User Manual

LBI-38636S
15
2. The loss of the circuit at 2175 Hz. Do not forget
the long-term variation of up to 4 dB more.
3. The requirement that the "Secur-it" burst must
arrive at the base station at no less than -20 dBm.
Normally, most systems will not crowd these limits.
However, if the result is a few dB short, consider adding
C-1 conditioning (at an added cost). Resist the natural
desire to just turn up the tone sending level as this will
cause improper system operation.
Increasing the level will cause the "Secur-it" tone
burst to go into limiting in the phone company equipment.
The limited tone causes the "Secur-it" tone filter in the
base station to ring. This will result in picking up or
dropping out functions, which were not selected. NEVER
allow the "Se-cur-it" tone to be in limiting.
There is an easy way to check and see if the "Secur-
it" tone is in limiting. With the phone lines connected to
the equipment at both ends connect an AC voltmeter
across the phone line at the base station. Arrange to send a
burst of "Secur-it" tone long enough to measure the
incoming level on the AC voltmeter. Then arrange to send
a burst of 1950 Hz "Function" tone long enough to
measure the incoming level on the AC voltmeter. If the
1950 Hz tone does not arrive 10 dB (1 dB) less than the
"Secur-it" tone, then the "Secur-it" tone is in limiting. It
will become necessary to lower the sending level at the
remote controller until it is below limiting.
If the audio is high enough to cause the telephone
equipment to go into limiting, it will cause amplitude
distortion. On a high loss line the amplitude distortion
will cause the "Hold" tone (2175 Hz) to vary and the
transmitter to drop out.
On remote systems using tone control, care must be
used when connecting two telephone lines in tandem. For
example, for a base station and two remotes, a phone line
is ordered to connect the station to the first remote, and a
second line to connect the second remote to the first
remote. The loss of each line is now added together and
the tones from the second remote can not operate the base
station. The installer can either specify a low loss on each
line, or run each line directly to the base station.
A check with the phone company can determine
which approach is the least expensive over a period of
time; i. e., an analysis of non-recurring costs versus
recurring costs over the expected length of time the circuit
will be used.
Voting System Considerations
A voting system uses a continuous 1950 Hz tone on
the telephone line when the receiver is squelched. This
voting tone is normally sent from the station to the voting
selector 3 dB lower than the 1000 Hz test tone level. Most
telephone lines have a frequency response which
attenuates the 1950 Hz tone with respect to a 1000 Hz test
tone, therefore care should be taken to ensure that the
correct levels are received at the voting selector.
If the telephone company will not allow a continuous
tone as high as -8 dBm to be sent, then a lower loss circuit
should be requested or C-1 conditioning added.
When ordering phone lines for a voting system, if
possible, all lines should be of the same type. Different
telephone line responses will cause the voter to prefer one
signal over others.
It is improper system design to have the received
signal selection biased by a "poorer" telephone circuit.
Many telephone companies will add pads to build out the
lines. If this is considered when the lines are ordered, it
should not be difficult to build all of the lines out to have
the same frequency response.
Ordering Voice Grade Telephone Lines
If a standard voice grade circuit is ordered, and the loss is
not specified, the following will normally result:
1. Loss at 1000 Hz will be 5 to 10 dB; normally 10
dB
2. Long-term variation 4 dB
3. Amplitude distortion (frequency response)
Referenced to 1000 Hz; + = more loss 300 to
3000 Hz: -3 to +12 dB 500 to 2500 Hz: -2 to + 8
dB
4. Noise: 31 dBrnC maximum
5. Frequency translation error: 5 Hz
6. Normal impedance: 600 ohms
7. Maximum permitted signal into the line: -6 dBm
to -13 dBm in-band three second average (the
level arriving at the carrier equipment cannot be
more than -13 dBm).
By adding C-1 conditioning, the loss changes to:
Amplitude distortion (frequency response) Referenced to
1000 Hz; higher frequency = more loss 300 to 2700 Hz: -
2 to +6 dB 1000 to 2400 Hz: -1 to +3 dB
One added advantage to C-1 conditioning is that the
voice quality will be improved by boosting the high
frequency components.