Voltage to Frequency Converter

[km]

Hi, how to convert the output of a Voltage to Frequency converter (Square wave) into Triangular and Sine wave?

 

[akg]

u can get a triangular wave by passing it through an integrator , to get sine wave u may need a tuned(high Q) amplifier or logrithmic wave shaping network(won't give perfect sinewave , also will have lot of harmonics)

 

[audioguru]

You can get a "perfect" sine-wave by using a switched-capacitor Butterworth lowpass filter IC. Its oscillator can also be voltage-controlled so that its cutoff frequency tracks the frequency of the VCO.

 

[km]

How

"switched-capacitor Butterworth lowpass filter IC"...

How does it works actually? The output of the VFC (square wave) directly apply to the "Clock" input?

 

[audioguru]

The clock frequency for the MAX7410 is 100 times its cutoff frequency. The output has a tiny amount of the clock frequency in it which is easily removed with a series resistor and a capacitor to ground from the output.

 

[Roff]

u can get a triangular wave by passing it through an integrator , to get sine wave u may need a tuned(high Q) amplifier or logrithmic wave shaping network(won't give perfect sinewave , also will have lot of harmonics)

The problem with the integrator is that the amplitude of the output will proportional to 1/frequency. A fixed cutoff frequency filter will have amplitude problems also. As Audioguru said, a switched-capacitor filter is a good solution, but you have to run your V-F converter at 100X for the clock, and then divide by 100 for the signal. This may not be practical, depending on the desired signal frequency.
The easiest solution may be to build or buy a V-F converter thay includes a triangular wave as part of the oscillator.

 

[km]

Waveform conversion: Square to triangular

I found this circuit on the internet - is about converting Square to Triangular wave:

**broken link removed**

Can i input the square wave of the VFC into this circuit to get the triangle wave (for wide range of frequencies)?

 

[Roff]

I found this circuit on the internet - is about converting Square to Triangular wave:

**broken link removed**

Can i input the square wave of the VFC into this circuit to get the triangle wave (for wide range of frequencies)?

Assuming it works, you can use it with the limitations noted in this quote from the design article:

It should be noted that the amplitude adjustment response for this circuit is not instantaneous, although appropriate selection of the integrating and averaging capacitor values will minimize amplitude shifts during frequency excursions. Abrupt changes in the input frequency will result in prolonged settling times that may render the circuit unsuitable for certain music applications. Response may possibly be improved with a more complex servo amplifier that adds proportional and derivative terms to the integral term.

In other words, if the input to your V/F converter has high bandwidth (fast rise and/or fall times, or other high frequency components), the triangle wave amplitude will not be able to keep up with the rapid frequency changes.
What kind of V/F converter are you using? Maybe we can come up with a substitute that will generate a triangle wave as part of the oscillator.

 

[km]

Thanks

Thanks for the instant reply!

I'm using AD537 monolithic V-F converter. The frequency range: 0-150kHz.

https://www.analog.com/en/prod/0,2877,AD537,00.html

 

[Roff]

Thanks for the instant reply!

I'm using AD537 monolithic V-F converter. The frequency range: 0-150kHz.

https://www.analog.com/en/prod/0,2877,AD537,00.html

Fig. 28 in the app note you referenced shows how to get a triangle wave. You don't need an AD521 for the instrumentation amplifier (IA). You can use 3 op amps with GBW of 10MHz or more and make your own IA. Do a Google search for "instrumentation amplifier" if you don't know how to do it. You will need a negative supply to do this.
What are you using this for? If you don't need the high precision of the AD537, I could probably show you how to do it much less expensively.

 

[km]

Programmable Frequency Generator

Programmable Frequency Generator - Range: 0-100kHz...

I'm using AD537 to generate the frequency. At the same time, I must able to display sine & triangular wave as well.

 

[Roff]

Do you know about function generators? That's what you are trying to make. Google max038, ICL8038, xr8038, xr2206, and "function generator". When posting a question, you should post more information than you think we will need in order to help you.

 

[km]

Sorry for not giving sufficient information.

Ya! i know i'm doing sth like function generator. I'm using parallel port to control the frequency.

 

[Roff]

Sorry for not giving sufficient information.

Ya! i know i'm doing sth like function generator. I'm using parallel port to control the frequency.

All the chips I mentioned seem to be voltage-controllable, and they have triangle and sine outputs. Can the AD537 do something that they can't?

 

[km]

IA

Fig. 28 in the app note you referenced shows how to get a triangle wave. You don't need an AD521 for the instrumentation amplifier (IA). You can use 3 op amps with GBW of 10MHz or more and make your own IA. Do a Google search for "instrumentation amplifier" if you don't know how to do it. You will need a negative supply to do this.

1. Why need Op-Amps with GBW 10MHz?
2. What do you mean by "You will need a negative supply to do this."?
3. How do I choose the resistor values for the IA design?

Please guide me.

 

[Roff]

1. Why need Op-Amps with GBW 10MHz?
2. What do you mean by "You will need a negative supply to do this."?
3. How do I choose the resistor values for the IA design?

Please guide me.

The schematic for the sawtooth generator is Fig. 28 in AN-277.

1. You can use slower op amps, but you will like the 100 kHz triangle wave fidelity from the 10MHz op amps better than if you used, say, 1MHz op amps, especially if you make the IA with a gain of much more than 3.

2. Look at the schematic in Fig. 28. It uses a negative supply becuse the output voltage is symmetrical around zero volts.

3. Did you Google "instrumentation amplifier"? There are plenty of sites with the gain equation. To calculate the gain you need, look at the differential waveform across the timing cap with an oscilloscope and measure the amplitude of the triangle waveform. As an example, if the triangle is 1V p-p, and you want 10V p-p, you need a gain of ten.

Please tell us why you don't use a function generator chip.

 

[km]

Thanks

Hi, actually i'm doing my project halfway...

My project title: Programmable Frequency Generator using Printer Port.

Initially, I got the design idea from "EDN" where a DAC is used to interface the parallel port with the VFC.

Now, i'm done with the frequency generating part. Then i faced the problem of getting Triangle n Sine waves. That's why i came to here for help.

About the function generator chips, i found out these two ICs are available in my country:-

1. https://www.electro-tech-online.com/custompdfs/2006/06/29113.pdf
2. https://www.electro-tech-online.com/custompdfs/2006/06/72349.pdf

Can i use these chips together with my existing circuit? (because i don't want to waste my effort) Please advice.

 

[Roff]

Hi, actually i'm doing my project halfway...

My project title: Programmable Frequency Generator using Printer Port.

Initially, I got the design idea from "EDN" where a DAC is used to interface the parallel port with the VFC.

Now, i'm done with the frequency generating part. Then i faced the problem of getting Triangle n Sine waves. That's why i came to here for help.

About the function generator chips, i found out these two ICs are available in my country:-

1. https://www.electro-tech-online.com/custompdfs/2006/06/29113-1.pdf
2. https://www.electro-tech-online.com/custompdfs/2006/06/72349-1.pdf

Can i use these chips together with my existing circuit? (because i don't want to waste my effort) Please advice.

Either of those chips would be great if you were starting from the beginning, but they will not help you get a triangle or a sine wave from the AD537. Your best bet is the instrumentation amp approach.
Hopefully you've learned that you need to think a project through before starting to build it.

 

[km]

Just to have more information...
If i'm using a function generator chip, how can i used it with the parallel port? I still need a DAC for voltage control?

 

[Roff]

Just to have more information...
If i'm using a function generator chip, how can i used it with the parallel port? I still need a DAC for voltage control?

Yes. I'm pretty sure all available function generator chips are voltage-controlled. I hope you know that a DAC is a digital-to-analog converter. Knowing this, you should be able to look at the function generator datasheets and see if they are voltage-controlled. I don't mind helping you, but I don't want to hold your hand just because you're a little insecure. Try to find the answers yourself first, and then post more specific questions about the parts you don't understand.