Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
60 kHz difference is coupled across T2030 to a differential
pair Q1020/Q1030, then amplified to TTL levels by amplifier
Q1040 and applied to the clock input of flip-flop U1050B,
part of the Phase/Frequency detector.
The Phase/Frequency detector consists of flip-flops
U1050A and U1050B, NAND gate U2050B, and inverter
U2050A. Now, if the loop had been locked, the two flip-flop
clock input signals would have been edge-coincident. Pin 4
and 5 inputs of U2050B would have moved high and after
the signal at TP1058 goes low, the NAND gate would have
reset both flip-flops. The result would have been a series of
pulses of equal amplitude and width from each of the flip-
flops. This would cause equal voltages to be applied to the
Error Amplifier, and the Controlled Oscillator frequency
would shift.
It is assumed, however, that the -r- N signal is 50 kHz
and the difference frequency from the collector of Q1040 is
60 kHz, for this description. Thus, the output of Q1040 is
leading the -r N signal. U1050B sets first placing a high at
the inverting input of U3075 which pulls the output of U3075
low until U1050A sets. A short time later, U2050B resets
both flip-flops and U3075 will switch back to balance until
the next correction cycle. This continues to occur until the
two signals applied to the Phase/Frequency Detector are
edge-coincident.
The correction voltage in this example from U3075 is ap
plied to the frequency-determining components of the Con
trolled Oscillator, and its frequency shifts downward. The
frequency of the oscillator will continue to decrease until the
output of U3075 is stable.
The Error Amplifier, which is part of the Error Amplifier
assembly, is described here because it is an integral part of
the inner loop. The stage consists of differential amplifier
U3075 and surrounding components. As the signals driving
the amplifier continue toward one direction, U3075 contin
ues to drive the oscillator down in frequency. The circuit
consisting of VR2065, CR3069, R2067, and C2072 clamps
the output to prevent the varactor diode from becoming for
ward biased and stopping the oscillator.
Strobe Driver Circuit
The Strobe Driver circuit consists of
h
- 5 counter U1022,
bandpass filter FL2064, source follower Q2091, and AND
gate U1091A and U1091B.
The Controlled Oscillator signal is applied to the clock
input of counter U
1 0 2 2
which is wired to divide the input
signal by five. The STROBE ENABLE 1 line from the Error
Amplifier permits the counter to operate when the line is low
and is the means by which the microcomputer can shut off
or turn on the strobe pulses. The output of the counter,
which ranges from 5.006 MHz to 5.019 MHz, is coupled
through an impedance matching network consisting of
C2030, L1031, C2033, and C1032. This circuit raises the
line impedance to about 8200 Ω. The signal is then passed
through monolithic bandpass filter FL2064, through another
impedance matching network, to the gate of Q2091. The
signal is coupled from the source of Q2091 to the inputs of
U1091A and U1091B, both of which are configured as buff
ers. U1091B drives the Phase Gate circuitry, and U1091A is
reserved for future applications. Capacitors C1032 and
C2105 are selected to provide maximum signal amplitude at
TP2087.
DIGITAL CONTROL <9>
The Digital Control section of the 492/492P provides the
operator/492 and digital controller/492 interfaces. It trans
lates changes in front-panel controls and instructions re
ceived via the accessories interface or GPIB interface (492P
only) into codes that control the instrument via the instru
ment bus.
The Digital Control section simplifies operating and pro
gramming the 492 and 492P. Unless overridden by the oper
ator, the microcomputer automatically selects secondary
parameters. Some examples are: when the operator selects
span, the microcomputer chooses an
appropriate
bandwidth; when the operator changes the reference level,
the microcomputer trades off input attenuation and IF gain.
In the 492P, the microcomputer can handle some oper
ations automatically. Some examples are: the micro
computer can set PEAKING for best response; the
microcomputer can search digital storage for signals and
change FREQUENCY and REFERENCE LEVEL to zoom in
on signals it finds.
The digital control operating program is defined by the
meaning of the controls and commands given in the operat
ing and programming manuals and is not further defined
here. The following description focuses on the hardware.
The following circuits make up the digital control section:
1
) microcomputer, including processor and memory
boards;
2
) addressable registers on the instrument bus;
3) front panel Accessories interface;
4) GPIB interface (492P only).
REV AUG 1981
5-77
