Tyco Mastr III, Руководство по установке

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LBI-38636S
17
The telephone company wants to see a maximum three-
second level of -13 dBm at the carrier equipment as
measured on a modified Western Electric 3-type Noise
Measuring Set. The telephone equipment will limit the
audio if the signal is above -13 dBm at the carrier input.
This does not mean that the maximum that can be applied
into the two-wire end is -13 dBm. If the radio equipment
is a good distance from the carrier equipment, there will
be some line loss. If the loss is 5 dB, for instance, then -8
dBm could be applied into the two-wire end. Therefore,
the telephone company will have to be asked in each case
what level is allowed to be applied at the two-wire end.
If the telephone company checks and finds that too
much audio is being applied into the carrier equipment,
they will put a pad into the circuit to cut the audio down.
When the phone company is asked what levels can be
applied to the line, they will either provide a level in
V olume U nits ( VU ) or test tone. VU is average voice,
which is generally considered to be 10 dB below test tone.
Test tone is a 1000 or 1004 Hz tone used to line up the
circuit. Test tone is normally given in dBm. If the radio
installer isn’t careful, he and the phone company will be
talking 10 dB apart. If the phone company says the limit
is 0 VU, use +10 dBm for the line-up.
The two wire ends of these lines are normally
designed to work with a 600 ohm impedance in and out.
The transmit and receive carrier equipment gains are set
up for 600 ohm terminations. If the line to the carrier
equipment is fairly long, the impedance at which at the
two-wire end is not very critical.
If the two-wire end is close to the carrier equipment,
however, then the impedance is critical. If the impedance
is not 600 ohms, it can cause the gain of the carrier
equipment to go up or down. In some cases, feedBack
(oscillations) from the receive path to the transmit path
will be present. A common problem that causes
oscillations in the carrier equipment is gain change,
whether from misalignment or other reasons.
American Telephone and Telegraph Company has
published a reference for Voice Grade Lines entitled,
"
Private Line Interconnections, Voice Applications "
(Publication Number 43201). It covers several types of
private line interfaces. There is no publication that covers
radio control alone. However, there are several
parameters provided in the publication that are important
to note.
The 1000 Hz loss design objective is 0 to 10 dB. If
the loss is not specified, there will be a loss of 10 dB at
1000 Hz in most cases. The phone company allows itself
a
SHORT-TERM fluctuation of 3 dB and a LONG-
TERM variation of 4 dB. If a 10 dB loss line at 1000 Hz
is specified, up to 14 dB loss can be expected, and the
phone company would still be within their design limits.
The loss between 500 and 2500 Hz can be +2 dB and
-8 dB relative to 1000 Hz loss. Note that the phone man
may refer to this as -2 and +8 in the telephone company
way of talking. The loss between 300 and 3000 Hz can be
+3 dB to -12 dB relative to the 1000 Hz loss. This says
that if there is a line with 10 dB of loss at 1000 Hz, a loss
of as much as 18 dB at 2500 Hz, and 22 dB of loss at
3000 Hz can be expected. A loss 4 dB of long-term
variation should be added to this.
Noise on this type of line is measured at each end
with a Western Electric 3-type Noise Meter. The
allowable level of a line from 0 to 50 miles is 31 dBrnC,
and for a line from 51 to 100 miles is 34 dBrnC. If this
type meter is not available, an AC- VTVM can be used. If
there is a noise reading of -50 dBm or less, generally this
is considered an acceptable circuit.
Tone Remote Control Systems
In contrast with DC systems, where audio level
setting is not as critical, it is important that levels in tone
applications be set properly. Failure to do so results in the
control function not working properly. For example, after
the installation when the user has gained a little
experience, the user may find that they are not always
picking up the function selected. A little extra time spent
at the installation will save many problems of this type
later.
This equipment is designed so that the tone sequence
consists of either two or three parts. The first part is the
"
Secur-it " tone (2175 Hz) which is sent at the highest
level for approximately 125 milliseconds. This is
followed by the "
Function " tone which is sent at a level
10 dB lower for approximately 40 ms. In the case of a
transmit function, the "
Function " tone is followed by
2175 Hz " Hold " tone at a level 30 dB down from the
" Secur-it " tone burst (therefore, it is 20 dB down from the
" Function " tone burst). This tone continues for the
duration of the transmit function. The average voice (0
VU) is sent at the same level as the "
Function " tone,
therefore, the test tone for the voice is sent at the same
level as the "
Secur-it " tone.
The " Secur-it " tone must arrive at the base station at
no less than -20 dBm. The transmit " Hold " tone must
arrive at the base station at no less than -50 dBm. The test
tone for the voice must arrive at the base station at no less
than -20 dBm. Therefore, the limits of system operation
are usually established by only three things:
1. The maximum level at 2175 Hz that the phone
company will allow to be sent from the most
distant point in the system. Normally this will
not be higher than 0 dBm. In some cases it can
even be less, or on rare occasions it can be +5 or
+10 dBm.