Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
precise current that ranges from 1 mA at +10 V to 0 current
at -1 0 V for application to the normalizer IC to generate
the baseline perturbation signal. Actual signal scaling is
done by the U3045A/Q3038 current driver. The output sig
nal is applied to the normalizer SWP IN input, pin 5. The
second
current driver,
U3045B/Q3038, generates a 2 mA
reference current for the normalizer. Horizontal Freq adjust
ment R1069 in the input translation circuits allows for shift
ing the 19 evenly spaced points up or down in frequency for
compensation flexibility.
Normalizer IC U2039 operates as a shaper and contains
19 bi-polar transistors that turn on then off in a sequence as
the current input to pin 5 decreases from 1 mA to 0 mA. The
collector of each of these IC transistors is connected to a
potentiometer that allows for output
trimming
as shown on
Diagram 23. Potentiometer R1061 is active with no current;
R1013 is active at 1 mA. The trimming operation will be de
scribed later in these paragraphs.
From the normalizer, the output is applied through a
jumper switch to buffer amplifier U2055B, which has a gain
of five, then to offset amplifier U2055A. This amplifier has a
gain of two, but its primary purpose is to offset the 0 to
+ 5 V (normal); 0 to - 5 V (invert) buffer output to the levels
required by the Log Amp
circuits. The
range required
by the
Log Amp is - 5 V (min). The output voltage is a series of
linear interpolations of the voltage between adjacent trim
ming resistors at the outputs of the normalizer. Compensa
tion adjustment R1065 allows for setting correct
interpolation.
Jumper switch P2060 selects the input side of buffer am
plifier U2055B. This provides the means to invert the buffer
output. During calibration, the procedure is as follows: Lev-
eler Disable
jumper
P3035 is removed, a test signal with a
normal uncorrected baseline waveform is displayed and
stored, the Mode jumper (P2060) is removed and reinstalled
in the invert position, the disabled jumper is reinstalled, the
stored waveform is displayed with the normalizer output
waveform superimposed, and the 19 potentiometers are
adjusted for exact compensation. The Mode jumper is then
removed and reinstalled in the normal mode position. Thus,
the output on the Video 1 line is opposite in polarity and
equal in amplitude to the undesired variations.
As stated previously, significant compensation is re
quired only on band 4. Selection of band 4 is indicated by
data bit 0 switching to a 1 (see the leveling table at the top
right corner of Diagram 23). When DBO is a 1, pins 3 and 2
of switch U2015 are connected and the output from the
offset amplifier (U2055A) is supplied out as the VIDEO 1
signal at edge connector pin 49.
Minor compensation is required for band 1 only when
preselection is specified (Option 01). With Option 01, a mi
nor slope caused by the 1.8 GHz lowpass filter and 2 GHz
limiter is corrected by adding two resistors in series between
the PRESELECTOR DRIVE signal input and the VIDEO 1
output signal. These two resistors, R4023 and R3026 (note
that R4023 is selected at factory calibration), form a voltage
divider with R4046 and are inserted by connecting pins 6
and 7 of switch U3025. This switch is controlled by inverter
Q4025, which is, in turn, activated by data bit 6 being a 0.
As shown in the VIDEO BLANKING table on Diagram 23,
data bit 6 is a 1 except when Option 01 is selected.
Video Filter Circuits
Video filtering provides selection of one of six
bandwidths,
under
the
control
of
the
analyzer
microcomputer. As shown in the VIDEO FILTER table on
Diagram 23, data bits 1 through 4 select any of six
bandwidths: 30 kHz, 3 kHz, 300 Hz, 30 Hz, 3 Hz, and
0.3 Hz. Either wide or narrow-band filtering is selected at
the front panel (30 kHz, 3 kHz, and 300 Hz are defined as
wide-band; 30 Hz, 3 Hz, and 0.3 Hz are defined as narrow
band), and the microcomputer makes the selection, based
on such factors as sweep rate and total dispersion. With no
video filtering (all data bits equal 0), the video system
bandwidth is 500 kHz, as determined by circuits that follow
the Video Processor, which has an internal bandwidth of
3 MHz.
Two signal inputs can be applied to the Video Filter cir
cuits: EXT VIDEO and INTL VIDEO. The EXT VIDEO signal,
from the rear panel auxiliary connector, is applied to pin 15
of switch U3063 through edge connector pin 53. The INTL
VIDEO signal from the Video Amplifier circuits (via the front
panel LOG CAL control) is applied to pin 2 of switch U3063
through edge connector pin 51. Note that the two left sec
tions of switch U3063 are normally held energized (pins 2
and 3 connected, pins 15 and 14 disconnected) by the +5 V
supply through resistor R3064. If the EXT VIDEO SELECT
line (also from the rear panel auxiliary connector through
edge connector pin 55) is grounded, those switch sections
are de-energized and the External Video signal is applied
through, or around, the filter to become the VIDEO FILTER
OUT signal at edge connector pin 57. This is shown in the
simplified schematic diagram of Fig. 5-14.
As shown in the figure, when no filtering is selected (all
data bits equal 0), either the internal or external signal is
applied around the filter because the two right sections of
switch U3063 are not energized by data bit 1. When data bit
1 is high (1), filtering of some value will be selected by bits 2,
3, and 4, which control three sections of switch U2015 to
add or delete filter time constant.
5-36
REV FEB 1983
