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Fundamentals of the
Electronic Counters
Application Note 200
Electronic Counter Series
Frequency
Counted
Input Conditioning Main Counting Display
Gate Register
Input Signal
Main Gate
Flip-Flop
Time Base
Dividers
Time Base
Oscillator
1
Table of Contents
Fundamentals of the Conventional Counters ........................................................................................... 3
The Reciprocal Counters ........................................................................................................................... 20
Time Interval Measurement ...................................................................................................................... 24
Automatic Microwave Frequency Counters ........................................................................................... 35
Introduction
Purpose of This Application Note
When Hewlett-Packard introduced its first digital electronic counter, the HP 524A in 1952, a
milestone was considered to have been laid in the field of electronic instrumentation. Frequency
measurement of up to 10 MHz or a 100-ns resolution of time between two electrical events became
possible. Since then, electronic counters have become increasingly powerful and versatile in the
measurements they perform and have found widespread applications in the laboratories, produc-
tion lines and service centers of the telecommunications, electronics, electronic components,
aerospace, military, computer, education and other industries. The advent of the integrated circuit,
the high speed MOS and LSI devices, and lately the microprocessor, has brought about a prolifera-
tion of products to the counter market.
This application note is aimed at introducing to the reader the basic concepts, techniques and the
underlying principles that constitute the common denominator of this myriad of counter products.
Scope
The application note begins with a discussion on the fundamentals of the conventional counter,
the types of measurements it can perform and the important considerations that can have signifi-
cant impact on measurement accuracy and performance. Following the section on the fundamen-
tals of conventional counters comes a section which focuses on counters that use the reciprocal
technique. Then come sections which discuss time interval counters and microwave counters.
2
Fundamentals of the Conventional Counters
The conventional counter is a digital electronic device which measures the frequency of an input
signal. It may also have been designed to perform related basic measurements including the period
of the input signal, ratio of the frequency of two input signals, time interval between two events
and totalizing a specific group of events.
Functions of the Conventional Counter
Frequency Measurement
The frequency, f, of repetitive signals may be defined by the number of cycles of that signal per
unit of time. It may be represented by the equation:
f= n/t (1)
where n is the number of cycles of the repetitive signal that occurs in time interval, t.
If t = 1 second, then the frequency is expressed as n cycles per second or n Hertz.
As suggested by equation (1), the frequency, f, of a repetitive signal is measured by the conven-
tional counter by counting the number of cycles, n, and dividing it by the time interval, t. The basic
block diagram of the counter in its frequency mode of measurement is shown in Figure 1.
Frequency
Counted
Input Conditioning Main Counting Display
Gate Register
Input Signal
Main Gate
Flip-Flop
Time Base
Dividers
Time Base
Oscillator
Figure 1. Basic block diagram of the conventional counter in its frequency mode of measurement.
The input signal is initially conditioned to a form that is compatible with the internal circuitry of
the counter. The conditioned signal appearing at the door of the main gate is a pulse train where
each pulse corresponds to one cycle or event of the input signal. With the main gate open, pulses
are allowed to pass through and get totalized by the counting register. The time between the
opening to the closing of the main gate or gate time is controlled by the Time Base. From equation
(1), it is apparent that the accuracy of the frequency measurement is dependent on the accuracy in
which t is determined. Consequently, most counters employ crystal oscillators with frequencies
such as 1, 5 or 10 MHz as the basic time base element.
3
The Time Base Divider takes the time base oscillator signal as its input and provides as an output a
pulse train whose frequency is variable in decade steps made selectable by the Gate Time switch.
The time, t, of equation (1) or gate time is determined by the period of the selected pulse train
emanating from the time base dividers. The number of pulses totaled by the counting register for
the selected gate time yields the frequency of the input signal. The frequency counted is displayed
on a visual numerical readout. For example, if the number of pulses totaled by the counting
register is 50,000, and the selected gate time is one second, the frequency of the input signal is
50,000 Hertz.
Period Measurement
The period, P, of an input signal is the inverse of its frequency.
P =1/ f
(2)
P = t /n
The period of a signal is therefore the time taken for the signal to complete one cycle of oscilla-
tion. If the time is measured over several input cycles, then the average period of the repetitive
signal is determined. This is often referred to as multiple period averaging.
The basic block diagram for the conventional counter in its period measurement mode is shown in
Figure 2. In this mode of measurement, the duration over which the main gate is open is controlled
by the frequency of the input signal rather than that of the time base. The Counting Register now
counts the output pulses from the time-base dividers for one cycle or the period of the input signal.
The conditioned input signal may also be divided so that the gate is open for decade steps of the
input signal period rather than for a single period. This is the basis of the multiple period aver-
aging technique.
Period measurement allows more accurate measurement of unknown low-frequency signals
because of increased resolution. For example, a frequency measurement of 100 Hz on a counter
with 8-digit display and a 1-second gate time will be displayed as 00000.100 KHz. A single period
measurement of 100 Hz on the same counter with 10 MHz time base would display 0010000.0