DB315A Datasheet, PDF (6/11 Pages) CML Microcircuits – Low Distortion Sinewave Output

CTCSS Encoder
6
MX315A
4. Application
The MX315A is dedicated to Continuous Tone-controlled Squelch Systems (CTCSS) in radio applications.
However, it can be used wherever encoding of low-frequency tones is required, such as intercoms, door-entry
systems and various industrial applications.
The performance of a CTCSS system can be degraded if speech frequencies in the signaling spectrum are
not removed prior to transmission. This can be accomplished by filtering the microphone signals to attenuate
frequencies below 250 Hz. Figure 2 illustrates the addition of TX Tone Output to the filtered microphone
signals prior to modulation. Figure 3 illustrates the TX Tone Output sequence and a typical spectral analysis.
Where two or more circuits are required to use a single oscillator (i.e. repeater applications), the signal at
Xtal can be used to drive one additional Xtal/Clock input. Any further circuits can be driven from the buffered
Xtal output of the second device.
The program code can be set on the D0-D5 inputs by hardwired logic levels or SPST switches to V_SS, as
illustrated in Figure 2 (allowing the internal pull-up resistors to program a logic “1”).
The MX315A provides both a TX Enable input and a TX Enable input. Either input can be used to enable the
tone output, with the unused pin left open circuit (internal resistors establish a valid logic level and prevent
damage). Any configuration of PTT switch or TX signal can therefore be interfaced.
TX ENABLE
RISE-TIME
ENVELOPE
TX TONE
OUTPUT
D.C. GROUND
D.C. GROUND
0
FUNDAMENTAL
10
20
30
TX TONE SPECTRUM
40
50
60
70
f₀ 2f₀ 3f₀ 4f₀ 5f₀ 6f₀ 7f₀ 8f₀
FREQUENCY
Figure 3: Tone Encoding Sequence and Spectral Response
4.1 Interfacing and Electromagnetic Capability
The MX315A requires a clock of 1MHz, which is internally converted to logic level square waves.
Consideration should therefore be given to possible interference problems with RF or IF circuitry caused by
1MHz or its harmonics.
A decoupling capacitor can be used to reduce ripple on the power supply. This will reduce the level of
superimposed noise on the supply caused by internal switching transients (particularly at 1MHz and f₀).

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