Research of electronic frequency counter characteristics

Work purpose

1.1 Obtaining the professional knowledge with digital frequency measuring device work. Be able to choose counting time (), period factor (n) and marks generator frequency ().

1.2 Research electronic frequency counter (EFC) basic characteristics in different working mods. Be able to choose working mod that will provide minimal error result of frequency measurement, time intervals and frequencies ratio.

 

Key features

2.1 Modern EFC based on micro processing schemes with hard logic are the multifunctional devices. The transition from one function to another is implemented by installed software or by switching keys.

Functional scheme is shown on pic. 1 relate to EFC with hard logic.

2.2 “FREQUENCY MEASURMENT” mode perform the direct comparing of researched signal frequency with standard frequency value (quartz-crystal generator). Here that gives opportunity to find with digital method the number that will show in how many times bigger than .

On pic. 2 are shown the diagrams of EFC voltages that reveal the point of digital method – the calculation of pulses quantity N coming on counting block (11) during time – “MEASURING TIME” or “COUNTING TIME”.

From diagram follows that the actual value of researched signal frequency equal to:

 

.

 

Measuring of researched signal frequency value:

 

.

 

The value of absolute error of discreteness:

 

 

The maximum value of relative error of discreetness defined by next formula:

 

where, N – quantity of pulses that comes on the counting block of EFC during counting time.

Total error of frequency measuring:

 

 

where – error component causing by quartz-crystal generator.

 

2.3 “PERIOD MEASURMENT” mode perform comparing of researched signal period with standard time interval. Diagrams of EFC voltages are shown on pic. 3. We can see from diagrams that actual value of researched signal period is defined by:

 

 

where – period of standard (master) signal;

N – marks quantity that comes on counting block;

n – period factor.

Counting time is defined by:

 

 

The measuring period value of researched signal:

 

 

The values of relative and absolute error of discreetness are equal to:

 

 

where

Total relative error of measuring:

 

 

where – transformation error caused by ratio of signal/obstacle voltage.

 

2.4 In “FREQUENCIES RATIO” mode the voltage with high frequency (the position of key on switch “1”) is applied on the input “1” (pic.1). In channel 1 this voltage transforms in sequence of short pulses with repetition rate. This pulses comes on the first input of time selector while on the second input is applied the control pulse with duration . Formation of control pulse is performed in channel “2” from the signal with lower frequency applied on the input “2”.

 

From pic. 4 follows equality:

 

 

The actual value of frequency ratio equal to:

 

 

The measuring value of frequencies ratio:

 

 

Counting time:

 

 

The absolute error of discreetness:

 

 

The relative error of discreetness:

 

 

Where

 

Total error of measurement:

 

 

 

Key questions

3.1 Why EFCs are related to high accuracy and universal measuring tool?

3.2 What is the main point of measuring method of EFC?

3.3 Explain the principle of EFC work by modes:

- frequency;

- time interval;

- frequency ratio.

3.4 List the errors that occur in EFC in different modes of work.

3.5 Find out methods of decreasing of measurement errors.

3.6 Explain how the total error determines in different work mods.

3.7 Explain how to determine the position of decimal point in different work mods.

 

Home task

4.1 Research the structural scheme of given EFC and the principle of work in different modes. To draw the structural scheme of EFC.

4.2 Using the technical characteristics of EFC choose time counting that provides the minimal error in frequency measuring mode. The result show in table 4.1. The variants of the tasks given in table 1 (appendix A); here .

 

Table 4.1 – Basic characteristics evaluation in “FREQUENCY MEASURMENT” mode.

ƒs, kHz	Тcount, ms	Expected result, kHz	∆ N, Hz	N	δ N

 

4.3 Using the technical specifications of EFC choose the parameters of period measurement mode that provide minimal measurement error of interval. The result is show in table 4.2. It is recommended to provide calculations on same frequencies as in table 1 (appendix A).

 

Table 4.2 – Basic characteristics evaluation in “PERIOD MEASURMENT” mode.

ƒs, kHz	n	Т м	Expected result (ms, µs)	∆ N	N	δ N	Тcount, ms

 

4.4 Using technical specifications of EFC choose parameters of the frequency ratio mode that will provide minimal measurement error. The results show in table 4.3 (the frequencies are given in appendix A).

 

Table 4.3 – Basic characteristics evaluation in “FREQUENCIES RATIO” mode.

ƒ1/ƒ2	n	Expected result	∆ N	N	δ N	Тcount, ms

 

 

Laboratory task

5.1 Research the counter. Establish a correspondence between governments, shown on the front panel of the device and the structural scheme (pic 1).

5.2 To make the external research of EFC in self-control mode.

5.3 To make research in all work modes.

5.4 In frequency measurement mode perform validation of feature selection on the frequencies given in table 1 (appendix A). The results show in table 5.1.

 

Table 5.1 – Basic characteristics evaluation in “FREQUENCY MEASURMENT” mode.

ƒs, kHz	Тcount, ms	Result, kHz	∆ N, Hz	N	δ N

 

5.5 In period measurement mode perform the same measuring. The results show in table 5.2.

 

Table 5.2 – Basic characteristics evaluation in “PERIOD MEASURMENT” mode.

ƒs, kHz	n	Т м	Result (ms, µs)	∆ N	N	δ N	Тcount, ms

 

 

5.6 In frequencies ratio mode perform the same measuring. The results show in table 5.3.

 

 

Table 5.3 – Basic characteristics evaluation in “FREQUENCIES RATIO” mode.

ƒ1/ƒ2	n	Result	∆ N	N	δ N	Тcount, ms

 

6. Protocol of the laboratory work (№.... name...)

6.1 The purpose of the experiment.

6.2 List of equipment used in the Table. 6.1.

 

Table 6.1 - List of equipment used

Name of the device	type	serial number	Metrological characteristics
Accuracy	Range	Frequency

 

6.3 Measuring chart.

6.4 To build in one system of coordinates of graphic arts of dependence of relative error of δ _N from frequency in the mode of measuring of frequency and period. To draw a conclusion about the choice of the mode of operations of EFC.

 

 

LABORATORY WORK №7a