Section IV Model 24CA
Paragraphs 4-5 to 4-22
4-5. Stray field leakage from the instrument has 4-15. The spectrum of harmonically related fre-
been held at a sufficiently low level so that an ex- quencies is heterodyned with the swept RF output of
ternal 20 db pad, such as the 509B Attenuator, can the buffer amplifier in a mixer stage to produce a
be used for making measurements below the range series of audio frequency beat notes , passing a zero
of the direct output of the generator. beat, as the swept frequency passes through the ref-
erence frequencies. These audio frequency beat
4-6. In shunt with the output system is a crystal notes ("birdiesTt) are fed to an audio frequency Adder
diode amplitude modulator adjusted for modulating Amplifier.
the output to 30%. A 1000 cps audio oscillator
drives the amplitude modulator. 4-16. The rectified response from the circuit under
test is fed into the Test Signal Amplifier and com-
4-7. When the CW Signal Generator is used in con- bined with the birdie markers, thus producing a
junction with the self -contained crystal-referenced "COMPOSITE SIGNAL OUTPUT" which provides
harmonic generator, the frequency Cali bration can accurate identification of the instantaneous frequency
be checked by means of phones or an oscilloscope being applied to the input of the system under test.
connected to the calibrate output jack for indicating The t'birdiet' markers are not added to the RF output
zero beats. of the 240A and therefore will have no effect on the
circuits under test.
4-8. SWEEP SIGNAL GENERATOR. 4-17. The reference frequencies ('tbirdieslt) can be
4-9. The radio frequency signals described above used for calibrating the center frequency dial of the
signal generator.
may be caused to deviate about the center frequency
by varying the magnetic flux passing through the
ferrite cores upon which are wound the five oscilla-
tor coils. The current through a coil and yoke
assembly establishes the correct operating point
for the saturable reactors. 4-1 8. PIP MARKER GENERATOR.
4-19. A unique feature of this instrument is the in-
4-10. The current waveform necessary to produce
linear deviation of frequency is generated from the clusion of two sharp, rectangular "pip" markers
output of an adjustable-frequency multivibrator cir- whose horizontal position on the oscilloscope trace
cuit covering the frequency range from 20 to 70 cps. can be independently controlled and whose position is
synchronized with the beginning of the sweep trace.
It may be synchronized to the.line frequency. A tri- This system makes it possible to accurately mark the
angular waveform of voltage appears at the front center of the birdie-type markers on a display
panel of the instrument for driving the horizontal in-
put of the oscilloscope. pattern. When the crystal reference birdie markers
are turned off, the clean-cut, pip markers will re-
4-11. The width of sweep is adjustable and is sym- main on the oscilloscope display. These pip markers
metrical about the center frequency shown on the fre- identify the center of the reference frequencies, with
quency tuning dial. A binding post on the front panel crystal accuracy, without disturbing the pass-band
permits the use of external sweeping voltages in place pattern.
of those generated internally. An automatic gain con-
trol system holds the output constant while the fre- 4-20. Either or both of the pip markers can be
quency is being swept. used as interpolation markers to identify frequencies
other than those provided by the crystal markers, if
4-12. The attenuator output system and level- a ruled grid is used on the oscilloscope screen. The
monitoring meter apply equally to the CW, AM and two markers can be displaced completely to the left-
Swept outputs. hand end of the trace, if their use is not required.
4-1 3. REFERENCE FREQUENCIES AND ADDER
STAGE.
4-21. POWER SUPPLY.
4-14. A set of reference frequencies ('birdiesTt),
which are harmonically related, is generated by a 4-22. Filament power for critical tubes is ob-
system of three separate oscillators referenced to a tained through a voltage-regulating transformer
prtz crystal. A front panel control permits the which must match the frequency of the power source:
choice of the harmonically related frequencies spaced 60 cps for the Model 240A; 50 cps for the Model
2.5 Mc, 0.5 Mc, or 0.1 Mc. The number of "birdies" 240AP. High voltage and bias supplies contain elec-
depends on the bandwidth of the signal circuit under tronic glow. discharge type voltage regulator sys-
test. tems to assure stable operation from 105-125 volts.
4 -2 |
ACEC
