Tektronix 492, 492P, Руководство по обслуживанию, Страница: 169 / 303

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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
The + 15V1 voltage provides operating bias for the oscil­
lator. The supply is protected by VR1010, C1016, and
R1011. The second supply, -+15V2, is for future applica­
tions. CR1018 and CR1019 stop transient voltages from
entering the tune voltage coils. It also protects the driving
circuits from the transients induced when degaussing.
Relay K1015 is closed when the FM Coil is used to tune
the oscillator. To prevent the tune volts coil from moving the
oscillator frequency while the FM Coil is in operation, C1012
and C1014 are connected across the tune coil. The heater
keeps the YIG sphere at a constant temperature for best
stability.
Transfer Switch
The Transfer Switch is a three-port coaxial switch that
permits application of 1st IF signals from inside or outside
the analyzer. This feature is primarily used for by-passing
the 1st Converter circuitry. The function is controlled by cir­
cuitry on the RF Interface board. It is automatically actuated
when waveguide bands are selected, or the front-panel EXT
MIXER push button is pressed.
1.8 GHz lowpass filter, a 1.7— 18 GHz filter, and a 3dB
attenuator.
Coaxial relays S10 and S11 switch the stage input and
output to select either the lowpass filter and Limiter or the
Preselector and 3 dB attenuator. The relay coils are driven
by cirduitry on the Preselector Driver board. The lowpass
filter path is used only on band 1; the Preselector operates
on all the other bands.
The 2 GHz Limiter operates from 100 kHz to 2 GHz. It
has a linear two-port transfer characteristic of unity (minus
1 dB) until the input exceeds +5 dBm. Above this point, the
internal detector diodes conduct, reflecting part of the RF
input energy back to the source. As the input level rises, the
limiter reflects more signal, thus limiting the amount that can
pass through.
The 1.8 GHz lowpass filter strips the incoming signal of
any frequencies above 1.8 GHz and passes the signal below
1.8 GHz on to the output segment of the selector switch
(S11).
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Directional Filter
The Directional Filter (FL16) couples the 2072 MHz signal
to the 2nd Converter via the lowpass and bandpass filters.
As intermodulation products (IM) flow through FL16, they
induce a selected current into a one-wavelength distributed
ring, which couples the 2072 MHz IF signal out to FL11, the
lowpass filter. The remainder of the IM products pass on
through, since the ring is excited only with 2072 MHz sig­
nals. The bandwidth of this unit is approximately 45 MHz.
The unfiltered signals are passed on to the Diplexer.
High IF Filters
The 2072 MHz signal from the Directional Filter is passed
through FL11, a lowpass filter that rejects all signals above
4.5 MHz. The second filter, FL14, rejects intermodulation
products both above and below 2072 MHz.
Diplexer and Filter
The Diplexer filters the 829 MHz IF signal from the mixer
output and sends it to the 2nd Converter through FL15. The
Diplexer also provides a good match to the 1 st Mixer IF port
at frequencies above 1 GHz. This match is important for the
overall flatness and frequency response of the analyzer.
Preselector
The Preselector, which is included when Option 01 is in­
stalled, consists of two selector switches, a 2 GHz limiter, a
5-6
The Preselector is a 1.7— 18 GHz YIG Filter that pro­
vides high selectivity and image-frequency rejection. Tuning
current, which is near 500 mA at 21 GHz, is provided by the
Preselector Driver circuits. The Preselector operates on
bands 2, 3, 4, and 5. The signal from the Preselector passes
through a 3 dB attenuator to the output section of the Filter
Selector switch. The attenuator isolates the Preselector,
which is sensitive to loading on its output.
Φ
2ND CONVERTER
CIRCUITS
Two 2nd Converter systems are used in the 492/492P
Spectrum Analyzer. One converts 2072 MHz to 110 MHz;
the other converts 829 MHz to 110 MHz. Only one convert­
er is operational at any time, and is selected as a function of
the measurement band being used. The selection of the IF
for each band is shown in Table 5-2 along with the center
frequency range and the local oscillator frequency range.
Note that this table includes Option 01 characteristics.
Two IF's are used by the 2nd Converter for the following
reasons:
REV AUG 1981