Philips AN1651 manual Fiber Optic Receiver for Low Frequency DataFigure, VOX Audio System

Philips Semiconductors	Application note
Using the NE/SA5234 amplifier	AN1651

				+4.5V
	10kΩ	10kΩ
10kΩ			4
	NE5234				+	±	16	12
						19
	100kΩ							R3
	+			+				12kΩ
1μF	A1			A2
	±			±	+4.5V
	2.2kΩ				RA			11
MIC					18kΩ
		0.15μF			SENS.		NE578
	220Ω
					ADJ.			4.7μF
					25kΩ
						RS		10
		D2			0.47μF
	Ct		D1		25kΩ			1nF
	+			±				9
	A3			A4		8
RD	±			+	4.2V		7
			10kΩ
					OFF	ON
	X1		11	2.2MΩ	0V
								40.2kΩ
								SL00651

Figure 23. VOX Audio System

Other critical parameters in this type of circuit are the attack and decay times of the RC network which controls the operation of the voice operated switch. Attack time determines how quickly the circuit activates after a quiet period, and the decay time sets how long the transmitter channel stays active between words. It is important to reach an optimum balance between the two time constants in order to allow unbroken transmissions of good quality and no lost syllables. A 100 to 1 attack/decay ratio is used in this particular application and this is primarily set by the value of RA and RD. A typical delay of two seconds is easily accomplished. Due to extremely high input impedance of the buffer stage A3, RD may be in the 1 to 2MΩ range allowing a reasonable value of storage capacitor to be used.

The Audio Channel

Audio input from the preamplifier, A1, is fed directly to Pin 14 of the NE578 compandor. Referring to Figure 24, which shows the internal diagram of the device, it can be seen that this is the compressor portion of the NE578. There is the option in this system to operate either in a 2:1 compressor mode or an automatic level control mode, (ALC). The compressor mode simply makes a 2:1 reduction in the amplitude dynamic range of the input signal and brings it up to the chosen nominal 0dB output level which is programmable from 10mVRMS to 1VRMS. In this particular example it is programmed for a 0dB level of 0.42VRMS which is approximately 1VP-P. This allows for a standardized output level with good characteristics for FM modulation where peak deviation must be controlled. Figure 25 shows the input-output characteristics of the compressor and ALC.

The compressor also has an attack time determined by capacitor C6 on Pin 11. Attack time is 10k * C6, decay time equals four times this value. An auxiliary amplifier stage is used following the NE578 in order to allow bandwidth and special forms of equalization to be implemented. Note that 2:1 compression in a transmission will enhance the channel dynamic range and may be used with no further processing at the receiver, but feeding the received signal through the complimentary 2:1 expandor will achieve even greater enhancement of the recovered audio. The NE578 contains both operations in the same package. Please refer to Philips Semiconductors applications note AN1762 by Alvin K. Wong for complete information on these compandor circuits using the NE578.

Fiber Optic Receiver for Low Frequency Data (Figure 26)

This application makes use of the NE/SA5234 to detect photo-optic signals from either fiber or air transmitted IR (Infra-red) pulses. The signal is digitally encoded for the highest signal-to-noise ratio. The received signal is sensed by an IR photo diode which has its cathode biased to half the supply voltage (2.5V). The first gain stage is configured as a transimpedance amplifier to allow conversion from the microampere diode current signals to a voltage output of approximately 10mV0-P. The second stage provides a gain-of-ten amplifier to raise this signal level to 1V peak amplitude. This stage is directly coupled from the preamplifier stage in order to provide the necessary common-mode voltage of 2.5V. Its gain control network is capacitively coupled to prevent DC gain as is required in single supply configurations. Since this is essentially a pulse gain stage, low frequency gain below the signal repetition rate is not needed. The third stage acts in a limiting amplifier

1991 Oct 14