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Old 09-30-2011, 08:46 PM
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Default Frequency Counter

Hi everyone!

I know I haven't been very active on here for a week or so now, but I'm still interested in learning from you folks. I kind of feel a bit bad asking for lots of help, but not helping out with some of the more basic questions which appear on here, so please forgive me for that.

It's the weekend now, so I'll finish my maths homework and have a look at the boards to see if there are any questions I can help with.


Anyhow, here's my question.

I'm considering making a frequency counting circuit, with a seven segment display output. I'm not too concerned about the range of measurable frequency with the project, but I would like to have a fairly wide range, to test anything in the near future.

A quick search on Google revealed this circuit;

http://www.eleccircuit.com/wp-conten...-451174190.gif

However, there's a couple of things I don't understand, and would appreciate some help with.

1. what are the switches along the top for? That's a lot of switches to have on the control panel, so I can't believe that they are for general operation.

2. As for the actual input, would that be via the 'load'? If not, what would be the input to the circuit?

3. What does it mean by UP/DN?

4. What does it mean by latch?


My apologies for the number of questions, I have tried my hardest to get a grasp of this circuit (by looking at datasheets, etc) but have so far failed.

I'm also sorry if I haven't provided you with sufficient information. Please ask for more if you need some.

Thanks for your time guys,

Sparky
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Old 09-30-2011, 08:50 PM
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Hello,

The switches on top are there for adding or subtraction the MF frequency when used in a radio.
The counter will be connected to the mixing oscillator wich is the MF frequency off.

The up/down is what it says , count up or down.

The latch is to store the counts into the display.

Bertus
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Old 09-30-2011, 08:58 PM
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Thanks for the uber quick reply Bertus!

So this isn't the complete circuit?
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Old 09-30-2011, 09:01 PM
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Hello,

No, You will need a timebase with the signals for the gate and latch.

Bertus
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Old 09-30-2011, 09:12 PM
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Frequency counters are one of the simpler circuits to build for DIY test equipment. If you go to 7 digits you should consider a crystal oven, such as the one The RB has at his website.

http://www.romanblack.com/xoven.htm

Basically all the time base does is open a gate for a even increment of a second or less.

It is also a good idea to reverse it and allow access to the counter. That way you can do odd items like count coils of wire while winding them, or measure periods of waveforms.
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Old 09-30-2011, 09:19 PM
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A lot can be gleaned by looking over the 74190 datasheet. It's an Up/Down Decade Counter. So UP/DN controls the direction it will count. CLOCK is the signal that will be used to increment/decrement the counter. And LOAD forces the counters to take the parallel inputs and uses that to determine the count (thereby overriding the counting function).

Since the parallel inputs are connected to the DIP switches you can then assume that they are used to test the circuit by setting a particular value for each digit.

LATCH does what you'd expect; it latches the value onto the display.
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Old 09-30-2011, 09:28 PM
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The OP may not fully understand what you are talking about.

If you are planning to build a straight forward frequency meter, you don't need the switches.

I built a frequency counter similar to this one to be used on an analog radio receiver (anyone remember those?). You selected the radio station by turning a tuning dial which had a string attached to it and moved a pointer across the front of a scale (like a ruler) to indicate the station frequency. This was in the days before digital readouts. The string from the tuning dial was also wrapped around a wheel that was attached to the spindle of a variable air-gapped capacitor. The capacitor was part of an oscillator circuit whose frequency was controlled by the capacitor, and hence the tuning dial. (Are you still with me?)

So, the objective of the digital display is to show the frequency of the station you wanted to listen to. This feature is particularly important if you were a DX'er, a person who searches the radio bands for far away radio stations. A DX'er would usually have a station guide, a listing of all radio stations from around the globe, showing operating frequency, power transmitted and program times (the definitive one being WRTH - World Radio TV Handbook). So a DX'er, would be searching for a distant radio station based on the transmitted frequency.

So the frequency meter gets its input from the oscillator called the local oscillator. For the AM band (or Medium Wave band) it is common to set the local oscillator to 455KHz above the transmitted signal. When the local oscillator signal is mixed with the incoming radio signal, you get a 455KHz signal (plus a whole bunch of other unwanted signals). The beauty here is that if you tune all your intermediate amplifiers to the one 455KHz signal you will pick up the single radio station that you are interested in. The 455KHz is called the intermediate frequency or IF.

The problem with your frequency counter is that it is now reading 455 too high. If you can preload your counters with -455, you would in fact be able to subtract this 455 and therefore be able to display the proper station frequency.

Why the latches? You want to be able to display your results even while you are counting. Hence the latches, holds, freezes the display to show the last results while the new results are being counted.



Here is a picture of my DX-150B Communications Receiver with my homemade frequency readout. It has a three-way switch that allows me to preset the counter to 455, 0, or 99545 in order to add or subtract the IF of 455KHz.

Hope this helps.
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Old 10-01-2011, 09:42 AM
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Thanks to everyone to who posted on this, it's been really helpful.

Mr. Chips has hit it on the head though, I didn't know what some of you were talking about... I tried to understand, really!

Mr. Chips, I'm not looking to use it to measure an incomming radio signal, as your fine example does. I thought it might be nice to have something which I could link a couple of probes to, which I could use to measure the frequency of a signal some where in my circuit.

Kind of like an oscilloscope, but without all the hassle of the seconds per division and guesstimating that it about 3/4s along the line...

I understand that this won't be very 'simple', but I'm up for the challenge.

So what sort of circuits will I need for the clock and latch inputs? Is it a simple as a switch and resistor?

Sparky
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Old 10-01-2011, 02:51 PM
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A frequency counter circuit is a relatively simple project to tackle. It is a classic design exercise in a college or university level course on digital electronics. For this reason, I cannot post a full solution on this forum.

Students are generally asked to present three drawings in their report, 1) block diagram, 2) timing diagram and 3) schematic diagram.

To understand what goes into the timing diagram, you have to imagine that the user of a frequency counter is not interested in seeing the counter display 0-1-2-3- etc. The counter should only display the final count.

The basic timing sequence is:
RESET-COUNT-RESET-COUNT...
The time duration of COUNT will determine the display resolution and the number of digits to display.

To incorporate the display latches into the design, the timing diagram would be modified as follows:
RESET-COUNT-LATCH-RESET-COUNT-LATCH...

Note: All timing diagrams would be drawn as volts vs time such as in an oscilloscope waveform.
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Old 10-01-2011, 04:10 PM
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To keep your counter/display circuit less complex, you might consider using a pair of 4553 3-digit BCD counters and a pair of 4543 7-segment display drivers. That's just four ICs (plus three driver transistors & a number of resistors) for a 6 digit display.

Have a look at this datasheet:
http://www.onsemi.com/pub_link/Colla...MC14553B-D.PDF
Page 6, figure 5.

You'll also want to add some functions like an overflow indicator (after all, you want to know if the incoming signal is higher frequency than you can count, right?), a prescaler (a /n circuit) so that you can count higher frequencies at the cost of lower resolution, and possibly other features.
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