4

0.1

0.2

0.3

0.5

2.0

1.0

7.0

Frequency kHz

3.0

-40

-30

-20

-10

-3

0

+5

Gain

dB

Audio Input Level = 275mVrms

V

= 3.75V

DD

Fig.4 Typical Audio Frequency Response of a Single Scrambled or De-scrambled Channel

0

Lowpass Filtered Voiceband

for Input to

Balanced Modulator

1000Hz

2000Hz

3000Hz

0

Carrier Frequency (3300Hz)

Generated On-Chip for Input to

Balanced Modulator

1000

2000

3000

0

Bandpass Filtered Voiceband

Presented at the Ouput

after Mixing with the Carrier Frequency

in the Balanced Modulator

1000

2000

3000

Speech Band

Input Voiceband Frequencies

3300

A

B

C

D

A

B

C

D

(1)

(2)

(3)

(4)

How the Inverter Works

Carrier Frequency

minus Input Voice Frequency equals Scrambled Voice Frequency

System Gains

When calculating the external components for the operation of the FX118 the following points should be considered:

(a)

The Input Lowpass Filter has a (typical) gain of 0.5dB.

(b)

The Balanced Modulator has a (typical) attenuation of 4.0dB.

(c)

The Output Bandpass Filter has a (typical) gain of 4.5dB.

Application Information ......

Fig.5 A Pictorial Explanation of the FX118 “Scramble Operation”