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Help: Voltage to Frequency converter

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muso52

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Gentlemen,

Wishing to design a circuit that converts voltage to frequency over a range of 0 to 10hz, I have been looking at the Microchips TC9400 range. However as this device employs charge pumping circuitry it appears the lowest fout this device will go down to is 1hz. With its isolated digital outputs and zero offset ability this chip would have been ideal. I would appreciate if anyone could suggest an alternative device.

Thanks in advance.

Willy
 
Gentlemen,

Wishing to design a circuit that converts voltage to frequency over a range of 0 to 10hz, I have been looking at the Microchips TC9400 range. However as this device employs charge pumping circuitry it appears the lowest fout this device will go down to is 1hz. With its isolated digital outputs and zero offset ability this chip would have been ideal. I would appreciate if anyone could suggest an alternative device.

Thanks in advance.

Willy

hi,
I have used the TC9400, ok.

You could have a divide by 10 IC after the TC.

E
 
Hi Gents,

Many thanks for your replies,forgive me for not replying sooner, watching Wimbledon!
Come on Andy!

Anyway, Eric. Have you used the TC9400 down to 0hz and is my understanding of the device wrong? Secondly, I am a bit confused by the use of the divide by ten circuit on the output. My understanding would be that all freq's would be divided by ten no? Also and in view of your knowledge of the device it is my assumption that a voltage can be applied to the zero offset to alter the point at which conversion starts to take place. I would appreciate you comments very much.

Willy
 
hi muso,

If you span the TC9400 for 1Hz thru 100Hz and divided by 10, you will have a range of 0.1Hz thru 10Hz, in 0.1Hz increments.

Spanned for 1Hz thru 1000Hz divided by 100, will give 0.01Hz thru 10Hz, in 0.01Hz increments.

Will either 0.1Hz or 0.01Hz be acceptable as your lowest frequency rather than 0.0Hz.

E
 
Hi Eric,
As allways thanks for your comments. Having given some thought I had sort of managed to understand what you mentioned. Being a controls guy as opposed to full time electronics is there such a thing as a divide by100 chip or would it be 2 divide by 10's?
Maybe a quick look around will enlighten me. I think I should be able to disable the device at the bottom of its range using a flip-flop and Nand gate.
thanks,
Willy
 
hi,
The HEF4518 is a CMOS dual BCD counter.

Connect the two counters in series for 100 divide.

E
 

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  • HEF4518.pdf
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there really isn't a way to get any F/V converter to go all the way to 0hz. you get into the territory of seconds, minutes, hours, etc... there are also scale factors, as many F/V converters frequency range is usually somewhere between a 10:1 to 100:1 ratio (Fmax:Fmin). the scaling factor of anything and zero is infinity, and really isn't practical.
 
Hi Eric ,
Many thanks for the info. I am fortunate that in the TC9400 specs .pdf there is an application diagram + values for Cint, Cref at the frequencies suggested.(no maths, yippee!)
As f OUT is going to TTL will see if a dual BCD counter is available in that format.

Unclejed, appreciate what you are saying but 1hz really wasnt low enough . 1 hz = 1cps and no good to me.

Willy
 
you could use a standard F/V with a VCO/PLL that does a x10 or x100 multiplication, it would require some means of switching the PLL into the signal chain when the frequency is below the range of the F/V. the way it would work is the VCO output is frequency divided by 10 or 100, and that frequency divided output is compared against the input signal in the phase comparator of the PLL. the raw VCO output is sent to the F/V. there was a similar contraption in Radio-Electronics magazine many years ago, only the VCO output was sent to a frequency counter to give a reading of millihertz on an instrument where it was impractical to wait for 10 or 100 second gate times to get a count.
 
I used an integrator and a comparator to make V to F converter. Goes perfectly from 0 to 10Hz.

VtoF.png

Waveforms. Red is the output of the integrator.
VtoFwaveform.png

EDIT: That circuit is from the book "The Art of Electronics. 2nd Ed." Page 240.

VtoFpage240.png
 
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sorry, my mistake.... OP was looking for a VCO... somehow i got it backwards:confused:
 
Hi MisterT,

Many thanks for your info.At first glance the circuit would seem ideal. So, can you please answer a few questions?
The circuit diagram you posted. Are the component values for a circuit generating 0 to 10hz?. The reason I ask is that the graph you have included would appear to show an output of 20hz
at I believe a nominal 5v in. It is my intention to source the circuit from a 50k pot across V+ and ground .
What would you recommend for Q1?
I note from the text you sent that fout is relative to the ratio of Vin/Vout. Now 5v in divided 5 V supply = Zero!!?? What determines the relative output frequency at full input voltage? I am guessing that C1's value would be calculated for a fixed output F. But how do I find value if this is correct?
Again thanks for your interest.

Willy
 
The values in my simulation are just some values I experimented with.

There are few things that affect the frequency range:
- The capacitor value and the input resistance (charge rate).
- The resistor value that discharge the capacitor (this also affects pulse width.)
- The comparator hysteresis levels.. this affects how much the capacitor need to bee charged and discharged before the comparator changes state (or how sensitive it is).

Hard to say how would you calculate precise values for certain frequency range. I have not played with that circuit in 6 months. I don't remember the details.

I think almost any logic level transistor should be ok for Q1.. or the bibolar substitute shown in the last picture. I used just some transistor I found. I don't know if it was bad or good, but I was happy. My circuit did not need to bee very accurate.

EDIT: If the voltage in comes from a human operated potentiometer, then you could just calibrate the input voltage to the right voltage range which gives 0 to 10 Hz output. Then the oscillator circuit itself would not need to be that accurate.
 
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