TM 11-6625-255-14/TO 33A1-13-170-1
CHAPTER 6
FUNCTIONING OF EQUIPMENT
61. Circuitry
mum gain of the two amplifiers is 40 dB. When
NOTE
the spectrum analyzer is used for making distor-
Preference symbol followed by another
tion readings, the bridge is first switched out of
reference symbol in parentheses, indi-
the circuit. A reference reading is obtained on the
cates that the latter is used in all models
meter in the voltmeter circuit; the bridge is
other than the TS-723A/U; for example
switched into the circuit and is tuned to reject the
C18 (C38) means that C18 is used in the
fundamental frequency, thus allowing the re-
TS-723A/U, and C38 is used in all other
maining harmonics to be measured. The meter
models, unless otherwise specified.
reading obtained is the percent of distortion, or
the dB difference, between the fundamental fre-
The block diagram for Spectrum Analyzer TS-
quency and its harmonics.
723( *)/U is shown in figure 6-1. For more detailed
b. Vtvm Section. The vtvm section consists of
overall circuit information, see figures FO-2 and
a voltage divider stage V11, a two-stage voltage
amplifier V12 and V 13, and a meter rectifier cir-
major circuits: a frequency selective amplifier, a
cuit V14 (CR1 and CR2). Voltage divider stage
vacuum-tube voltmeter (vtvm) circuit, and a
V11 is basically a cathode follower tube with
power supply.
multitapped cathode resistors R58 and R88 (fig
a. Frequency Selective Amplifier Section. The
frequency selective amplifier section consists of a
cathode resistors are R92 through R98. This stage
three-stage preamplifier, a Wien bridge circuit,
provides a high input impedance for the vtvm sec-
and a bridge amplifier. The preamplifier stage
tion. The two-stage voltage amplifier amplifies
amplifies the incoming signals and passes them to
the output of the voltage divider stage and applies
the bridge. The bridge may be switched in or out
this amplified ac voltage to the meter rectifier cir-
of the circuit as required. When switched into the
cuit. A negative feedback path (fig. 6-10) for re-
circuit, it is tuned to reject the fundamental fre-
turning output voltage to the input of amplifier
quency of the incoming signal and allow all of the
V12 is provided by the meter rectifier circuit. This
remaining harmonics to pass through to the
negative feedback voltage controls the gain of the
bridge amplifier. When it is switched out of the
amplifier. Tube V14 (CR1 and CR2) functions as a
circuit, all frequencies in the incoming signal are
Full-wave rectifier and converts the ac output
passed from the preamplifier directly to the bridge
voltage of the two-stage voltage amplifier to the
amplifier. The bridge amplifier amplifies the out-
dc voltage required for actuating the meter. The
put of the bridge when the bridge is switched into
meter is a milliammeter calibrated in rms volts
the circuit, and acts as the final two stages of am-
and decibels. The vtvm circuit may be used
plification when the bridge is switched out of the
independently of the frequency selective ampli-
circuit. The bridge is responsive to input signals
fier. When used in this way, the input signals to
which have fundamental frequencies from 20 to
b e measured are connected to the METER
20,000 Hz. When the spectrum analyzer is-used as
binding posts and fed directly into the voltage
a vtvm, the bridge is switched out of the circuit
divider circuit.
and the frequency selective amplifier may be used
as a 20 or 40 dB gain amplifier for the vtvm cir-
c. Power Supply. The power supply circuit
supplies regulated and unregulated B+ voltages to
cuit. The gain of the amplifier is controlled by the
the other circuits in the spectrum analyzer. In-
amount of negative feedback applied to the input
of the preamplifier from the output of cathode
coming ac line voltage passes through power
follower V6 in the bridge amplifier. The negative
transformer T1 (fig. 6-12). Stepped up ac volt-
ages obtained from the high-voltage secondary
feedback is controlled by an adjustable feedback
network. When the function switch is set to SET
windings of T1 is rectified in full-wave rectifier
LEVEL, the negative feedback is attenuated
V7. Unregulated B+ voltage (after the filter) from
V7 is supplied to the vtvm circuit and to series
enough to establish 20-dB maximum gain in the
regulator V8. Tube V8 supplies regulated B+ to
combination of the preamplifier and bridge ampli-
fier. When the switch is set to NOISE, the maxi-
the amplifier circuits. Dc error amplifier circuit V9
6-1
