Simpson 479 RF Generator Instruction Manual

Simpson 479 RF Generator

Instruction

The Simpson Model 479 RF Generator is a vintage electronic test instrument designed as a combined AM/FM and TV signal generator for servicing, alignment, and testing of radio and television receivers. It includes RF signal generation, marker generation, audio modulation, and crystal calibration functions.

Specification

Key Features

Multi-Function Signal Generation: Combines RF signal generation, FM sweep functions, AM modulation, audio output, and crystal calibration in one instrument for servicing applications
Wide Frequency Coverage: Provides multiple frequency bands for testing broadcast receivers, FM equipment, and television circuits.\
Crystal Calibration: Built-in crystal reference allows improved frequency accuracy during adjustment and measurement procedures.
Adjustable Signal Output: Output level controls allow technicians to set suitable signal strength for different receiver testing requirements.
FM Sweep Capability: Includes a variable sweep generator function for alignment of FM and TV receiver circuits.
Audio Modulation Source: Integrated 400 Hz audio oscillator provides modulation for receiver testing.
Service Equipment Design: Built for laboratory and workshop use in electronic repair environments.

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CRYSTAL CALIBRATOR

The Simpson Model 479 Signal Generator has been designed carefully to supply all
the necessary signal sources for the proper alignment and servicing of TV and FM
receivers. For your convenience, markings on the FM Generator tuning dial allow you
to tune it according to the frequency which you require, or according to television channel number for RF signals through both VHF and UHF ranges.
There are two separate tunable oscillator sections. Each oscillator section is provided with a large, precision vernier dial having a 20:1 knob-to-pointer ratio and a
1000 division logging scale. They are easy to read and easy to set to any exact frequency within the range of the generator.
Everything possible has been done to make the Model 479 the most accurate, flexible and convenient instrument available. Each part of this instrument has been considered carefully for long life and stability. Many of the vital components are manufactured under rigid supervision within our own plants in order to insure lasting accuracy and many years of uninterrupted service.
DESCRIPTION
The Model 479 is arranged in two major sections as shown in figure 1. The left
hand section contains a crystal calibrator, a 400 сycle audio oscillator, and a threerange r-f generator which can be amplitude modulated with the output of the 400 cycle oscillator.
The desired type of signal is selected by the SIGNAL switch (left). The SIGNAL
switch has five positions, named OFF, UNMOD. R.F., CAL., MOD. R.F., and AUDIO.
When the switch is in the OFF position, the entire A.M. Generator is inoperative. When the switch is in the UNMOD.R.F. position, an unmodulated r-f signalis available through the
OUTPUT jack and cable. The amplitude is controlled with the SIGNAL ATTENUATORS,
with both fine and coarse adjustments (center and right). When the switch is in the
CAL. position, the output of a 5.0 mc. crystal oscillator is mixed with the output of the
R.F. Generator to produce a “beat”according to the information in table 1. The beat
pattern can be observed on an oscilloscope connected with the VERT. AMPL. and
HORIZ. AMPL. cables. By using table 1 and the oscilloscope, anyfrequency within the
range of the instrument can be produced quickly and precisely. When the switch is in
the MOD. R. F. position, the r-f signal is amplitude modulated 30% with a 400 cycle
audio frequency and the modulated signal is available through the OUTPUT jack and cable. The amplitude is controlled by the SIGNAL ATTENUATORS. When the switch
is in the AUDIO position, a 400 cycle signal is available through the OUTPUT jack and cable. The amplitude is controlled by the SIGNAL ATTENUATORS.
A potentiometer and a five-position switch together comprise the SIGNAL ATTEN- UATORS. The switch, at the right, is the coarse amplitude selector for the output of
the a-m generator, and the potentiometer acts as a fine adjustment on amplitude.
The A.M. GENERATOR RANGE switch, located just below the center of the dial, selects each of the three bands of radio frequencies. The tuning knob varies the fre- quency throughout each band.
Band A. Fundamental 3.3 to 7.8 mc.
Second harmonic 6.6 to 15.6 mc,
Band B. Fundamental 15 to 38 mc.
Second harmonic 30 to 76 mc.
Band C. Fundamental 75 to 125 mc.
Second harmonic 150 to 250 mc.

SIGNAL switch to CAL.

Prepare the Model 479 for calibration, turn the POWER switch to OPERATE;
SIGNAL switch to CAL.; SIGNAL ATTENUATORS to a low setting; and the A.M. GENERATOR RANGE switch to A, B, or C depending on the frequency to be established.
Connect the VERT. AMPL. cable to the vertical input of the oscilloscope. Connect the
HORIZ, AMPL. cable to the horizontal input of the oscilloscope if a 60 cycle sine wave
sweep is desired (calibration beats can be observed with either a 60 cycle sweep or
linear sweep in the oscilloscope). With horizontal deflection on the oscilloscope due to
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either the internal sweep or the 60 cycle sine wave sweep, advance the vertical amplifier gain of the oscilloscope and slowly rotate the a-m generator tuning knob while
observing the oscilloscope screen. At various tuning points a pattern will appear on
the oscilloscope screen. Rotate the dial slowly through the area in which a pattern can
be seen. Firsta high frequency appears, then as the knob is rotated slowly, note that
the frequency reduces to zero, thenincreases to a high frequencyagain and disappears.
The patterns are the results of beat frequencies developed between the a-m oscillator
and the 5.0 mc crystal oscillator.
The point at which the pattern reduces to zero frequency is known as zero beat and
is the point at which the two oscillators are in step. The zero beat point is identified
easily by the fact that the slightest movement of the dial in either direction will cause
the pattern to increase in height and in frequency. At zero beat the pattern is, essentially, a straight line. At the higher frequencies it is sometimes difficult to bring the
pattern down to exact zero beat, but this is not important so long as it is brought down
to within two or three hundred cycles. This shows three to five cycles on a 60 cycle
sweep.
Note that some points on the dial will produce much larger patterns than others.
This is due to the order of harmonics of the two oscillators producing the beat pattern.
The lower harmonics result in a stronger beat pattern. Some of the weaker patterns
may require a higher setting of the vertical gain control of the oscilloscope.
Table 1 has been developed to assist the operator in identifying the frequencies
where beat patterns occur and the oscillator harmonics which produce them. The
frequencies preceded by an asterisk (*) will produce the stronger patterns and should
be used wherever possible.

DETERMINING AN EXACT FREQUENCY

There are two methods by which a given frequency setting may be obtained. They
somewhat similar but one is simpler, while the other yields more accurate results.
The first method is the simpler and, with practice, can produce acceptable results
for most purposes. The process consists of first determining the number of logging
scale divisions which correspond to a one megacycle frequency difference which includes the desired frequency; second, mathematically figuring the number of logging
scale divisions the desired frequency is away from a crystal check point (see table 1);
third, turning to the crystal point and observing its logging scale reading; and fourth,
adding or subtracting the determined number of scale divisions to or from the reading
at the crystal check point. When the logging scale is set to the reading obtained in the
fourth step, the oscillator will be tuned to the desired frequency.
A step-by-step example of the first method follows. Assume that a frequency of
20.75 mc. is desired in the A.M. Generator. Note that table 1 shows a strong calibration check point at 20 mc. Set the A.M. GENERATOR RANGE switch to B, the
SIGNAL switch to CAL., and the SIGNAL ATTENUATORS low to see the zero beat indications on an oscilloscope with the VERT. AMPL. and HORIZ. AMPL. cables connected. Have the POWER switch in either STAND BY or OPERATE position for at least
15 minutes before beginning the calibration to allow the Model 479 to warm up, and set
it in the OPERATE position to calibrate.
1. Observe the tuning arc of range B from a positiondirectly in front of the pointer
(to avoid parallax error) and set the pointer over the 20 megacycle mark on the dial.
Record the logging scale reading for this setting. Ona sample unit the setting was 36.0
(use your readings, since there will be variation from one unit to another which does not
affect the accuracy in any way). Set the pointer exactly over the 21 megacycle mark on
the dial. Again record the logging scale reading. The sample unit read 40.45 for this
setting. Subtract the first reading from the second to obtain the number of scale divisions which correspond to one megacycle. 40.45 – 36.0 is 4.45 divisions.
2. Determine the frequency difference, in megacycles, between the desired frequency and a check point (table 1); then multiply this difference by the result of step i
above. In the example, the desired frequency of 20.75 mc. is .75 mc. away from the
strong calibration check point at 20 mc. The result of step 1 shows thatin this area of
the sample unit, a change of 4.45 scale divisions corresponds to a change of one megacycle. Multiply .75 x 4.45 to get 3.33 divisions.
3. With the aid of the oscilloscope, tune the generator to its zero beat position for
the chosen calibration check point and record the logging scale setting for this position.
In the example, the sample unit was tuned to 20 megacycles and the logging scale read
36.2 divisions.
4. Add or subtract the, results of steps 2 and 3. Add if the check point frequency is
lower than the desired frequency, or subtract if the check point frequency is the higher.
This sum or difference is the logging scale setting to use for the desired frequency. In
the example, add (because the check point is below 20.75 mc.) 3.33 to 36.2 to obtain
39.53 divisions.

CRYSTAL CALIBRATING POINTS

Figure 2 is an illustration of the logging arcs as they are used in both a-m and
f-m generator dials. The upper arc of each dial is divided into 10 equal divisions
marked from 0 to 100. On the knob shaft is another dial marked in 100 equal divisions.
The gear ratio between the knob shaft and the pointer is such thatone revolution of the
knob shaft moves the pointer through one of its ten divisions. Thus each division of the
logging scale is effectively divided into 100 parts and the entire arc into 1000 parts.
The minor divisions may be divided visually for further increasing the number of logging
points and the resulting accuracy of calibration information. For example, the reading
on the logging scale in figure 2 is 22.5. The main pointer shows that the setting is 20
plus some additional amount, and the dial on the knob shaft shows that the additional
amount is 2.5. If the knob were turned slightly counterclockwise so the dial setting
were half way between 2.5 and 2.6, it could be read as 2.55 and the indicated setting
would be 22.55 divisions. Take advantage of the visual division of these

There are two methods by which a given frequency setting may be obtained. They
somewhat similar but one is simpler, while the other yields more accurate results.
The first method is the simpler and, with practice, can produce acceptable results
for most purposes. The process consists of first determining the number of logging
scale divisions which correspond to a one megacycle frequency difference which includes the desired frequency; second, mathematically figuring the number of logging
scale divisions the desired frequency is away from a crystal check point (see table 1);
third, turning to the crystal point and observing its logging scale reading; and fourth,
adding or subtracting the determined number of scale divisions to or from the reading
at the crystal check point. When the logging scale is set to the reading obtained in the
fourth step, the oscillator will be tuned to the desired frequency.
A step-by-step example of the first method follows. Assume that a frequency of
20.75 mc. is desired in the A.M. Generator. Note that table 1 shows a strong calibration check point at 20 mc. Set the A.M. GENERATOR RANGE switch to B, the
SIGNAL switch to CAL., and the SIGNAL ATTENUATORS low to see the zero beat indications on an oscilloscope with the VERT. AMPL. and HORIZ. AMPL. cables connected. Have the POWER switch in either STAND BY or OPERATE position for at least
15 minutes before beginning the calibration to allow the Model 479 to warm up, and set
it in the OPERATE position to calibrate.
1. Observe the tuning arc of range B from a positiondirectly in front of the pointer
(to avoid parallax error) and set the pointer over the 20 megacycle mark on the dial.
Record the logging scale reading for this setting. Ona sample unit the setting was 36.0
(use your readings, since there will be variation from one unit to another which does not
affect the accuracy in any way). Set the pointer exactly over the 21 megacycle mark on
the dial. Again record the logging scale reading. The sample unit read 40.45 for this
setting. Subtract the first reading from the second to obtain the number of scale divisions which correspond to one megacycle. 40.45 – 36.0 is 4.45 divisions.
2. Determine the frequency difference, in megacycles, between the desired frequency and a check point (table 1); then multiply this difference by the result of step i
above. In the example, the desired frequency of 20.75 mc. is .75 mc. away from the
strong calibration check point at 20 mc. The result of step 1 shows thatin this area of
the sample unit, a change of 4.45 scale divisions corresponds to a change of one megacycle. Multiply .75 x 4.45 to get 3.33 divisions.
3. With the aid of the oscilloscope, tune the generator to its zero beat position for
the chosen calibration check point and record the logging scale setting for this position.
In the example, the sample unit was tuned to 20 megacycles and the logging scale read
36.2 divisions.
4. Add or subtract the, results of steps 2 and 3. Add if the check point frequency is
lower than the desired frequency, or subtract if the check point frequency is the higher.
This sum or difference is the logging scale setting to use for the desired frequency. In
the example, add (because the check point is below 20.75 mc.) 3.33 to 36.2 to obtain
39.53 divisions.
Note that the logging scale readings are for a sample unit only. Do not use these
readings. Obtain the logging scale readings for your Model 479 and use them in a similar way. Although you will be using some frequency settings repeatedly, do not rely
on the stability of the instrumentover long periods of time; the components are subject
to normal deterioration and will cause slight changes of logging scale settings in time.
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The second method is differentfrom the first only in the fact that two crystal check
point settings are used in place of two dial markings. First, determine the number of
logging scale divisions which correspond to the frequency difference betweentwo crystal
check points surrounding the desiredfrequency; second, mathematically figure the number of logging scale divisions the desired frequency is away from one of the check
point frequencies; third, add or subtract the determined number of scale divisions to or
from the reading at the crystal check point. Addif the lower check pointis the reference,
or subtract if the higher check point is the reference. When the logging scale is set to
the reading obtained in the third step, the oscillator will be tuned to the desired frequency.
The accuracy obtained by this method is better than 0.1%
A step-by-stepexample of the second method follows. Again, assume thata frequency
of 20.75 mc. is desired in the A.M. Generator. Note that the two nearest strong crystal
check points are 20.0 and 21.67 mc. (table 1). Thereare weak check points at 21.0 and
21.25 mc., but these are notrecommended because they are close together and difficult
to identify. Set the A.M. GENERATOR RANGE switch to B, the SIGNAL switch to CAL,.
and the SIGNAL ATTENUATORS low to see the zero beatindications on an oscilloscope
with the VERT. AMPL. and HORIZ. AMPL. cables connected. Have the POWER switch
in either STAND BY or OPERATE position for at least 15 minutes before beginning the
calibration to allow the Model 479 to warm up, and set it in the OPERATE position to calibrate.
1. With the aid ofthe oscilloscope, tune the A.M. GENERATOR around the 20 mega- cycle point for the zero beat indication. Record the logging scale setting for the zero
beat position. On the sample unit, the reading was 36.2 divisions (use the reading on
your own Model 479; this is for an example only). Retune the A.M. Generator around
the 21.67 mc. point for the zero beat indication. Record the logging scale setting for
this zero beat position. On the sample unit, the reading was 43.3 divisions. Subtract
the first reading fromthe second for the number of logging scale divisions between the
check point frequencies. For the example, 43.3-36.2 is 7.1 divisions.
2. Determine the frequency difference between the desired frequency and either
check point frequency. In the example, the desired frequency (20.75 mc.) is .75 mc.
above the lower check point and is .92 mc. below the upper check point. Next find the
frequency difference between the two check points. In the example this is 1.67 megacycles. By ratio and proportion, the frequency deviations can be translated into scale
divisions for the logging scale;