Heathkit IM-4180 - Page 15

Page 52
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remaining cells in the series string will begin charg-
ing it in the reverse direction. If this condition occurs,
and you attempt to recharge the series string, the
reversed cell can be permanently damaged. The re-
versed cell can usually be saved if you first short it to
remove all its charge, then recharge it in the normal
manner. However, repeated cell reversal will increase
the internal cell pressure and cause the cell to vent
and expell electrolyte. This is a permanent failure and
the cell must be replaced.
APPLICATIONS
The input signal may be coupled to the Deviation
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signal generators and other low-power sources. For
medium power sources, an attenuator should be used
in the signal line. If relatively high-power sources are
tr ha tactad mall
to be tested, a small antenna such as the Heathkit
Model SU-510 can be used. High-level sources may
slightly affect the local oscillator, but will not affect
the deviation measurement. At higher UHF frequen-
add cama noise to thea
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ciae. high
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deviation measurement and will affect its accuracy.
At these frequencies, you will attain the greatest accu-
racy with low-level signais. In some cases, high-
nawar AM gianale i in vour area can affect tha onera-
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_tion of the Deviation Meter if you do not use shielded
inputs. NOTE: If the deviation readings are several
times greater than normal in a strong RF field, move
from the signal source.
Access tones used in
ally deviate several k
the 7 5 kHz devi ati ion range Continuone tones cucha
. anon r qugc. TON nuous (ones Such as
those used in tone squelch circuits, deviate less and
should be measured on the 2 kHz range. Since these
tones are “sub-audibie,” a iow pass filter is used in
n two-way communications usu-
kilohertz and may be measured on
To describe the frequency response on the 2 kHz
oa naif. warioh
Tange, 4na if you wish to use that r range with a mod-
ulating signal above 300 Hz, the effect of the low pass
filter may be corrected by multiplying the deviation
reading as follows:
ee
l= ih xX Vv 1+(Ce5)
where D, = deviation reading, and F,, = modulating
frequency.
The FM Deviation Meter has a bandpass similar to
that found in a monaural FM broadcast receiver. If
you are measuring stereo multiplex broadcasts, the
A SIGNAL COUPLER FOR DEVIATION MEASUREMENTS
ON MEDIUM POWER TRANSMITTERS MAY BE MADE By
BREAKING OFF THE MALE PIN OF A BNC TEE-CONNECTOR,
THE
ATTENUATION
IS
APPROXIMATELY
55
dB
AT
25
MHz
AND GOES DOWN AT ABOUT 6qdR/OCTAVE. THE TEST
SETUP IS AS SHOWN.
(B) BNC CONNECTOR
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Prorat MINA nays An
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pe
‘ DEVIATION
Meter will detect the deviation in the main (L + R)
channel and will reject the deviation in the sub (L —
R) channel. Thus, the instrument will “see” only half
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the modulation. For accurate deviation measure-
ments, input signals should be from monaural
sources; simply switch the stereo generator to the
“MONO” mode of operation
When you are measuring the deviation at high fre-
quencies, care shouid be taken io be sure thai ihe
Deviation Meter is not subjected to shock or vibration;
a slight disturbance of the local oscillator frequency
will show up as a deviation reading.
! ases nect the Deviation
Meter directly into a tre ransmission line to continu-
monitor medi ium power panna tere: This
the center pin
signal sfiemastind iis it appears at the RF INPUT
of the Meter. The attenuation is thus approximately
55 dB at 25 MHz and ne approximately 6 dB per
octave. Part B of Pictorial 8-3 shows a typical in-line,
continuous monitoring setup for your FM Deviation
oe
IVLULEL.