Digital Electronics: Principles, Devices and Applications

682 Digital ElectronicsThe resolution of reciprocal counters can be further enhanced by using a technique calledinterpolation. It is possible to achieve a nine-digit resolution with a 10 MHz clock using interpolationtechniques that otherwise would require a clock of 1 GHz. This is particularly important when we arelooking for a given resolution in shorter gate times. The details of interpolation techniques are beyondthe scope of this text.16.14.4 Continuous-Count CountersThe counter architectures discussed in the previous paragraphs had a counter that counted for a knownperiod equal to the gate time. These counters have a dead time when the gate is disabled. Such counterscould miss vital information that could be important to the measurement. The continuous-count counterarchitecture is based on the fact that, if different measurements of a certain parameter of a signal werenot disjoint and the relationship that they had were made use of, the measurement resolution couldbe significantly enhanced by applying what we call curve-fitting algorithms. These counters have allthe attributes of reciprocal counters, with the additional ability of reading the event, the time and thecounter without having to disable the gates.16.14.5 Counter SpecificationsThe data sheets and manuals of universal counters contain detailed specifications of the instrument.The important ones include sensitivity, bandwidth, resolution, accuracy and throughput.16.14.5.1 SensitivityThis refers to the smallest signal that the instrument can measure and is usually expressed as mV(RMS) or peak-to-peak. A sensitivity of 10–20 mV (rms) is typical. In the majority of measurementsituations, sensitivity is not the issue.16.14.5.2 BandwidthThe bandwidth of the counter is its front-end bandwidth and is not necessarily the same as the maximumfrequency that the counter is capable of measuring. Measuring a signal frequency higher than theinstrument’s bandwidth only reduces its sensitivity specification and requires a larger minimum inputsignal. However, the bandwidth does affect the measurement accuracy in the case of some parameters.Rise time is one such parameter. Thus, it is always preferable to choose a counter with as high abandwidth as possible. Bandwidth is not explicitly mentioned in the specifications. However, it can beestimated by looking at variation in sensitivity across the frequency range of the instrument.16.14.5.3 ResolutionResolution refers to the minimum resolvable frequency increment (in the case of frequencymeasurement) and time increment (in the case of time interval measurement). The resolution is usuallyvery close to the least significant digit and is often ±1 count or LSD. Noise in the input signal, noisein the front end and input signal slew rate are some of the factors that affect resolution.