Triplett CableRater- PN: 3279 - Manual (Page 20)

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duce any more output level. In such a case, the CableRater will measure the Gain
as being lower than expected. If saturation is suspected, reduce the input level
to the amplifier with an attenuator, then measure the gain of the amplifier with
the CableRater, adding the attenuator loss to the Gain displayed on the CableRater
to obtain the correct amplifier gain. For example, insert a 10dB attenuator be-
tween the Transmitter’s output and the amplifier’s input, then measure the out-
put signal of the amplifier with the CableRater, and add 10dB to the CableRater’s
reading to obtain the actual Gain.
Important Note:
Some amplifiers may be excessively noisy or may oscillate if they are operated
with their input disconnected. This does not damage them, but it can cause
unusual readings when measuring amplifier gain. For example, the amplifier can
appear to be amplifying a signal, because a CableRater Receiver connected to
the amplifier output indicates a signal is present... but the Transmitter test signal
is not being applied to the amplifier’s input... the amplifier input is open. Try
connecting a termination to the input of the amplifier to see if it behaves cor-
rectly... i.e. the CableRater connected to its output reads Underrange or a value
less than -15dB. If appropriate behavior is obtained, the amplifier may be OK
once it is incorporated into the cable distribution system.
Some amplifiers can become unstable when the CableRater Receiver is con-
nected to their output. The CableRater Receiver does not have a ‘wideband’ 75
Ohm input impedance (75 Ohms is the standard impedance of a cable TV distri-
bution system). The CableRater Receiver uses a ‘narrowband’ 75 Ohm input
impedance. This sometimes causes an amplifier to oscillate or be overly noisy. If
the CableRater Receiver indicates the presence of a signal on the output of an
amplifier, and the input has been terminated as previously described, the Re-
ceiver may be causing the amplifier to oscillate. Usually, when this occurs, it is
necessary to properly terminate the amplifier so it won’t oscillate. The amplifier
can be terminated, and the CableRater connected to the amplifier to perform a
measurement, by inserting an in-line attenuator between the output of the am-
plifier and the input of the CableRater Receiver. Use whatever attenuator is handy,
in the range of 3 to 10dB. Assuming that this hookup makes the amplifier stable,
a gain test can be performed as previously described, but now the loss of the in-
line attenuator is added to the CableRater reading to obtain the amplifier Gain.
For example, if the CableRater reads 14.0dB, and a 6dB in-line attenuator was
used, the amplifier’s Gain is 20.0dB.
9.8 Testing Distribution System Loss
9.8.1 Signal Level
A Cable TV distribution system is usually designed to apply a signal of 0dBmV to
10dBmV to each TV, VCR, STB (Set Top Box), or other device.