Design required for VCO 7.7MHz - 8.7 MHz

Thread Starter

Nomi 1114

Joined Dec 4, 2017
86
Dear all,

hope you all are fine and enjoy your time. Here i would like to ask for favor, I have designed Voltage Control Oscillator (VCO) by using LM358 Op Amp. at input i used Ramp pulse (5V, 5Hz) and at output i obtained square wave that is okay. but the problem is frequency requirement: for this signal the output frequency is 335.61Hz (5V) while i need 8.7 MHz at output and at input 180Hz .. i am too much confused how can i do it .. if some body can help me in this regard then i shall be highly grateful to him/her. i can share my schematic also or someone recommend one circuit.

Regards
 

dendad

Joined Feb 20, 2016
3,331

Thread Starter

Nomi 1114

Joined Dec 4, 2017
86
Sharing your schematic that you designed choosing a LM358 dual low power Op Amp configured as a voltage controlled oscillater
Is a must.
Dear Delta prime,

First of thank you very much for your kind reply. secondly please find the attached schematic and result for VCO, the output frequency is around 325 Hz. but i mentioned before i need 7.7 MHz - 8.7MHz , where frequency varies according to input voltages...

Regards
 

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Thread Starter

Nomi 1114

Joined Dec 4, 2017
86
https://inspectapedia.com/emf/Electromagnetic_Frequency_Hertz_Definitions.php

What is your application for this VCO?
Are you concerned with stability and repeatability?

Have you considered an Arduino and a DDS ?
https://www.instructables.com/id/Arduino-30MHZ-DDS-Signal-Generator-In-12/

I use these si5351 boards for radio VFOs..
https://www.instructables.com/id/Arduino-Frequency-Synthesiser-Using-160MHz-Si5351/
Dear dendad,

Thanks for your concern, i need this VCO for Electronic Article Surveillance system, for academic learning purpose.. i don't have any concern for stability .... but i need such kind of results as mentioned in attached file... please suggest me how can i design the required cct .. your help should be highly appreciable ... we need to use sawtooth wave for at input ...



Regards
 

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jpanhalt

Joined Jan 18, 2008
8,781
Your capacitor in that circuit is huge for the frequencies you want. Here are some links that may help you :

http://ww1.microchip.com/downloads/en/Appnotes/00866a.pdf
https://www.allaboutcircuits.com/worksheets/opamp-oscillator-circuits/
http://www.learningaboutelectronics.com/Articles/Op-amp-oscillator-calculator.php#answer1

Notice the capacitors used in the first link are pF's for frequencies of 100 kHz. The last link is a calculator for components in a different configuration. Finally, as pointed out, that op-amp you have chosen will not work.
 

Thread Starter

Nomi 1114

Joined Dec 4, 2017
86
I set V2 to 5V and played with R2 to get neat 8.7mhz. Then set V2 to 0V and played with R3 to get near 7.7mhz.
View attachment 197129
Output is square wave 0 to 5V or could be 0 to 3.3V.
View attachment 197130
Thanks for your kind reply .... output squre wave voltage is 5 V is good enough ... but if bit less, then also no problem !!! But my question is can we use triangle/ sawtooth waveform for this cct holding frequency 180 Hz .... Output is fine coz already square wave ...
Regards
 

Thread Starter

Nomi 1114

Joined Dec 4, 2017
86
Your capacitor in that circuit is huge for the frequencies you want. Here are some links that may help you :

http://ww1.microchip.com/downloads/en/Appnotes/00866a.pdf
https://www.allaboutcircuits.com/worksheets/opamp-oscillator-circuits/
http://www.learningaboutelectronics.com/Articles/Op-amp-oscillator-calculator.php#answer1

Notice the capacitors used in the first link are pF's for frequencies of 100 kHz. The last link is a calculator for components in a different configuration. Finally, as pointed out, that op-amp you have chosen will not work.
Agreed, then please suggest me an op-amp which can work for my requirements !!
 

Thread Starter

Nomi 1114

Joined Dec 4, 2017
86
From post #13. I made V2 a 180hz sawtooth and it works.
The supply should be 3.3 to 5V and the output will be 0V to supply. I think you will use 5V.
Dear i worked on it ... it seems good.. Thank you very much ... secondly i need more purification if i need any help i must contact you , secondly i wanna ask can we observe the clearly variation in out freq during change in input voltages in LTspice (in our case as in fig hard to see the variation), as in attached fig.
 

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