MX118
FULL-DUPLEX SCRAMBLER FOR CORDLESS TELEPHONES
Features
Full-Duplex Audio Processing
On-Chip “Brick-wall” Filters (300-3000 Hz)
Low Voltage CMOS
High Baseband and Carrier Rejection
Excellent Audio Quality
ECPA* Qualified Voice Protection
Applications
Cordless Telephones & Wireless PBXs
Battery Powered Portability
MX118P
(16 pin PDIP)
MX118DW
(SOIC-16)
Description
The MX118 is a full-duplex frequency inversion scram-
bler that secures cordless telephone conversations. The
two audio paths, C1 and C2, are identical and independ-
ent. Each consists of the following:
1) A 10
th
order lowpass filter cut off at 3.1 kHz.
2) A balanced modulator with high baseband and carrier
rejection.
3) A 3.3 kHz inversion carrier (injection tone).
4) A 14
th
order bandpass filter (300-3000 Hz).
5) Input op-amps with externally adjustable gain.
The MX118 uses CMOS switched-capacitor filter
technology and operates from a single supply in the range
of 3.0 V to 5.5 V. The inversion carrier's frequency and
filter switching clock are generated on-chip using an
external 4.433619 MHz crystal or clock input.
Figure 1 - Simplified Signal Paths
* Electronics Communications Privacy Act (Title 18 US Code 2510 et seq.).
MX118
Figure 2 - Block Diagram
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Component
Value
R1
1.0MΩ
C1
1.0µF
C2
33.0pF
C3
47.0pF
C4
0.47µF
C5
1.0µF
C6
1.0µF
X1
4.433619 MHz
Tolerances: R = ±10%, C = ±20%
Notes:
1. Xtal circuitry shown is in accordance with MX-COM's Application Note on Crystal Oscillators (see Appendix).
Figure 3 - Recommended External Components
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MX-COM, INC.
MX118
Pin Function Chart
Pin
1
2
3
Function
Xtal:
This is the output of the clock oscillator inverter.
No Connection.
C1 Filter Output:
This is the output of the channel 1 input filter. It should be coupled to pin 4 (C1
Balanced Modulator Input) by a 1.0
µF
cap. See Figure 3.
C1 Balanced Modulator Input:
This is the input to the channel 1 balanced modulator. Internally
biased at V
DD
/2, it should be coupled to pin 3 (C1 Filter Output) by capacitor C
2
.
V
SS
:
Negative supply (GND).
C1 Output:
This is the analog output of channel 1. It is internally biased at V
DD
/2.
C1 Gain Amp Output:
This is the output pin of the channel 1 gain adjusting op-amp. See Figure 4
for test gain setting components.
C1 Input:
This is the analog signal input to channel 1. This input is to a gain adjusting op-amp whose
gain is set by internal components. See Figure 4.
C2 Input:
This is the analog signal input to channel 2. This input is to a gain adjusting op-amp whose
gain is set by internal components. See Figure 4.
C2 Gain Amp Output:
This is the output pin of the channel 2 gain adjusting op-amp. See Figure 4
for test gain setting components.
C2 Output:
This is the analog output of channel 2. It is internally biased at V
DD
/2.
V
BIAS
:
This is the analog bias line at V
DD
/2. It should be coupled to V
SS
by a 1.0
µF
or greater
capacitor. See Figure 3.
C2 Balanced Modulator Input:
This is the input to the channel 2 balanced modulator. Internally
biased at V
DD
/2, it should be coupled to pin 14 (C2 Filter Output) by capacitor C
7
.
C2 Filter Output:
This is the output of the channel 1 input filter. It should be coupled to pin 13 (C2
Balanced Modulator Input) by a 1.0
µF
capacitor. See Figure 3.
V
DD
:
Positive supply of 3.0 V to 5.5 V.
Xtal/Clock:
4.433619 MHz or an externally derived clock is injected at this pin. (See Figure 3.)
4
5
6
7
8
9
10
11
12
13
14
15
16
MX-COM, INC.
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MX118
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MX-COM, INC.
MX118
Application Information
Figure 5 - Typical Audio Frequency Response of a Single Scrambled or De-scrambled Channel
System Gains
When calculating the external components for the operation of the MX118 the following should be considered:
a) The input Lowpass Filter has a typical gain of 0.5 dB.
b) The Balanced Modulator has a typical attenuation of 4.0 dB.
c) The Output Bandpass Filter has a typical gain of 4.5 dB.
How
Inver
verter
How the Inverter Works
Carrier Frequency
minus
Input Voice Frequency
equals
Scrambled Voice Frequency.
Figure 6 - An Explanation of the MX118 Scramble Operation
MX-COM, INC.
Page 5
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