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voltage controlled oscillators


luzell

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hi guys, nid ur help.,pls., cud anyone pls share his circuit designs for voltage controlled oscillator using 74LS124 IC and having a base frequency of 10KHz., i hope u geniuses wud help me., dis my proj design and it wud be pass first wik of sept.,pls., need ur help badly.,thank you very much..

luzell

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Hi Luzell,
Sorry, but the old 74LS124 was replaced by the improved 74LS629 many years ago. The datasheet tells you exactly what you need and has circuits. Search at Google.com.

Hi Kevin,
Most commercial function generators use VCOs without phase detectors. The LM331 VCO also works very well.
If you use a phase detector then you will also need a stable (crystal) oscillator for comparison. Then the circuit will be a single or switched frequency phase-locked-loop instead of a variable VCO.

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Kevin,
A phase detector produces a DC correction voltage to adjust the frequency and phase of the VCO so that it is exactly the same as the crystal oscillator. You could put a switch-selected digital divider between the VCO and phase detector so that the VCO is an exact multiple of the crystal oscillator's frequency. Otherwise, Luzell can use a 1MHz crystal oscillator and a divide-by-100 digital divider so that a phase detector will lock his VCO to exactly 10KHz. But since he said, "base frequency of 10KHz", then maybe he isn't allowed to do that.
A phase detector doesn't allow the VCO's frequency to be smoothly variable, like the VCO can do with a variable DC input.

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Hi MP,
Thanks for the good demo of a 74LS124 that is used as part of a phase-locked-loop, whose VCO follows the exact frequency and phase of a variable clock oscillator. But the VCO's frequency can simply be set with a DC voltage from a pot, then the clock oscillator and other parts aren't needed.

Hi Luzell,
You didn't ask for a phase-locked-loop, but please take your pick: a complicated phase-locked-loop, or a simple VCO.
I'll bet that you need a phase-locked-loop circuit and a master oscillator with decade divider circuit so that the VCO frequency can be switched from 10KHz on up in 10KHz increments (10KHz, 20KHz, 30KHz, etc.).

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audioguru, you are welcome. Also note that I posted exactly what luzell asked for instead of telling him to try something else. You might consider this sometime. There are times when students come onto the forum looking for help with an assignment they do not understand. Telling them to try something else is not at all helpful.

MP

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