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8922A
-004 Option
Logarithmic Analog Output
604-1.
INTRODUCT ION
604-2. The Logarithmic Analog Output Option
provides a non-isolated output voltage which varies
continuously as the logarithm of therms input. Scaling is:
OV de output corresponds to 0 dB which is 200 µV rms
input to the 8922A while 13. l V de output = 131 dB =
700V rms input. A continuous frequency response of
circuits with a wide dynamic output is easily plotted on an
XY recorder using this option. The option's output is
non-isolated.
604-3. SPECIFICAT IONS
604-4. Specifications for the Logarithmic Analog
Output Option are given in Section l of this manual.
604-5.
604-6.
604-1.
1.
INSTALLATI ON
Install the option as follows, referring to Figure
Remove the top cover (see Access Procedures).
2. Remove the plate located at the top of the
8922A's rear panel.
3. Install the banana jack plate with the red
banana jack to the right (when viewing the 8922A
,from the rear).
4. Secure the Logarithmic Analog Output
Assembly to the top of the transformer bracket (see
Figure 604-1) using the two screws provided.
5. Plug P501into1501 (located on the Main PCB
Assembly).
6. Solder the Logarithmic Analog Output
Assembly's red output lead to the red banana jack,
and the black lead to the black banana jack.
7. Verify operation using the calibration
procedure presented in Section 4 of this manual.
604-5.
OPERATION
604-6. Once installed, the Logarithmic Analog Output
Option requires no operator attention other than
ensuring that no voltage is ever applied to the option's
output banana jacks.
604-7.
THEORY OF OPERATION
604-8. The Logarithmic Analog Output Option,
illustrated in Figure 604-2, utilizes the logarithmic
characteristics of a P-N junction to develop an output
proportional to the logarithm of the de input from the
thermal sensor.
604-9. The de output voltage of the thermal sensor
develops a collector current in one-half of a dual
transistor. The resulting emitter base voltage is compared
to the reference Vbe of the second half and scaled up
accordingly. This voltage in turn develops a current which
is summed with range information to produce the
logarithmic output.
604-10. The output of the sensor covers one decade (.I
to IV) in any one range. Scaling is such that one decade
corresponds to 2V or 20 dB (.IV= l dB) at the output. "O"
dB corresponds to 200 µ V and each range increase
produces an additional 2V at the output. Transients
during range changes are eliminated by a sample and hold
circuit.
604-1