER ENABLE line to cause the RF input
jack to be switched to the 50 ohm power termination. For modes other than the power meter, an Input Detector
on the RF Input jack detects when the input power has exceeded 200 mW and then switches the input to the
load.
5-37.
The switch is a single pole double throw configuration so that when switched to the RF load the path to
the Step Attenuator and Converter is open circuited. However, leakage across the open switch provides
sufficient signal for operation of the normal monitor functions.
5-13
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