2.2. High-pass timing discrimination

In high-pass timing discrimination the timing point is generated from a unipolar input pulse using a high-pass filter, as shown in Fig. 6a. The zero-crossing point of the bipolar output signal defines the timing point, which is insensitive to variation in the amplitude of the input signal as long as the signal is processed in a strictly linear manner, i.e. the pulse is not distorted in the receiver. Thus the discrimination will not have the same walk error as leading edge discrimination. When the amplitude of the signal varies, the slew rate and over/underdrive around the timing point will change. As the propagation delay of a comparator depends on these parameters, the timing comparator will generate walk error basically for the same reasons as explained above (Binkley & Casey 1988). The walk error is nevertheless much smaller than that of a leading edge discriminator and its level depends on the speed of the comparator and decreases with the scaling of process technologies.

The walk error can be partly compensated for with an offset voltage arranged at the input to the timing comparator, so that a signal with a small amplitude will have its zero-crossing point slightly earlier than signals with larger amplitudes, as shown in Fig. 6b (Kinbara & Kumahara 1969). The compensation is not ideal, but it does reduce the walk error significantly in the useful region, as shown in Fig. 7.

The effect of walk error compensation is shown in Fig. 8, where the walk error is measured for an integrated receiver channel with four offset voltages (Paper VIII). The increase in the error at larger signal amplitudes is caused by distortion of the pulse in the amplifier channel.

A noise comparator is needed in the high-pass timing discriminator to prevent noise from causing false triggerings, whereas the leading edge discriminator requires no noise comparator at all. In practise the minimum usable SNR is about 10 for a high-pass timing discriminator (Ziemer & Tranter 1985) and should be slightly more for a leading edge discriminator so that the walk error and jitter should not be too high. On the other hand, the maximum signal level in high-pass timing discrimination is limited by the linear dynamic range of the receiver and usually has to be extended with gain control structures.

The jitter in high-pass discrimination depends on the shape of the pulse and on the bandpass corner frequencies of the receiver. Simulations indicate that a high-pass corner frequency of about 30% of the receiver channel bandwidth will give a good compromise between small walk error and small jitter and the level of the jitter is comparable to that in leading edge discrimination.

As the walk error is an important topic in the design of laser radar receivers, a new idea for combining timing discrimination (high-pass timing discriminator) and time interval measurement was developed to avoid the problem, as presented in section 3.1.3.