RCA 54-45 Manual page 30

First, as a means of comparison, the square wave output from the Audio Generator is viewed on
the scope. The horizontal sweep of the scope should be adjusted so that at least two full cycles can
be seen on the screen. ( Fig. 6a shows one full cycle of a perfect square wave).
connected to the output of the amplifier under test so that the modified square wave can be viewed on
the screen. Possible output wave shapes are shown in Fig. 6b to 6i, and the significance of each
waveshape is explained below.
E Q U I PM ENT
AU DIO
TO BE TESTED ___.
G E N ER ATOR
Figure 5 . Equipment Test Block Diagram
Fig. 6b shows "rounding" of the leading edge of square wave. This indicates a drop off in gain
at high frequencies . "Rounding" will generally be observable when there is a substantial drop in the
gain by the tenth harmonic (or less). Therefore, if a 2kc square wave fed to the amplifier is repro­
duced on the ' scope without "rounding" , the amplifier is flat to 10 x 2kc
Fig. 6c shows the effect of increased gain and Fig. 6d shows the effect of decreased gain at the
square wave frequency. Fig. 6e indicates lowered gain in a narrow frequency band.
wave frequency is brought into this narrow frequency band, Fig. 6d will result.
The effect of phase shift in the amplifier is shown in Figs . 6f and 6g.
there is phase shift in the leading direction, the square wave will be tilted as in Fig. 6f.
phase shift in the lagging direction, the top of the square wave will be tilted as in Fig. 6g. The steep­
ness of the tilt is proportional to the amount of phase shift.
amplifiers, although the ear is not entirely insensitive to it.
however, phase shift should be at a minimum.
Fig. 6h shows the pulse output from the amplifier that results when the square wave has under­
gone differentiation. This will happen when the grid resistor or the coupling condenser is too low in
value or if the coupling condenser is partially open.
damped oscillations following the leading edge. This results when a high frequency square wave is fed
to an amplifier in which distributed capacitances and lead inductances resonate at low frequencies. In
television and 'scope amplifiers it may result from an undamped peaking coil.
High fidelity audio amplifiers may be given a rapid check by testing first with a square wave of
fundamental frequency not less than 3 to 4 times the low frequency limit of the amplifier (3db point),
and then with a square wave of fundamental frequency which may be anywhere between 1/100 to 1/10
of the high frequency limit of the amplifier depending upon how many harmonics are considered neces ­
sary to produce an acceptable version of a square waveform.
frequency from 40 to 60 cps and 1000 to 2000 cps are employed to cover the range up to 20, 000 cps .
To insure correct results, the following is suggested: Connect the proper value of load across
the amplifier output terminals ; use a low capacitance cable for connecting the generator to the ampli­
fier input; set the generator output to an ample value, but be sure not to overload the amplifier.
square wave signal is fed to the amplifier input and the ' scope is connected across the amplifier load
Use the internal linear sweep to observe the waveform. Note that tone controls have a very marked
effect on the square wave response and should be set to the "flat" positions unless it is desired to
observe their effect. Note, also, that low fidelity and p. a. amplifiers will not reproduce the square
waveform.
Video amplifiers may be square wave tested in the same manner as described for testing audio
amplifiers. The test frequencies might be 60 cps for the low end and 25 , 000 cps for the high frequency
end.
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cY
SCOPE
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Figure 6. Sample Output Wave Shapes
Lastly, Fig. 6i, shows a square wave with
2 9
The 'scope is then
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20kc .
=
If, at low frequencies,
Phase shift is not important in audio
In television and 'scope amplifiers,
Usually, square waves of fundamental
If the square
If there is
The