Tektronix 1L40, Инструкция по эксплуатации

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SECTION 3
CIRCUIT DESCRIPTION
Type 1L40
Change information, if any, affecting this section will be found at the
rear of the manual.
Introduction
The Type 1L40 Spectrum Analyzer is a swept IF spectrum
analyzer, covering the frequency range from 1.5 GHz to
40 GHz by means of selectable input mixer assemblies.
This section describes the functional operation of each
major circuit and its relation to the unit operation. A block
diagram is first described to show the relationship of the
circuits, then each circuit within the functional block is
described in detail.
BLOCK DIAGRAM DESCRIPTION
A functional block diagram of the Type 1 L40 is illustrated
in Fig. 3-1 and in the Diagrams section of the manual.
The RF input section contains a local oscillator, a diplexer
and a mixer assembly. The mixer assembly is selected
according to the desired input frequency range. For the
frequency range 1.5 GHz to 12.4 GHz a coaxial mixer
assembly is plugged into the input receptacle and the signal
source is connected to the input of the mixer. This is
replaced with a W aveguide M ixer Adapter, a two-foot coax­
ial cable and one of three W aveguide Mixers for the fre­
quency range 12.5 GHz to 40 GHz. The diplexer is a direc­
tional coupler which couples the local oscillator signal to the
mixer assembly and the mixer output frequency (heterodyne
frequency between the local oscillator and the incoming sig­
nal frequencies) to the 280 MHz low pass filter.
The 280 MHz low-pass filter and the 150 MHz-250 MHz
bandpass filter reduce the spurious signals outside the 100
MHz IF bandpass and in most cases prevent them from
reaching the 2nd mixer stage.
The wide band IF is converted to 75 MHz in the 2nd mixer
by mixing the IF with the output from a swept oscillator.
The swept oscillator is swept from a center frequency of
275 MHz by the sawtooth voltage that supplies the horizon­
tal sweep to the oscilloscope CRT beam. The width or
frequency dispersion the oscillator is swept depends on
the setting of the DISPERSION selector. This can be varied
from 0 dispersion to more than 100 MHz. The 75 MHz
IF is the difference frequency between the swept oscillator
frequency (at a given instant of time) and a frequency within
the 150 MHz to 250 MHz wide band IF. For example: When
the swept oscillator frequency is 225 MHz, the 75 MHz IF
represents the conversion between 225 MHz and 150 MHz.
The swept oscillator frequency of 225 MHz occurs when the
CRT beam is at the sweep start. As the sweep travels across
the graticule, the swept oscillator frequency increases and the
75 MHz IF represents the conversion between the swept oscil­
lator frequency and a higher frequency portion of the wide
band IF response.
Calibrated attenfuation (in 1 dB steps to 51 dB) is pro­
vided by the IF attentuator. The signal output from the
attenuator is then amplified and applied to the 3rd mixer
stage, where it is mixed with 70 MHz and converted to a
3rd IF of 5 MHz. The output of the third mixer is applied
to a 5 MHz, narrow bandwidth IF amplifier. The output
of the 5 MHz IF am plifier is applied to the variable resolu­
tion circuit whose bandwidth can be varied from less than
1 kHz to more than 100 kHz.
The signal output from the resolution amplifier is ampli­
fied, detected and applied through a logarithmic, linear,
or square law voltage divider circuit to the vertical
am plifier of the indicator oscilloscope.
The phase lock circuit locks the local oscillator to either
an internal 1 MHz reference oscillator or an externally
applied reference frequency. This stabilizes the oscillator
and reduces incidental frequency modulation.
CIRCUIT DESCRIPTION
RF Section
The RF section (See RF section schematic) contains the
mixer assembly, the local oscillator and phase lock circuit,
a diplexer, a resistive coupler, a mixer peaking circuit,
and a 280 MHz low pass filter assembly.
The local oscillator for the Type 1L40 consists of a triode
with its plate and cathode circuits connected to tunable
resonant cavities. Frequency of oscillation can be tuned
from 1.7 GHz to 4.2 GHz by means of the RF Center Freq
control. The oscillator frequency is changed by varying
the length of the cathode and plate cavities. Harmonics
through the 10th of this fundamental frequency range are
used to heterodyne with the incoming RF to provide an
overall frequency range from 1.5 GHz to 40 GHz.
The 10 volt filament or heater supply for the oscillator
tube (V42) is provided through the series-parallel current
paths consisting of V620 filament and two dropping resis­
tors, R47 and R49.
Two probes in the oscillator chamber couple the oscil­
lator signal to the phase lock circuit and through a hybrid
directional coupler to the mixer assembly. The directional
coupler, or diplexer, see Fig. 3-2, couples the LO signal
to the mixer port (OUT) and the mixer output jack through
to the IF port. This permits the use of external mixer
assemblies.
Four mixer assemblies are required to cover the frequency
range of the unit. A coaxial mixer is used for the 1.5 GHz
to 12.4 GHz frequency range. Three waveguides mixers with
frequency ranges of 12.4 GHz to 18 GHz, 18 GHz to 26.5
GHz and 26.5 GHz to 40 GHz, are used to cover the higher
frequencies.
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