# 3-digit Frequency Counter

1. ## Re: 3-digit Frequency Counter

Practically 7490 is good for upto 40MHz. A 4 digit display showing for example 36.76MHz can be achived. Similarly setting duty cycle to 95% (5% time for counting and rest of the time for display) will divide by 10.
1.2MHz display with 50Hz clock with 95% duty cycle.

After adding decimal point for making count from 0 to 9.999MHz and adjusting initial rising pulse and with 5% duty cycle.

Resolution is now 1KHz. That means you will get 0 reading until your input pass 999Hz. Display will show 1 at 1000Hz-1999Hz.
There are frequency dividers used for higher frequencies to divide incoming signal before entering frequency counters often called prescaler.
http://en.wikipedia.org/wiki/Prescaler

From initial posts,

It is fine to reduce the time window so that it will divide by 10 or 100 or 1000, in case your incoming frequency is too high for your 3 digits to accommodate.

Likewise you can extend the time window to 10 seconds, so that the counter can handle an incoming frequency slower than 1 Hz.
It is fine to reduce the time window so that it will divide by 10 or 100 or 1000, in case your incoming frequency is too high for your 3 digits to accommodate.

Likewise you can extend the time window to 10 seconds, so that the counter can handle an incoming frequency slower than 1 Hz.
Sorry sir, didn't understand this.
Does it make sense now?

2 members found this post helpful.

•

2. ## Re: 3-digit Frequency Counter

Dear All Experts
Thanks for your replies. if i will set my clock to 500khz. what is the frequency where it start count and what is the end frequency. i mean what is the frequency range which will show on display.now can u explain the complete circuit operation of each ic step by step because on monday my lab viva day. i have to explain each and every step of circuit. it is so helpful for me and i again thanks in advance please helped me. please help.
Dear Eshal are you from Karachi University??

3. ## Re: 3-digit Frequency Counter

Try to understand by revising the thread. You can ask if you have a confusion. Simulate it yourself with different inputs and try to explain the response you get.

4. ## Re: 3-digit Frequency Counter

sir please tell me when we have set Clock to 0.5 Hz its mean we can start count from 1 hz. if we apply 10 hz signal on pin 14 then clock on pin 3 and pin 6 is ground. when clock is low and pin 6 is also low then it start counting, the clock is same for all ICs. my confusion is this when first IC reach at count 9 and how it is zero and increment to 1 because 7490 reset all outputs to zero when pin 3 clock is high and pin 6 is already low. but in this case pin 3 is low and pin 6 is also low then how first digit display zero and second display one. please help me out. again thanks in advance. my main confusion is how first ic show 0 in 10hz or 50 hz etc. i little bit understand the increment of next stage but how the first ic show 0 i am confused.

- - - Updated - - -

I understand how it reset to zero. it is a decade counter so after nine it reset to 0. is it right or no and on high to low pulse is next ic pin 14 so it read 1. right

5. ## Re: 3-digit Frequency Counter

Now you came to right conclusion. Count is 0-9 not 1-10. A single digit can only show 0-9 not 10. At tenth pulse it shows 0 but a carry pulse is there for another counter for tens of count.The significant digit of 10 that is 1. 0 is least significant digit, the right most. It shows unit value.
http://en.wikipedia.org/wiki/Least_significant_bit

http://pages.cs.wisc.edu/~cs354-2/cs...s/numbers.html

6. ## Re: 3-digit Frequency Counter

sources:
http://en.wikipedia.org/wiki/Frequency_counter
http://www.ehow.com/list_7500738_fre...fications.html
http://www.ehow.com/how-does_5164850...ter-works.html
http://www.ehow.com/how_5022745_use-...y-counter.html

Frequency is the number of repetitions of a periodic process per 1 second and is measured in Hertz (Hz). Light or sound are examples of such periodic processes and are called electromagnetic waves. Frequency is a way to distinguish among different types of waves. For instance, the frequency of sound is between 20 to 20,000 Hz, while the frequency of light is about one trillion times higher. It can be calculated either from the energy or the wavelength using the fundamental physical constants.

A frequency counter is an electronic instrument, or component of one, that is used for measuring frequency. Frequency is defined as the number of events of a particular sort occurring in a set period of time. Frequency counters usually measure the number of oscillations or pulses per second in a repetitive electronic signal.
A frequency counter may be the simplest piece of electronic test equipment to operate, but perhaps the hardest to understand. You apply a signal to the input, and the frequency appears on a digital display. Although the manual may have several pages of specifications, you can understand them better if you break them down into those that will suit your application.

Frequency Range
The range tells you what frequencies it will measure. Audio frequencies run from 20 HZ to 20,000 HZ. Microwave frequencies run up to 30 or more GHz. Some counters have models or plug-in units for various frequency bands. Expect the price to increase with higher frequencies and wider ranges.

Power Range
The power range tells how strong a signal the counter will read. Also, some signals ride on DC voltage that the counter must withstand. If you want to read the frequency at the output to a transmitter amplifier, you need a counter that can withstand high power. But that's not necessary at the input to the amplifier.

Accuracy
Accuracy tells how closely the counter measures the actual signal. Specifications can be illustrated in percentages or parts per million. Although price usually increases with accuracy, technological advances tend to keep lowering the price of units offering adequate accuracy.

Resolution
The resolution specification tells how pure the signal must be for the counter to read it. Complex formulas describe resolution specifications and circuits, but your application really determines what resolution you need. Usually it isn't too great of a concern for most frequency-measurement applications.

Environmental
Environmental specifications tell what climates in which you can operate and store the counter, and include temperature, humidity and altitude. Some counters also include specifications for dust, pollutants, physical shock or vibrations.

Operating principle

Most frequency counters work by using a counter which accumulates the number of events occurring within a specific period of time. After a preset period (1 second, for example), the value in the counter is transferred to a display and the counter is reset to zero. If the event being measured repeats itself with sufficient stability and the frequency is considerably lower than that of the clock oscillator being used, the resolution of the measurement can be greatly improved by measuring the time required for an entire number of cycles, rather than counting the number of entire cycles observed for a pre-set duration (often referred to as the reciprocal technique). The internal oscillator which provides the time signals is called the timebase, and must be calibrated very accurately.
The accuracy of a frequency counter is strongly dependent on the stability of its timebase. A time base is usually a fixed frequency or a fixed period in time used as a comparison against which other frequencies or timed events are calculated. A time base can be established to meet your requirements for purposes of measurement.

Input
You connect the signal you're measuring to the frequency counter's input. Since you may be sending it any kind of waveform, the frequency counter conditions the signal and converts it to rectangular pulses, which are easily counted. The input will have a range of sensitivities depending on the model, typically from a few millivolts to a few volts peak-to-peak.

Gate
Before the signal is counted, it passes through a gate circuit. It's a precisely timed digital switch that is either open or closed. It has two main modes: frequency count and event count. Frequency count lets you measure the frequency of the input signal. In this case, the gate will open for a time period, passing the signal to the counter and letting the counts accumulate. For example, the gate may turn on for 1 second, so the counter counts pulses from your signal for that long. After 1 second, the gate closes and lets the counter display its result. Then it resets the count to 0 and opens again.

Event count keeps the gate open, letting you count all the signal events you send to the counter. You can reset the count with a manual push-button switch or external pulse. Event counting lets you determine the total number of events occurring in an arbitrary time period, from seconds to weeks or longer.

Counter
The counter circuit has a set of divide-by-10 stages. The total number of stages is the number of display digits minus 1. The stages are chained together, so the first stage divides by 10, the next by 100 and so on. The counter outputs are used to drive the display.

Latch
The latch is a simple memory circuit that holds the last count. Typically, the counter will be counting for the current gate period, but the latch will have the results from the last count. When the current gate period is over, it resets the latch and gives it the latest count. This improves readability. Without the latch, you'd see a fluttering of numbers as the counter counts from 0 to your frequency every second. With the latch, you see only the actual frequency, with updates every second.

Display
The frequency counter's display is a set of decimal digits driven by the counter and latch. A typical frequency counter has six or seven digits. They may be seven-segment LED, LCD or other technology. In addition to showing the frequency count, the display may have indicators for mode, gate time or battery condition.

Quartz Standard
The frequency standard determines the accuracy of the gate. A typical standard is a quartz crystal in a heated chamber. Keeping it at a known temperature improves the counter's accuracy. If the crystal runs at a frequency of 100,000 hertz, a separate set of dividers reduces it to 1 pulse per second. A separate counter may divide it further to 1 pulse every 10 seconds. The longer the gate is open, the more precise the frequency measurement can be, with the trade-off that it takes longer.

Usually any frequency counter has some sort of input signal conditioning that turns your input signal into a square-wave. So it just counts the cycles.
If you fed white noise into something like that, you would just see garbage random numbers. There is really no point in trying to measure the frequency of a complex rapidly fluctuating signal, because it probably does not have one.

Of the things you'll find on a technician's workbench, a frequency counter is one of the easiest to use. Their main purpose, measuring frequency, is done by setting a few front panel switches. Spending a few minutes with a frequency counter and a test oscillator will show you what you need to know.
Things You'll Need
Frequency Counter
Oscillator or Function Generator
Cable with BNC female connectors at both ends

Instructions
1: Connect oscillator to frequency counter using the BNC cable.
2: Turn oscillator and frequency counter power on.
3: Select a pure, unmodulated waveform from the oscillator: sine, triangle, or pulse.
4: Set the oscillator's amplitude (output level) about halfway. Set its frequency to about 1000 Hz.
5: Set the frequency counter to the lowest frequency range. Set its gate to one per second. If it has a frequency/period mode, set it to frequency.
6: If the frequency counter has a "Hold" button, press it. The display should hold the same count. Press "Hold" again to resume normal operation.
7: Change the counter's mode to "Period," if it has that mode. It should now display a time interval of about .001 seconds.
8: Change the oscillator frequency. The counter should show the new frequency momentarily.
9: Change the "Gate" setting. The display should update less often but with higher resolution.

4 members found this post helpful.

7. ## Re: 3-digit Frequency Counter

Thank you very much. i m so glad to see detail information. its very good for me to prepare for my viva. another question if i do not change clock from 0.5hz means my clock is fixed at 0.5hz and now i want to see frequency in Mhz, now tell me what is the circuit configuration to measure frequency in Mhz with same components. because my teacher ask this question that do not change clock and can i read frequency in Mhz??

- - - Updated - - -

Thank you very much. i m so glad to see detail information. its very good for me to prepare for my viva. another question if i do not change clock from 0.5hz means my clock is fixed at 0.5hz and now i want to see frequency in Mhz, now tell me what is the circuit configuration to measure frequency in Mhz with same components. because my teacher ask this question that do not change clock and can i read frequency in Mhz??

8. ## Re: 3-digit Frequency Counter

if i do not change clock from 0.5hz means my clock is fixed at 0.5hz and now i want to see frequency in Mhz, now tell me what is the circuit configuration to measure frequency in Mhz with same components
Reducing the duty cycle can accomplish this requirement. Instead of 50%, you can set 5%, 0.5%, 0.05% etc for higher divisions.

1 members found this post helpful.

9. ## Re: 3-digit Frequency Counter

can u explain how duty cycle effect on 7490. explain with different on times and off times and then explain how it will count in Mhz and also division. what is the effect of duty cycle on 7490

•

10. ## Re: 3-digit Frequency Counter

In the attachment in the post #41 it is implemented in same way. If you see properties of gating signal, it is set to 50Hz 5% duty cycle instead of 500Hz 50% duty cycle. Both settings have same result. Then again 5 Hz 0.5% duty cycle will have same result.

2 members found this post helpful.

11. ## Re: 3-digit Frequency Counter

I don't know how you make simple things. However, you have helped me a lot. I have no words to thank you. But you will always be the part of mine prayers.

•

12. ## Re: 3-digit Frequency Counter

Today is my Viva day. after viva i will again reply you how is my Viva. So much thanks for your replies. any suggestion for me then tell me. one question how u have set Helped Me sign i want to set in my profile. this is not related to this discussion.

1 members found this post helpful.

13. ## Re: 3-digit Frequency Counter

Originally Posted by engmunir
one question how u have set Helped Me sign i want to set in my profile.
You got one. We can't see our own "helped me" button, but everyone else can see it.

- - - Updated - - -

I clicked yours to demonstrate.

14. ## Re: 3-digit Frequency Counter

My project is completed and task successfully. Thank you all. especially, ALTERLINKS.
I am going to mark this thread as solved.

--[[ ]]--