Private Squelch Circuit ...... Application Notes
The FX375 Private Squelch Circuit utilizes Audio Frequency Inversion and Continuous Tone Controlled Squelch
System (CTCSS) techniques to provide secure voice communication on a common radio channel.
Clear/Private Switching
is controlled by the logic state of the Private Enable input. Table 1 shows that, in the
receive condition the signal path will only be inverted
when the programmed CTCSS tone is received.
Although other logic actions will enable the receive
path, privacy of the conversation is maintained at all
times.
Pre- and De-emphasis
(6dB/octave) filters are
included on-chip in the transmit path, so that the use of
this device will produce natural sounding audio (clear
or private modes) when installed in modern radio
communication transceivers, with or without existing
audio processing circuitry. The recommended layout is
shown in block form below.
Figure 5 shows the recommended positioning of the
CLEAR
To Transmitter
Stages
Amplifier
Pre emphasis
Band Pass
Filter
Band Pass
Filter
De-emphasis
Pre-emphasis
Modulator
CLEAR
From Receiver
Stages
Demodulator
Band Pass
Filter
Band Pass
Filter
De-emphasis
Amplifier
Fig.5 The Private Squelch Circuit Installed within a Typical Audio Stage
FX375 (shaded areas) when installed within the audio stages of a typical transceiver system. The accompanying
waveform diagrams indicate the relative "voice band amplitudes" at each stage of the receive or transmit process.
Installation Recommendations –
Care should be taken on the design and layout of the printed circuit
board taking into consideration the points noted below.
(a) All external components (as recommended in
Figure 2) should be kept close to the package.
(b) Tracks should be kept short, particularly the Audio
and V
BIAS
inputs.
(c) Xtal/clock and digital tracks should be kept well
away from analogue inputs and outputs.
(d) Inputs and outputs should be screened wherever
possible.
(e) A "ground plane" connected to V
SS
will assist in
eliminating external pick-up on input and output
pins.
(f) It is recommended that the power supply rails have
less than 1mV rms of noise allowed.
(g) Tx Tone Output loading – Large capacitive loads
could cause this pin to oscillate. If capacitive loads
in excess of 100pF are unavoidable, a resistor of
1kΩ or greater put in series with the load should
minimise this effect.
9
CMLMICRO [ CML MICROCIRCUITS ]
