Telephone Ring-tone Generation

Linear Technology Corporation

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Telephone Ring-Tone Generation –
Design Note 134
Dale Eagar
Requirements
When your telephone rings, exactly what is the phone
company doing? This question comes up frequently, as it
seems everyone is becoming a telephone company. Deregu-
lation opens many new opportunities, but if you want to be
the phone company your have to ring bells.
An Open-Architecture Ring-Tone Generator
Here is a design that you can own, tailor to your specific
needs, layout on your circuit board and put on your bill of
materials. Finally, you will be in control of the black magic
(and high voltages) of ring-tone generation.
Not Your Standard Bench Supply
Ring-tone generation requires not one but two high volt-
ages, 60VDC and – 180VDC (this arises from the need to put
87V
RMS
on – 48VDC). Figure 1 details the switching power
supply that delivers the volts needed to run the ring-tone
circuit. This switcher can be powered from any voltage from
5V to 30V and shuts down when not in use. Figure 2 is the
build diagram of the Transformer used in the switching
power supply.
60V
= PRIMARY GROUND
= SECONDARY GROUND
T
1
2
70T #34
7
5V TO
30V
5
V
IN
SW
2
0.01
FB
GND
3
10k
RING
10k
330Ω
0.1µF
DN134 F01
Quad Op Amp Rings Phones
When a phone rings, it rings with a cadence, a sequence of
rings and pauses. The standard cadence is one second
ringing followed by two seconds of silence. We use the first
1/4 of LT
®
1491 as a cadence Oscillator whose output is at
V
CC
for one second and then at V
EE
for two seconds. This
sequence repeats every three seconds, producing the all-
too-familiar pattern. The actual ringing of the bell is done by a
20Hz AC sine wave signal at a signal level of 87V
RMS
superimposed on – 48VDC. The 20Hz signal is implemented
with the second amplifier in the LT1491 which acts as a gated
20Hz Oscillator (see Figure 3).
The third amplifier in the LT1491 which is configured as a
lowpass Filter converts the square wave output of the
Oscillator to a sine wave by filtering out unwanted harmonics.
Finally, the 87V
RMS
and the – 48VDC parts are handled by the
fourth amplifier in the LT1491 and its steering of two external
, LTC and LT are registered trademarks of Linear Technology Corporation.
+
0.47µF
+
220µF
4
60V
1
3A
60V
5
20T #26
4
6
3
1
200T #34
8
+
0.47µF
–180V
10k
180V
VC
5
2
2k
4
1
MATERIALS
2
EFD 20-15-3F8 Cores
1
EFD 20-15-8P Bobbin
2
EFD 20- Clip
2
0.007" Nomex Tape for Gap
Start Pin 1 200T #34
Winding
Term Pin 8
1
1 Wrap 0.002" Mylar Tape
Start Pin 2 70T #34
Winding
Term Pin 7
2
1 Wrap 0.002" Mylar Tape
Connect Pin 3 1T Foil Tape Faraday Shield
1 Wrap 0.002" Mylar Tape
Shields
Connect Pin 6 1T Foil Tape Faraday Shield
1 Wrap 0.002" Mylar Tape
Start Pin 4 20T #26
Winding
Term Pin 5
3
Finish with Mylar Tape
Figure 1. The Switching Power Supply
Figure 2. Ring-Tone High Voltage Transformer
Build Diagram
08/96/134