measuring frequency 2

[buzzly]

Hi! Do any of you know of any methods of measuring frequency besides using a digital counter? r there any ways of improving the quality of the measurement like using a multiplying pll to increase accuracy? Thanks...

 

[pebe]

melone,
You seem to be going to great lengths to prove that a scope is the best tool to use. Yet you don't know just what buzzly needs to measure - or why. Info that buzzly has not submitted is:

What range of frequencies? What accuracy? Is waveform important? Is it a one-off measurement, or for continuous monitoring? What action is to be taken with the data? Are records to be kept?

Until this is known, the best instument to use cannot be decided.

 

[nbucska]

Pebe you are right
Pebe you are right


It is a BIG mistake to solve an unknown problem !
It is a BIG mistake to solve an unknown problem !
<nbucska@pcperipherals.com>

 

[melone]

PEBE and NBUCSKA you have both hit the nail on the head! If we don't understand the problem, why would ANYONE suggest a solution that has SO MANY INHERENT FLAWS? Why not err on the side of caution and suggest a solution that will work (regardless of the end users complexity, especially since it will probably change of the lifetime of his project), instead of trying to get fancy by making some pretty big assumptions?

You are right, I am going out of my way to prove that the scope is the best tool for the job, because the scope IS the tool for this job. Not only will it give the most accurate results, it offers more functionality than is currently required. My recommendation is based on what the user needs today, and what he is likely to need tomorrow (based on his VERY limited information). Frequency by itself does not give you the whole story! 2 signals can have the exact same frequency, but that doesn't mean that both signals are the same. Try putting a sawtooth waveform into a clock input that is designed for square waves.

 

[nbucska]

I think it is about time that someone answers to the original question:

The best way to improve the accuracy is :
1.) Use more accurate reference frequency
2.) and more time to measure.

Digital counter's accuracy is unlimited, given
ideal reference and infinite time.

<nbucska@pcperipherals.com>

 

[biomedicalEng]

hi buzzly

there are so many chips that measure the frequency directly
and you don't need to setup a microprocessor based circuit to measure frequency

here i introduce the most widely use frequency to voltage converter of the world!!!
it's also very low cost

national semiconductor's LM2907/LM2917 Frequency to Voltage Converter

General Description:
The LM2907, LM2917 series are monolithic frequency to
voltage converters with a high gain op amp/comparator designed
to operate a relay, lamp, or other load when the input
frequency reaches or exceeds a selected rate. The tachometer
uses a charge pump technique and offers frequency
doubling for low ripple, full input protection in two versions
(LM2907-8, LM2917-8) and its output swings to ground for a
zero frequency input.



Advantages:

* Output swings to ground for zero frequency input
* Easy to use; VOUT = fIN * VCC * R1 * C1
* Only one RC network provides frequency doubling
* Zener regulator on chip allows accurate and stable frequency

Applications:

* Over/under speed sensing
* Frequency to voltage conversion (tachometer)
* Speedometers
* Breaker point dwell meters
* Hand-held tachometer
* Speed governors
* Cruise control
* Automotive door lock control
* Clutch control
* Horn control
* Touch or sound switches


and many other user defiend applications!!!!

like your application

if you need more go to

http://www.national.com

and download it's data sheet.

 

[pebe]

melone,
If someone wants to know how to crack a nut, it is better to ask how big the nut is before offering him a sledge hammer.

 

[melone]

Or maybe, we should simply over up the solution that will just barely meet the requirements and offers a whole host of problems that require a varied set of troubleshooting skills to solve. Perhaps we can use some of our wisdom to look past the original post, and try to figure out what this person NEEDS and not only what he/asked for.

 

[buzzp]

Melone, How do you know the best tool for the job when you don't know what the job is?

 

[melone]

You guys are right....I should have simply answered the question as it was stated. I'm am sorry that considering an option that exceeds the stated requirements by allowing for future functionality is so unacceptable.

BTW, could someone PLEASE explain why the oscilloscope is so repulsive? I don't think that anyone has offered a good explanation of why one of the most commonly used pieces of lab equipment is no longer acceptable to measure frequency. Also, why do we think that we would NEVER actually want to look at the waveform? Call me old-fashioned, but I kinda like to know what I'm dealing with.

 

[ve7brz]

Keep it up guys, this is good fun! I draw your attention to the fourth posting in this thread. The fellow asked how to convert a period reading into a frequeny reading. Enough said!

 

[melone]

That is exactly my point. This person doesn't know what he/she needs, and I was trying to offer up a solution that allows for future expansion without having to go through the trouble of major hardware / software debugging.

 

[buzzp]

So we agree that no-one knows what exactly he wants to do and why. I simply gave another possible solution. Since the scope would be obvious and was already suggested, I didn't think it was necessary to repeat that suggestion. Measuring frequency with a micro is not that difficult (depends on experience level with micros, board layout, blah blah). I just don't appreciate the slams against my suggestions when it would be the BEST solution for some applications. The inherent flaws are minimal with micros if you know what your doing. And maybe this guy does not have several grand to shell out for a feature rich scope. Bottom line: no-one knows his application so no-one can say what is best so quit being so dang arrogant Melone!! Would you want to leave your multi-thousand dollar scope outside in a 3R box with temperatures down to 30 below or 130 above? Absolutely not! This is one example where a scope would not be the best choice. Also, a microcontroller circuit could be built for way under $50 to handle this application. If this needs to be done in more than one location at the same time, then a scope would not make sense unless the locations are such that a multi channel scope could be hooked to all at the same time. Obviously, there are many applications where a scope would be the worst choice and applications where the micro circuit would be the worst. I think you need to just relax and take a deep breath before you start going off and being so sarcastic, Melone.
Before I stoop to such a low level, this is my last post unless something meaningful is posted.

 

[melone]

I guess we agree to disagree on this point, and I appreciate your explanation of your posistion. I hope that you accept my apologies for the sarcasm.

However, let's examine why my suggestion make sense. 1) The user asked a vague question that suggests that they either don't know what they need, or they are looking for a unique solution. When the micro solution was suggested, he/she admitted that they have never used a micro before. Therefore, this person has probably never debugged a micro project before.

2) Let's go on the assumption that this person doesn't know what they need and after some troubleshooting / debugging, they decide that they need additional functionality. If it is easily added in software, then it doesn't cost anything except time. Otherwise, it would require more hardware and additional software (troubleshooting and debugging).

3) I have tried to be impartial and simply suggest a solution that will work for MOST situations. Then when I tried to get someone to explain to me why the micro solution is better, people got defensive and would not consider other options. I agree that in certain circumstances, the scope is the worst tool for the job. However, you must admit, that for most circumstances, it is the best. I took an educated guess at what the poster needed, and suggested a solution based on some assumptions. If ANYONE would have given an answer other than, &quot;a scope is not the only solution&quot;, I think this thread would have gone in a different direction.

Finally, I know that you, BUZZP, are probably pretty upset with this discussion, but I am very happy that there are people out there that are passionate about helping others! People like you are what makes this site so great, and I hope that our disagreements don't disuade you from helping others with your knowledge. I have read many of your posts and have always been impressed with your knowledge and understanding of technical matters.

 

[ve7brz]

Cool down Buzz,

I've had this thread marked and have been watching it faithfully. I can agree with the points all have you have made. If the Genie in the bottle were to grant me ONE wish it would be to walk into HP (or your favourite manufacturer of the best equipment) with a blank cheque.

On the other hand I'm quite competent to design and build equipment as good as or better than any of them could offer. However, time is money, and that approach wouldn't be economically sound.

What I ask you to take note of, is regardless of your approach, that you remember you're dealing with a neophyte here; someone who hasn't yet learned to convert a period measurement to a frequency measurement. What good is either top-of-the-line or sophisticated homebuilt equipment to him? This thread is great for those of us in a position to argue the virtues of one approach over the other, but keep it simple.

 

[itsmeRICK]

No shortage of solutions. Make a one-shot with a pulse width much less than the minimum period and pass the pulse through a low pass filter. For your application, a 25 us one-shot should work. Make the low pass filter corner (it can be a simple RC) somewhere around 1 kHz if that's fast enough for you.

 

[sunny123]

Hi folks!
I just rummaged through this thread for the first time and it is highly interesting.
I am far from being initiated into electronics. Hence, please excuse me if I am being irrelevent. But I can't help but suggest the following:
Convert the alternaing signal to a pulse using possibly a diode and associated circuitry which I suppose should not be complicated. Input this to an electronic switch compatible with Personal Computer (after moderating the pulse voltage/current values and/or matching the output and input impedences respectively of the circuit giving out pulse to that of the electronic switch). The switch then sends zeros and ones to the PC. And a suitable free software can count the pulses and the associated time and calculate the frequency.
This way, you can plot the graph, monitor continuously and format the display to your liking.
And foremost, a PC is the most likely available instrument anywhere.
Hope I am not too much off the mark.
With utmost humility.
Happy NEW YEAR to all.
sunny123

 

[IRstuff]

You could buy an A/D board with 100 kps performance for $395:

http://www.accesio.com/go.cgi?p=../analog_input/a1216e.html

TTFN