Section 730-514-202
9
The + 7TLP output of the RCV GN is routed to the RCV EQLR (receive equalizer) which provides amplitude
equalization for loaded and nonloaded cable facilities. The output of the RCV EQLR is applied to the SF RECEIV-
ER and the RCV ATN (receiver attenuator) thru the RCV FET switch. The RCV ATN provides up to 24dB of at-
tenuation in 0.1dB steps for setting the receive station equipment level. For 4W operation the output of the RCV
ATN is applied the 4W RCV STA. For 2W operation the output of the RCV ATN is applied to the 2W HYBRID and
then to the 4W/2W XMT STA. The net result of the RCV GN and RCV ATN stages is to allow the CFMs to ac-
commodate both line and station equipment levels between + 7 and –16dBm.
4.3 Signaling Interface
The CFMs provide E&M leads to the station equipment. The E&M leads can be conditioned to appear as a trunk
or signaling circuit. They can also be optioned for Type I, II or III E&M lead signaling modes. See Figure 2,
Figure 3 and Figure 4.
4.4 SF Receiver Circuitry
The SF receiver consists of the 2600Hz NOTCH FILTER, SF DETECTOR, SF TIMER, and the RCV FET
SWITCH. The input to the SF receiver comes from the output of the RCV EQLR. This insures that the receiver
input is at a + 7TLP. The idle SF tone is –20dBm0, which means that the low level or idle SF tone level is
–13dBm, while high level SF tone is –1dBm (–8dBm0). High level tone is received during dial pulsing and for 400
milliseconds following off-hook to on-hook transition. The output of the SF receiver is connected to the SF TIM-
ER.
The SF TIMER with the RCV FET SWITCH controls the insertion and removal of the 2600Hz NOTCH FILTER in
the receive voice path. The 2600Hz NOTCH FILTER is inserted in the voice path within 13 milliseconds of the SF
receiver detecting 2600Hz tone to prevent this signaling tone from reaching the 4W RCV STA. The 2600Hz
NOTCH FILTER is removed 50 milliseconds (nominally) after the SF receiver signals the loss of the SF tone, but
remains inserted for 225 milliseconds if SF tone was not present for 175 milliseconds.
The SF TIMER with the RCV FET SWITCH also controls the guard circuit of the SF DETECTOR. The guard cir-
cuit is disabled when SF tone has been received for 225 milliseconds or more. This places the SF receiver in the
broadband detection mode, allowing noise and frequencies other than 2600Hz to aid in maintaining the idle condi-
tion. The guard circuit is enabled 50 milliseconds (nominally) after the loss of SF tone, which places the SF re-
ceiver in the narrow band condition. With the SF receiver in the narrow band condition, SF signaling tone must be
10dB greater than the broadband energy for the SF receiver to recognize it as valid signals. This reduces talk-off,
while dial pulses and winks are easily passed because they are high level 2600Hz tone.
The SF TIMER contains a receive pulse corrector and an A relay driver circuit which corrects input pulsing. When
receiving a tone-on indication the A relay driver holds the A RELAY released. The absence of SF tone causes the
receive pulse corrector to turn on the A relay driver which operates the A RELAY. For receive pulse corrector re-
sponse to pulsed SF tone, see RECEIVE PULSE CORRECTION in Part 11.
4.5 SF Transmitter Circuitry
The SF transmitter consists of the 2600Hz OSCILLATOR; the XMT FET SWITCH; and the XMT tone on/off, XMT
level and XMT cut controls which are part of the SF TIMER.
The XMT cut control timing, controls the XMT FET SWITCH causing the transmit path to be cut and uncut in the
following manner: When the SF receiver is receiving continuous SF tone and the transmit signaling interface input
is at idle, the transmit voice path is cut continuously. If the receive SF tone is removed and the transmit signaling
interface input remains idle, the XMT cut is affected for approximately 550 milliseconds. If the receive SF tone is
on continuously and the transmit signaling interface input goes busy, the transmit voice path is cut for approxi-
mately 125 milliseconds. The transmit voice path is cut within 10 milliseconds of the transmit signaling interface
change of state from busy to idle.
The XMT tone on/off control places or removes SF tone from the transmit voice path toward the line. SF tone en-
ters the transmit voice path at a –16dB TLP; the SF tone is at –20dBm0. Therefore, low level SF tone is –36dBm
and high level SF tone is –24dBm.
The XMT level control operates the tone level control FET which is part of the XMT FET SWITCH. The XMT level
control closes the tone level control FET in a few milliseconds allowing high level SF tone from the 2600Hz OS-
CILLATOR to pass. The XMT level control opens the tone level control FET approximately 400 milliseconds after
the transmit signaling interface goes idle. This action allows dialing information to be transmitted as high level SF
tone.
