HP 8559A Operation And Service Manual page 417

MODEL 8559A
A D J U S T
FIGURE &66.10dB GAIN OPERATION IN LINEAR MODE, SIMPLIFIED SCHEMATIC
collector of 424 goes to 8 VT, turning the log diodes on. In Lin mode, the LOG/LIN line is at
-
turned off and current flows through R34 (LIN) to stages 6 and 7.
Log Mode Temperature-Controlled Variable-Gain Amplifier (J)
In Lin mode, when approximately 700 mV rms
voltage at the output of stage 7 (TP5) is about 1.5 rms. With the same input in Log mode, the output at TP5 is
about 2.OV rms. To maintain an equal relationship with maximum input signal (the trace at top display), the
output in Log mode must be attenuated. This attenuation is achieved with variable gain amplifier 47, the gain
of which is determined by the ratio of its collector load to its emitter load.
In Lin mode, the LOG/LIN line is a
+
mately
or approximately 0.3. In Log mode, the LOG/LIN line is at
U2b (TPI) is about -0.45V. Diode CR29 is forward biased and exhibits an ac resistance of about 100 ohms.
This resistance is in parallel with the 100 ohms of R104 for a total of 50 ohms. Since the collector load of 4 7 is
about 50 ohms, the gain becomes 0.15 (50/316). This gain depends upon the resistance of CR29, which is
established by SLOPE adjustment R23.
Detector (K)
The detector comprises a voltage-to-current converter, a half-wave rectifier, and a low-pass filter. The output of
the variable gain amplifier is applied to 46, where voltage variations are converted to current variations.
Transistor Q5 acts as a current driver for half-wave rectifier 44, while CRl biases 4 4 just below cutoff. When
the signal is positive going, 4 4 conducts; during the negative half-cycle, Q4 is cutoff. The detector's output goes
a
to the low-pass filter, series of pi-section filters that smooth the detector's output and remove
components.
SECOND STAGE
FEEDBACK P I T H
10 dBm) is applied to the input of the Log amplifier, the
(+
8 VT, CR4 is forward biased, and the output of U2b (TP1) is approxi-
-
+
.
SERVICE
8 VT, 424 is
-
signal