Frequency Counter Specifications

Wednesday, August 4th, 2021 - Electrical Note

Frequency counters specifications are usually divided into three sections: Input Characteristics, Operating Mode Characteristics, and General. The Input Characteristics section describes thecounter’s input signal conditioning: input amplifier performance and conditioning circuitry such as coupling selection, trigger level control, and impedance selection. The Operating Mode Characteristics section specifies how the counter performs in each of its operating modes or functions such as Frequency, Period, Time Interval, and Totalize. Range, Least Significant Digit Displayed (LSD Displayed), Resolution, and Accuracy are usually specified. The General section specifies the performance of the timebase and instrument features such as auxiliary inputs and outputs (e.g., markers, trigger level lights, arming inputs, timebase inputs and outputs), Check mode, sample rates and gate time selection.

Frequency Counter SpecificationsCounter sensitivity and hysteresis

Frequency Counter Specifications

Range Specification

Range of frequency over which input amplifier sensitivity is specified. If input coupling is selectable, then ac and dc must be specified separately. Although the specification states that the input amplifier has a range from dc to 100 MHz, it does not mean that measurements in all operating modes can be made over this range. Consult the individual RANGE specifications under the appropriate OPERATING MODE specification.

Sensitivity Specification

Lowest amplitude signal at a particular frequency which the counter will count. Assumes that the trigger level (if available) has been optimally set for a value equal to the midpoint of the input signal.
Sensitivity is actually a measure of the amount of hysteresis in the input comparator and may vary with frequency. Because of this, the sensitivity specification may be split into two or more frequency ranges.

Hysteresis is used to describe the dead zone of a Schmitt Trigger (or voltage comparator). Referring to Figure 1, you see that when the input is above EON, the output goes high. When the input voltage falls below EOFF, the output drops low. If you graph the input-output function, it resembles the familiar hysteresis loop between magnetizing force and magnetic flux in a magnetic material.

In order for the counter to count, the input must pass through both limits. The p-p minimum countable signal, defined as the counter’s input amplifier sensitivity, is equal to EON– EOFF. The rms sine wave sensitivity = \frac{E_{ON}-E_{OFF}}{2\sqrt{2}}

The input waveform must cross both hysteresis limits to generate acount. This imposes a limit on the “useful” sensitivity of counter inputs. In the upper waveforms of Figure 2, the noise is not of sufficient amplitude to cross both limits. No extra counts are generated and the frequency measurement is made without error (not the case, however, for reciprocal counters measuring frequency — trigger error causes measurement inaccuracies). The lower wave-forms show a more sensitive counter input. In this case, the noise does cross both hysteresis limits and erroneous counts are generated.

Sensitivity SpecificationNoise induced counting

Since the counter input does not respond to the rms value of the waveform but only the peak-to-peak value, the sensitivity specification should be volts peak-to-peak with a minimum pulse width. Since many applications involve measuring the frequency of a sinewave signal, the specification is also given in terms of volts rms sine wave. (Note, however, that a different waveform with the same rms voltage may not trigger the counter — the counter responds only to peak-to-peak.)

Example :

  • 10 mV rms sine wave to 10 MHz. (By looking at the RANGE specification, you see that in dc coupling the counter will count a 10 mV rms sine wave at any frequency between dc and 10 MHz and in ac coupling, it will count a 10 mV rms sine wave at any frequency between 30 Hz and 10 MHz).
  • 25 mV rms sine wave to 100 MHz. (For frequencies between 10 MHz and 100 MHz, regardless of ac or dc coupling, the counter will count a 25 mV rms sine wave)

Signal Operating Range Specification

If the signal peaks extend beyond the specified signal operating range, one or more operating modes may give incorrect results; for example, frequency miscounting or time interval inaccuracies.

Dynamic Range Specification

The minimum to maximum allowable peak-to-peak signal range, centered on the middle of the trigger level range. If the input signal exceeds this range, then the input amplifier may saturate, causing transitions of the input to be missed. The dynamic range is limited by the range over which the differential input of the amplifier can swing without saturation.

For some input amplifiers, the dynamic range puts a further restriction on the allowable signal peaks as specified by the signal operating range. The signal peaks must always stay within the signal operating range specification and the peak-peak value must stay within the maximum dynamic range specification.

Example :

  • 50 × 1 : 100 mV to 1 V p-p pulse
  • 50 × 10 : 1 V to 7 V p-p pulse
  • 1 M × 1 : 100 mV to 1 V p-p pulse
  • 1 M × 10 : 1 V to 10 V p-p pulse

Valid input signalValid input signal

Invalid input signalsInvalid input signals

Trigger Level Specification

For instruments with a trigger level control, the range over which trigger level may be varied should be indicated. Trigger level is usually the voltage at the center of the hysteresis band and physically is the dc voltage applied to one input of the input comparator.

For instruments with a readout of trigger level (a dc signal or voltmeter reading), the settability of the trigger level should be indicated as well as the accuracy. The settability specification indicates to what tolerance trigger level may be set and the accuracy specification indicates the worst case difference between the indicated trigger level and the actual trigger point

Damage Level Specification

Maximum input the counter can withstand without input failure.  The value may vary with attenuator setting and coupling selected.

Trigger level and actual trigger pointTrigger level and actual trigger point of frequency counters specifications

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