frequency meter in a digital function generator

[samfisher92]

im working out on a digital function generator.

I got the waveform generation stage worked out.

Now i need to work out the digital display which i pretty much i know going to have an ADC which is most slightly the max183 ic from maxim, to convert the analog ac voltage of the waveform to binary form and have a display driver to decode the binary and display it on some 7 segment led.

now i need to design another set of circuits to pick out the frequency of the waveform and display it on a LED display.

Any ideas?

On a side note, because the function generator is generating AC waveforms which have AC voltages, can an ADC really convert that AC voltage into digital form? does the ADC even care if its DC or AC in the first place?

[HalfSpace]

Hi samfisher92,

What do you want to display, just the frequency of the function generator?

HalfSpace 

[dannyf]

have an ADC

My experience with people who utilize adc to measure frequency is that they are either

1) extremely new to the field; or
2) extremely experienced and know exactly what they are doing.

[samfisher92]

have an ADC

My experience with people who utilize adc to measure frequency is that they are either

1) extremely new to the field; or
2) extremely experienced and know exactly what they are doing.

I am 1). I am new to ADC. I just recently started learning about the types of ADC like the sar type, dual slope and I learned the flash type in high school.

[samfisher92]

Hi samfisher92,

What do you want to display, just the frequency of the function generator?

HalfSpace 

Yup I just want to display the frequency of the  function generator

[PA0PBZ]

Hi samfisher92,

What do you want to display, just the frequency of the function generator?

HalfSpace 

Yup I just want to display the frequency of the  function generator

I'm not pretending to better know what you want than yourself, but from what I could decypher from your first post I got the impression that you want to display the output voltage using an ADC and some other circuit to display the frequency?

[samfisher92]

I'm not pretending to better know what you want than yourself, but from what I could decypher from your first post I got the impression that you want to display the output voltage using an ADC and some other circuit to display the frequency?

thats right. i wish to isolate the hardware needed to measure these 2 parameters.

[oldway]

Hameg is using an ADC to display the frequency in the function generator HM8030-2.
http://www.hameg.com/manuals.0.html?&no_cache=1&L=0

[Kalvin]

There are quite a few frequency counter designs in old hobby electronics magazines like Elektor, Everyday Practical Electronics etc. over the years. The older designs used quite many 74LS-series logic ICs and 7-segment displays. The newer designs are using microprocessors like Arduino etc. with LCD display.

The amplitude measurement can be done using either an analog meter or a digital display. Typically the output voltage of a function generator is given as peak voltage (instead of an average or RMS voltage) so you may want to construct a peak voltage detector which will then feed the voltage/amplitude meter/display.

There are also some complete kits and/or just plain PCB available if you are inexperienced and want to take a bit easier route than starting the design and building from scratch.

Br,
Kalvin

[samfisher92]

Hameg is using an ADC to display the frequency in the function generator HM8030-2.
http://www.hameg.com/manuals.0.html?&no_cache=1&L=0

I am really glad for your help. this is a really big clue and a big step into completing the hardware.

thanks a lot. 

[fcb]

LM2917 is still available.

[samfisher92]

There are quite a few frequency counter designs in old hobby electronics magazines like Elektor, Everyday Practical Electronics etc. over the years. The older designs used quite many 74LS-series logic ICs and 7-segment displays. The newer designs are using microprocessors like Arduino etc. with LCD display.

The amplitude measurement can be done using either an analog meter or a digital display. Typically the output voltage of a function generator is given as peak voltage (instead of an average or RMS voltage) so you may want to construct a peak voltage detector which will then feed the voltage/amplitude meter/display.

There are also some complete kits and/or just plain PCB available if you are inexperienced and want to take a bit easier route than starting the design and building from scratch.

Br,
Kalvin

I thought of a peak voltage detector to measure the V_peak of the waveform but I didnt give it much thought about integrating it into the whole project. And now since you made sense of it, well i think there's a real need to include it. well i didn't go into much research about the frequency counters using the 74LS chips because back in school I did learn about counter circuits specifically like the 74192 for the BCD type and 74193 for the binary type and some display drivers like the 7447 as the output IC. I used the CD4026 before too but im pretty out of touch with these ICs. haha all these knowledge have rusted anyway. trying to find my way back into doing such fun stuffs.

[samfisher92]

LM2917 is still available.

I guess that IC is not applicable to my project..

[fcb]

LM2917 is still available.

I guess that IC is not applicable to my project..

Huh?  What are you actually trying to do???  You talk about using an ADC to measure frequency - it's not the way I would do it, but hey, you know what you want??

The LM2917 converts a frequency into an analog voltage, which you would then read with your voltmeter>LED block (or whatever).

Perhaps you should describe exactly what you want or have built, along with specifications. It is less frustrating.

[oldway]

Have a look on the schematic of the Hameg HM8030 and you will understand how it works.
Frequency is proportional to voltage (VCO of the function generator), then, if you displays this voltage (with adequate voltage divisor), you are displaying the frequency.
It's not necessary to convert output of the VCO (frequency) to an analog voltage, you already have this analog signal.

I agree, that's not a high precision reading, as you are not really measuring the frequency, but that's enough....those 8000 series plug-in's are not intended for profissional use and linearity of the VCO is quite nice.
You could also use the same ADC circuit to display the output voltage. (with a switch frequency/RMS output voltage and an AD8436)
NB: AD8436 is limited at 800Khz.

[samfisher92]

LM2917 is still available.

I guess that IC is not applicable to my project..

Huh?  What are you actually trying to do???  You talk about using an ADC to measure frequency - it's not the way I would do it, but hey, you know what you want??

The LM2917 converts a frequency into an analog voltage, which you would then read with your voltmeter>LED block (or whatever).

Perhaps you should describe exactly what you want or have built, along with specifications. It is less frustrating.

well im trying to use an ADC to convert the frequency into a digital code. not sure if its a right thing to do but your idea of a F/V converter do sound like a way to work around that. I should read up on its theory of operation. Im pretty new to that type of converters.

Thanks thanks

[samfisher92]

Have a look on the schematic of the Hameg HM8030 and you will understand how it works.
Frequency is proportional to voltage (VCO of the function generator), then, if you displays this voltage (with adequate voltage divisor), you are displaying the frequency.
It's not necessary to convert output of the VCO (frequency) to an analog voltage, you already have this analog signal.

I agree, that's not a high precision reading, as you are not really measuring the frequency, but that's enough....those 8000 series plug-in's are not intended for profissional use and linearity of the VCO is quite nice.
You could also use the same ADC circuit to display the output voltage. (with a switch frequency/RMS output voltage and an AD8436)
NB: AD8436 is limited at 800Khz.

Yeah I went to look up on its schematics. I do agree that using the output of the VCO to measure the frequency is not really nice because its not really a direct kind of measurement. Also I went to look around Digikey and Mouser they sell only SMT-type VCOs. Im trying to do a fully through-hole kind of project right here.

fcb said something about F/V converter. I think its another approach to achieve that spec of displaying the frequency on a 7-Seg LED or a LCD even.

[oldway]

vco DIL8 : LM566C (has a triangular wave output)
vco DIL16 : 4046B or 74hc4046

[tggzzz]

LM2917 is still available.

I guess that IC is not applicable to my project..

Huh?  What are you actually trying to do???  You talk about using an ADC to measure frequency - it's not the way I would do it, but hey, you know what you want??

The LM2917 converts a frequency into an analog voltage, which you would then read with your voltmeter>LED block (or whatever).

Perhaps you should describe exactly what you want or have built, along with specifications. It is less frustrating.

well im trying to use an ADC to convert the frequency into a digital code. not sure if its a right thing to do but your idea of a F/V converter do sound like a way to work around that. I should read up on its theory of operation. Im pretty new to that type of converters.

Thanks thanks

Yes, but what do you want to achieve? Don't tell us your solution, tell us your problem!

 What range, resolution and accuracy do you need?

If you want to know the actual frequency, why not use a frequency meter that counts the cycles in one second, or similar?

If your signal generator is based on a crystal, your meter will not be any more accurate.

If you want to convert frequency to voltage and measure the voltage, how are you going to calibrate that conversion?

[oldway]

I got the waveform generation stage worked out.

Post the schematic of what you have done, it will be easier.

If you want to know the actual frequency, why not use a frequency meter that counts the cycles in one second, or similar?

A non reciprocal counter is too slow reacting in low frequency range to ajust the frequency. That's the reason why Hameg choosed the solution of ADC circuit.

[samfisher92]

LM2917 is still available.

I guess that IC is not applicable to my project..

Huh?  What are you actually trying to do???  You talk about using an ADC to measure frequency - it's not the way I would do it, but hey, you know what you want??

The LM2917 converts a frequency into an analog voltage, which you would then read with your voltmeter>LED block (or whatever).

Perhaps you should describe exactly what you want or have built, along with specifications. It is less frustrating.

well im trying to use an ADC to convert the frequency into a digital code. not sure if its a right thing to do but your idea of a F/V converter do sound like a way to work around that. I should read up on its theory of operation. Im pretty new to that type of converters.

Thanks thanks

Yes, but what do you want to achieve? Don't tell us your solution, tell us your problem!

 What range, resolution and accuracy do you need?

If you want to know the actual frequency, why not use a frequency meter that counts the cycles in one second, or similar?

If your signal generator is based on a crystal, your meter will not be any more accurate.

If you want to convert frequency to voltage and measure the voltage, how are you going to calibrate that conversion?

To answer your questions, I intend to make a function generator that works from 1 Hz to 1 MHz using one of the newer 555 chips (take a look at ICM7555 model from Intersil). I didn't think of using crystals because of its drift over time and those good ones are hard to come by and are very expensive. I want to keep the project as affordable for myself as possible.

You did make a good point about calibration. I have no idea of working out that calibration of the converter. This discussion is getting more and more interesting...

[samfisher92]

I got the waveform generation stage worked out.

Post the schematic of what you have done, it will be easier.

If you want to know the actual frequency, why not use a frequency meter that counts the cycles in one second, or similar?

A non reciprocal counter is too slow reacting in low frequency range to ajust the frequency. That's the reason why Hameg choosed the solution of ADC circuit.

I haven't really got time to draw the schematic out on my project book. But I know is that my function generator is based on the 555 timer with the necessary hardware to convert its sine wave output into triangle waveform, sawtooth, square wave like the integrator, differentiator.

And what do you mean by a non-reciprocal counter? Havent come across such a term before, mind explaining that?

[dannyf]

It is not difficult to make a frequency-to-voltage converter out of a 555.

[samfisher92]

It is not difficult to make a frequency-to-voltage converter out of a 555.

Yeah I heard its monostable operation can be made into such a converter and its pretty simple.

[oldway]

It seems you are looking for a very simple old school cheap function generator like this: