PDF AN1882 Data sheet ( Hoja de datos )

Número de pieza	AN1882
Descripción	NE568A as a Wideband FM Modulator
Fabricantes	Philips
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No Preview Available ! Philips Semiconductors The NE568A as a wideband FM modulator Application note AN1882 INTRODUCTION The NE/SA568A used as an FM modulator for frequencies up to 140MHz is described. This device is fabricated on 8GHz integrated bipolar technology and operates at 5V. The VCO gain constant, KO, is 4.2 x 10 rad/ volt-sec at a center frequency of 70MHz. This is equal to approximately 670MHz per volt. The gain, KO, is calculated from the magnitude of the frequency deviation as a function of the differential voltage present at Pins 17 and 18. Figure 1 shows the equivalent VCO circuit as used in this application. The internal phase detector reference inputs are hard wired to the VCO as shown in Figure 2. When used as a modulator the phase detector error voltage outputs (Pins 19 and 20) are no longer tied to the VCO inputs as with normal PLL operation. Instead these pins become the output for the modulator. -5V 350Ω X25 14 17 4.8V ICO 15 10mA 2x 2x MODULATOR INPUT 4.8V 18 500µA 2mA 4mA Figure 1. VCO 500µA SR01126 100Ω 5V 50Ω 20 LF-1 19 LF-2 100Ω VCO 4.8V VCO 4.8V 11 LIMITER 10 2mA Figure 2. Limiter — Phase Detector SR01127 Linearization of the Modulator In order to change the transfer function of the VCO-Phase Detector block from an exponential small signal function to a linear voltage-to-frequency converter, the output buffer amplifier signal is fed back from Pin 14 to Pin 18 (inverting input) of the VCO (see Figure 3). The output buffer amplifier has a gain of approximately 25dB. Pin 15 of the buffer amplifier allows for the addition of a shunt oriented passive filter to the feedback loop. The filter -3dB frequency is calculated based on a Pin 15 source impedance of 350Ω. A typical shunt lowpass filter consists of a 100pF capacitor from Pin 15 to ground and has a break frequency of 4.5MHz. 14 x25 15 RFB VMOD 18 – VCO + 17 PHASE DETECTOR IIN Figure 3. Closed Loop Linearization + OUTPUT – SR01128 Modulator Frequency Deviation The modulator frequency is determined by the frequency set capacitor, CO, attached to Pins 4 and 5. The value of the frequency set capacitor is calculated by the formula given below for FO equal to 70MHz. CO = (0.0013 /fO) Farad; @ fO = 70MHz, kO = 0.0013 A linearizing DC bias current is fed into Pin 17 through an RF choke and series resistor RIN as shown in Figure 4. This reduces the harmonic content of the output signal while also increasing its amplitude. The AC modulator output signal appears on Pins 19 and 20 superimposed on a DC common mode voltage 4.8V above ground. In order to provide a low impedance ground referenced output signal, a gain-of-two buffer amplifier (NE5539) is added. The output impedance is 75Ω. Note that the buffer amp differential input resistance is kept relatively high in order to reduce loading on the internal differential stage (RS = 200Ω.) This also decreases the harmonic distortion of the modulator output waveform. Harmonic distortion and added sideband interference may be further reduced by the addition of low pass filters around the buffer amplifier, but caution is required due to the critical stability requirements of very wideband amplifiers such as the NE5539. 1996 Jan 29 1 1 page

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