Tektronix 492, 492P, Руководство по обслуживанию

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Theory of
O p e ra tio n —
492/492P Service Vol. 1 (SN B030000 &
u p )
The 1st LO feeds signal to the Mixer in one of two ways:
directly from the Power Divider, or through the Phase Gate
and Bias Return if Option 03 is installed. (The Phase Gate
and Bias Return stages incur very little loss.) The Phase
Gate couples off a small amount of signal (approximately
0 dBm) to compare with a strobe signal from the phaselock
system. The output is an error signal that is used by the
phaselock system for determining the FM tuning current for
the 1st LO. The Bias Return provides high-pass filtering for
the 1st LO and a dc return for the 1st Mixer diodes.
The current tuned 1 st LO output is mixed with the incom­
ing RF, and the IM (intermodulation) products are routed
through the Transfer Switch to the 2.072 GHz Directional
Filter. (Option 08 deletes the Transfer Switch.) This filter is
broadband, and provides a constant match to the 1st Mixer
output at all frequencies. The filter couples the 2.072 GHz IF
to the 2nd Converter through a 4.5 GHz lowpass filter and
directly couples other IM products to the Diplexer. The
lowpass filter removes odd multiples of 2.072 GHz that are
re-entrant modes of the Directional Filter. The Diplexer pro­
vides dc path for mixer bias and rejects frequencies above
829 MHz.
The single balanced 1st Mixer affords less IM products
than an unbalanced mixer, so the conversion loss is inher­
ently less. It also cancels local oscillator signal feed-through
to the RF input port.
In standard instruments, an external mixer port and
Transfer Switch are included. Option 08 deletes these fea­
tures. The external mixer feature permits an external mixer
to be connected to the instrument to serve as the 1st IF
source. This feature is primarily used for waveguide mixers.
For Option 01 instruments, a Preselector or 1.8 GHz
lowpass filter is inserted in the RF signal path. The signal
passes through a low-band/high-band switch, which selects
the Preselector or the 1.8 GHz lowpass filter. The 2 GHz
Limiter protects the 1 st mixer diodes from signals 2 GHz
and above by reflecting RF energy back to the input source.
The 1.8 GHz lowpass filter attenuates signals above
1.8 GHz to reduce spurious responses caused by RF sig­
nals above 1.8 GHz feeding through to the 1 st mixer.
The Preselector is the signal path for frequencies from
1.7 to 21 GHz. The Preselector is a tunable filter that tracks
with the 1st local oscillator. This prevents other RF signals
from feeding through to the 1st mixer and eliminates
spurious responses from external sources. From the
Preselector the signal passes through a 3 dB attenuator,
which improves the return loss of the Preselector, to the 1 st
Mixer.
RF INTERFACE CIRCUITS
Introduction
Refer to the block diagram adjacent to Diagram 27. The
RF Interface circuits receive address and instruction data
from the Microcomputer, decode it, and control the RF
Attenuator, Transfer Switch, and IF selection. The circuit
consists of the Digital control circuits, which decodes the
address and control the input data to the buffer. The RF
Interface section also includes the driver circuits, which fur­
nish the current required to drive the three functions men­
tioned at the first of this paragraph.
Digital Control
Address decoder U2045 enables the data at the input of
U3046 whenever address 4F is selected by the Micro­
computer. Table 5-1 lists the purpose of each data line from
the buffer.
Transistors Q2025 and Q3028 are enabled by a negative
pulse from the microcomputer. The two transistors raise the
Vcc of the three attenuator drivers (U3034, U3029, and
U3038) to +16 V for about 100 ms; this furnishes sufficient
voltage to energize the attenuator solenoids. Each of the
attenuator driver output lines is protected by a diode from
the inductive kick that occurs when the solenoids change
state.
Transfer Switch
Amplifier U4023, transistors Q3025 and Q3024, plus re­
lated components form the driver circuit for the Transfer
Switch. To select the external mixer, the microcomputer
sets line 05 high. The change is coupled through C4026 and
R4012, which hold U4023 at a low output state for a few
milliseconds. This lets Q3025 conduct, and the Transfer
Switch selects the external source. If the microcomputer se­
lects the internal mixer, it pulls line Q5 low, switching U4023
in the opposite direction, which causes Q3024 to conduct.
The Transfer Switch energizes in the opposite direction, and
the internal mixer is part of the circuit. Diodes CR3018 and
CR3017 protect the transistors from voltage spikes induced
by the Transfer Switch when it changes state.
Timer
M1019 is an electrochemical timer. The current through
R1015 causes the copper band to progress along the scale
that is calibrated for a duration of 5000 operating hours.
5-4
REV AUG 1981