Tektronix 492, 492P, Руководство по обслуживанию, Страница: 192 / 303

Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
Also, for bands 4 through 10, a diode may be connected
to each decoder output to transmit that low signal via edge
connector pin DD to the gain control transistor in the Band
Leveling circuit to increase the gain in each of those bands.
Those diodes are CR3022, CR3023, CR3024, CR3025,
CR3031, CR3027, and CR3026, and are installed during fi­
nal instrument calibration.
5 Volt Regulator Circuit
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8
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The 5 Volt Regulator circuit (U3041) supplies the required
5 volt source for use in several sections of the VR system.
This is required because of noise in the 5 volt supply.
that gain was reduced to unity, the total gain reduction is
70 dB. With further increases in input signal level, three
more gain change steps take place. The gain of the first
three stages is reduced below unity approximately 7 dB for
each stage. This reduction starts with the first stage and
proceeds to the third, to provide an additional gain reduction
of approximately 20 dB.
Thus, as the input signal increases from -9 0 dBm to
+ 10 dBm, the gain through the amplifier decreases
logarithmically so that the output signal is exactly propor­
tional to the logarithm of the input. This is accomplished
through a system of series diode limiting in each stage, with
a second set of diodes for extra limiting in each of the first
three stages. Refer to Diagram 21 while reading the
following.
LOGARITHMIC AMPLIFIER AND
DETECTOR <£>
Refer to the block diagram adjacent to Diagram 21. The
Logarithmic (Log) Amplifier and Detector accepts input sig­
nals from the VR circuits, with a dynamic range to 90 dB. It
then amplifies these signals so the output is proportional to
the logarithm of the input, and applies the signals to a linear
detector to produce the video output signal. By controlling
the compression curve characteristics, each dB of change in
the input signal level results in an equal increment of change
in the output. Thus, in the 10 dB/division mode, each divi­
sion of displacement on the screen represents 10 dB of in­
put signal level change.
Log Amplifier Circuits
The Log Amplifier circuits logarithmically amplify the in­
put signal from the VR circuits and apply the output signal
to the Detector circuit. These circuits consist of seven ac
coupled amplifier stages. Each stage has two gain values
that depend on signal amplitude. In addition, the first three
stages have an extra automatically selected gain value. The
combined circuits provide high gain for low-level signals and
low gain for high-level signals. For the output signal to be
proportional to the logarithm of the input, more gain is re­
quired for a change from —90 dBm to —89 dBm than a
change from -1 dBm to 0 dBm. Thus, for a given stage of
the seven, the gain starts at approximately 10 dB for a low-
level signal and decreases to unity as the input signal level
increases. In the first three stages, the gain becomes less
than unity as the signal amplitude further increases.
The following description of a simple three-stage log am­
plifier with one gain step in each stage is provided as an aid
to understanding the concept of a logarithmic amplifier. For
the example amplifier shown in the following three figures
and described in the text, the gain of each stage is 3.16 V
(10dB) up to an output level of 1 volt peak, then unity for
output levels greater than 1 volt peak; that is, each stage
uses one breakpoint. The breakpoint voltage is used for
ease of illustration; the actual breakpoint voltage is signifi­
cantly lower.
Figure 5-9 illustrates the amplifier and the input signal
source. For purposes of discussion, assume that the source
has a step attenuator at the output that will allow increment­
ing the input signal in 10 dB steps. Table 5-6 shows the
progression of gain reduction above 1 volt at each amplifier
stage output. Note that with each input level change of
10 dB, the output change at point 4 is 0.684 volt. The gain
curve for one stage is illustrated in Fig. 5-10. Also note,
when the level at point 1 is increased beyond 1 volt it is
beyond the logging range of the amplifier. Similarly, if the
input level is decreased 10 dB below the nominal minimum
input level, the output increment is different. A curve of the
ends of the logging range is shown in Fig. 5-11.
From the VR circuits, the signal is applied to input
preamplifier Q3105 in the Log Amplifier circuits through co­
axial connector P621. The input preamplifier provides trans­
fer from 50 Ω input to the high impedance input of the 1st
amplifier stage. The input signal is also applied to transistor
Q2105, a common-base amplifier, that acts as a buffer to
supply the 10 MHz IF signal to the rear panel connector.
Input signal levels nominally range between —90 dBm
and 0 dBm. As the signal level increases, the gain decrease
begins with the final stage and proceeds in succession back
through the remaining six stages to the first. Since each
stage produced approximately 10 dB of gain initially, and
From the input preamplifier, the signal is applied to the
first of seven cascaded amplifiers that consist of Q3100/
Q1095, Q3090/Q1080, Q3075/Q1070, Q3055/Q1050,
Q3045/Q1035, Q3030/Q1025, and Q3015/Q6010, plus the
associated circuitry. These stages are similar, except that
REV FEB 1983
5-29