Electronic Design

Robust, low-cost continuous phase FSK modulator

In bandpass data transmission, one common way to design low-cost frequency-shift-keying (FSK) analog modulators is to use two independent oscillators that have outputs switched according to the binary input wave to be modulated. While this approach is simple, phase discontinuities during oscillator switching unnecessarily increases the bandwidth of the modulated signal. On the other hand, any any solution based on the synchronization of the oscillator increases the modulator cost.

A well-known alternative is the use of classical oscillators such as opamp or transistor-based astables, CMOS oscillators, or “linear” oscillators (e.g., the Wien oscillator), where a capacitor value is switched under the control of the binary wave. Because the capacitor voltage can’t change instantaneously, the result is a continuous phase FSK (CPFSK). Another common solution involves the direct use of a VCO.

In both of these alternative cases, the design is very sensitive to powersupply variations and the tolerances among the IC’s fabrication series or manufacturers. This sensitivity could be negligible in small production runs, but leads to problems in large factory productions of FSK modulators, which are usually solved by adding some adjustable circuit component. Apart from the augmented production cost due to this adjustment, experience shows that the adjustment margin can be different if the oscillator’s IC is purchased from different manufacturers.

Shown here is a phase-locked loop (PLL) based alternative for CPFSK generation. It exploits the low sensitivity (or robustness) of the set composed of the phase detector and VCO with respect to certain parameter variations, mainly in the lower part of its frequency band (compared with the sensitivity of other solutions, such as the direct use of the VCO). However, the main problem in using a PLL for FSK generation is that the PLL input is designed for sinusoidal waves, not for the bit stream to be FSK-modulated.

The proposed solution benefits from the double-balanced mixer commonly used as a phase detector in PLL circuits. This mixer is based on a constant-current source for a differential amplifier (Fig. 1). By externally sinking or sourcing bias current at the transistor Q0, which acts as the current source, the conversion gain (KD) of the phase detector is changed. In a simplified X model of the PLL (Fig. 2a) with zero input (regulation model), the change of conversion gain is similar to having an offset in the output of the phase-detector model (Fig. 2b). This offset modifies the voltage input of the VCO of the PLL, so the VCO output frequency is varied under the control of the bit stream applied to the current source. This structure is similar to some phase-lock angle modulators. However, in the proposed idea, it’s unnecessary to break the PLL with additional circuitry in order to introduce the modulating signal, hence a single analog PLL can be used.

Figure 3 illustrates a circuit based on the analog phase-locked loop NE564. The IC 4016 is a switch controlled by the binary signal to be FSKmodulated, which commutes the voltage at pin 2 of th NE564 between ground and 5 V × 2700O/(2700O + 6800O) = 1.42 V. This voltage at pin 2 modifies the current bias of the current source in the phase detector, and consequently the VCO output frequency varies without phase discontinuities. The VCO center frequency is fixed by C0, and the margin between the two FSK frequencies is adjusted by R1 and R2. For an FSK modulator with frequencies of 390 kHz for “MARK” and 560 kHz for “SPACE,’ the component values are as shown in Figure 3.

In this design, the classical PLL low-pass filter has not been included, as its presence is unnecessary for FSK generation. In addition, the filter increases the transfer function sensitivity with respect to the VCO parameters.

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