Interval times or time interval counters provide the ability to measure the time interval between edges on waveforms. They can be used for single waveforms, or by providing two separate inputs, they can use one waveform to start the time interval, and another to end the time interval measurement.
Timers use may of the same circuit blocks as those used in frequency counters. By re-arranging the blocks the same circuits can be sued in a slightly different manner to measure the time intervals. Accordingly frequency counters and interval timers are often combined in the same instrument - these test instruments are often referred to as a frequency counter timer.
These test instruments are widely available and often can be purchased relatively cheaply. Some of the major differences are the accuracy of the basic clock oscillator, the display, ease of operation and robustness of the test instrument.
Interval timer basics
The timer interval measurement capabilities of time interval counters can be used in a variety of ways.
- Time interval of waveform: The frequency timer counter can be used to simply measure the time interval of the incoming waveform. This is simply the reciprocal of the frequency. It typically measures the positive going edge of the waveform to the next positive going edge.
- Time interval of pulse: Rather than simply measuring one positive going edge to the next, it is also possible to measure a positive going edge to the next negative going one. Or alternatively it could measure a negative going edge to a positive one. In this way it is possible to use the frequency timer counter to measure the time interval of a pulse.
It can be seen that the two options are represented by the two times T1 and T2: T1 being for the timer interval + to - and T2 for - to + going edges of the waveform.
In this way the functionality of the instrument has been greatly enhanced by the simple addition of the selection of the waveform edge transition. This can be provided without much additional circuitry, and with the use of integrated circuits comes at virtually no additional cost.
- Time interval A to B: Many counter timers have two inputs. In this way it is possible to measure an edge on one signal to an edge on another. Normally it is possible to select either positive or negative going edges on each signal.
For this measurement there are four combinations that can be selected by using the different edges of the two waveforms. These are represented by T1 (A+ to B+), T2 (A+ to B-), T3 (A- to B+) and T4 (A- to B-), where A- is the negative going edge of waveform A, etc.
The time interval capability of a frequency counter timer, may not be used as often as the straight frequency counter measurement, but it is a key application in some areas. The time interval measurement capability is normally provided in the lower frequency counter timers aimed more for general purpose use. It is not normally included in RF or microwave frequency counters that are aimed specifically at the specific RF applications.
Frequency counter timer operation
Time interval is the reciprocal of frequency. As a result it is possible to make interval timer measurements using a frequency counter timer by simply reconfiguring some of the circuitry from those used for frequency measurements.
Like the frequency counter, the timer counter or interval timer has a number of blocks that make up the test instrument. They are very similar to those used in the counter, and just require reconfiguration to give the interval timing function. The operation of the various stages is summarised below:
- Signal input stages: In operation the interval timer counter take in the waveforms that require timing. Often timers have both A and B inputs. The input stages condition the input signals and convert them to the logic levels required within the interval timer counter.
- Gate Flip-Flop: The Gate Flip Flop then generates a pulse equal to the length of the timer interval required. This can easily be achieved using simple logic, although this functionality is normally contained within a special frequency counter timer integrated circuit. Selection of the positive and negative going edges is also selected here.
- Clock oscillator: The clock oscillator for the frequency counter timer or timer interval counter will typically be a crystal oscillator. In many cases this will be oven controlled, and many test instruments will also have an output to use this oscillator as the clock or reference source for other instruments. They may also be an input so that external clock oscillators can be used. This can enable some test systems to run off a single source thereby giving the advantages of synchronisation across the test system.
- Gate: The function of the gate within the interval timer is exactly the same as in the frequency counter, except that for the interval timer the inputs are effectively reversed. The circuit will be the same.
The edges to be timed produce a signal that enables the gate for the duration of the timing. During this time, the clock signal passes through. For example, if the signal to be timed is a second long, and the clock entering the gate is 1 MHz, then 1 000 000 pulses will appear at the output of the gate.
- Counter/ latch: The counter takes the incoming pulses from the gate. It has a set of divide-by-10 stages as a decimal based display is required. The number of stages within the overall counter is equal to the number of display digits minus 1. As the counters are chained the first stage is the input divided by ten, the next is the input divided by 10 x 10 (100) as it has been divided by two stages, and so forth. These counter outputs are then used to drive the display.
In order to hold the output in place while the figures are being transferred and displayed, the output is latched. Typically the latch will hold the last result while the counter is counting a new reading. In this way the display will remain static until a new result can be displayed at which point the latch will be updated and the new reading presented to the display. Often the latch may be incorporated into the display circuitry.
- Display: The display takes the output from the latch and displays it in a normal readable format. LEDs are still widely used although LCDs are more common because of the lower power consumption and the greater flexibility provided for additional characters and indictors. There is a digit for each decade the counter can display. Obviously other relevant information may be displayed on the display as well. The display will be programmed to place the decimal point in the correct position. For example for the 1 second time interval with a 1 MHz clock, 1 000 000 pulses are counted and the decimal point will need to be placed after the figure 1 to indicate 1.000 000 seconds.
Interval timers or timer counters are normally very easy to use. There are simple switches on the front panel or if it is a software instrument, upon the control panel to enable the different inputs and different edges, i.e. positive and negative going edges to trigger the start and stop points.
These test instruments can also measure time intervals very accurately. The measurement accuracy of the interval timer is dependent upon the clock oscillator and this can easily be accurate to one part in 106 or a few parts in 107 dependent upon the oscillator used. Accordingly these test instruments are capable of being used for high accuracy measurements in many applications.
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