Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
The calibrator output amplifier is a differential amplifier
(Q2031 and Q1031) that is overdriven. With low levels of
drive, this amplifier would operate as a small-signal amplifi
er. However, with the higher positive and negative levels
from reference amplifier Q2016, the transistors are either
driven hard or are not conducting at all. Since the transis
tors are overdriven, the current in the output side (Q1031) is
the dc bias current when that side is conducting. Changing
the bias current will therefore change the output voltage.
Thus, the output is determined by internal dc levels, not in
put signal levels. Potentiometer R1045 provides for adjust
ment of that quiescent current.
Calibration Output Amplifier
The output frequency is stable and rich in harmonics.
Thus, it provides a useful signal comb of 100 MHz markers
to approximately 2 GHz. At 100 MHz, the output level is set
by R1045 for -2 0 dBm which is applied to the front panel
CAL OUT connector through coaxial connector J1015.
Φ
IF SECTION
The IF section receives the 10 MHz IF signal from the 3rd
Converter, establishes the system resolution through
selective filtering, levels the gain for all bands, and
logarithmically amplifies and detects the signal to produce
the video output to the Display section.
System resolution is selectable, under microcomputer
control, among five bandwidths: 1 MHz, 100 kHz, 10 kHz,
1 kHz, and 100 Hz. (Some 30 Hz circuits are included for
future use.) This selection is done in the Variable Resolution
circuit block by two sets of filters. Bandpass filters are also
included at the circuits input and output.
Significant gain is provided in the resolution circuit block
by several stages of amplification. Also, the capability to
add other gain steps under microcomputer control is pro
vided by switching attenuators in or out of the signal path.
These attenuators, by being switched in combination, pro
vide for 10, 20, 30, or 40 dB of additional gain.
Leveling to compensate for instrument front-end losses
is also included in the resolution circuit block. Front-end
losses occur primarily in the higher frequency bands; there
fore, most band leveling amplification is required in those
bands.
In order that each division of signal change on the crt
screen be equal to that for each other division and be
equvalent to a similar signal level change in dB, a logarith
mic amplification of the signal is required. This is done by a
seven stage amplifier that produces an output that is pro
portional to the logarithm of the input. Thus, the screen dis
placement can be selectable as to amount of change per
divisions, and can be proportional to the input level change.
For instance, in the 10 dB per division mode, each division
of displacement in the screen represents a signal level
change of 10 dB regardless of whether it is at the top or
bottom of the screen.
Following the logarithmic amplifier, an area detector pro
duces a positive-going pulse output that is applied to the
display section as the VIDEO signal.
V a r i a b l e R e s o l u t i o n S e c t i o n
The Variable Resolution (VR) circuits provide selection of
resolution bandwidth under microcomputer control, and ap
proximately 35 dB of system gain. It consists of two sets of
filters and various gain leveling stages. Since the input to
the VR circuits is nominally at —35 dBm and the Log Ampli
fier input must be 0 dBm for full screen, the VR circuits must
provide the gain difference. Also, additional gain (up to
40 dB) is required for operation in the 2 dB/DIV or the linear
mode plus compensation for variations in front end losses.
Physically, the VR section consists of two sub-assem
blies that plug onto the analyzer mother board. The input
circuits are in one sub-assembly; the output section and
digital interface are in the other. Each of the sub-assemblies
consists of boards that plug onto a four-layer mother board
with a ground plane on both outside layers. Only power sup
ply and control voltages travel through the mother board. All
signal interconnection is via coaxial cable.
Circuits for the VR section are contained on three dia
grams: 18, 19, and 20. The following paragraphs describe
the circuits.
Input Circuit \ I 8 /
The VR Input circuit receives the —35 dBm 10 MHz sig
nal from the 3rd Mixer through J693. This signal is applied
to a two-pole, 1.2 MHz bandpass filter that augments the
1 MHz fitter that precedes the 3rd Mixer and provides initial
selectivity. This 1.2 MHz filter includes C1037 and C1031
and all of the components between. Filter tuning is provided
by variable capacitors C1033 and C1026.
REV AUG 1981
5-23
