luzell Posted August 20, 2004 Report Share Posted August 20, 2004 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 Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted August 20, 2004 Report Share Posted August 20, 2004 I have seen that VCO's don't operate well with a DC voltage applied to them. I think they have to work in conjunction with a phase detector. It would seem you could design one that works alone but the ones on the market require the phase detector. If you could post your design for one it would be helpful. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 20, 2004 Report Share Posted August 20, 2004 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. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted August 20, 2004 Report Share Posted August 20, 2004 Hello Audioguru. You can change the frequency of the VCO because the phase detector is programmed to reduce or increase the frequency produced by the crystal. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 20, 2004 Report Share Posted August 20, 2004 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. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted August 20, 2004 Report Share Posted August 20, 2004 I have seen frequency synthesizers that do the very same thing. They incorporate a phase detector, a programmable section, and a crystal. What more is there to it. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 20, 2004 Report Share Posted August 20, 2004 A frequency-synthesiser's VCO jumps abruptly only to pre-determined frequencies, a VCO with a variable DC control voltage smoothly glides to any frequency (within its range). Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted August 21, 2004 Report Share Posted August 21, 2004 I understand what you are saying, but have you proven this? What about the fact that I have applied DC voltages to a VCO and have got unstable output? By the way I have seen the schematic for a VCO and it looks quite complicated. Quote Link to comment Share on other sites More sharing options...
MP Posted August 21, 2004 Report Share Posted August 21, 2004 luzell,Sorry for the late reply. What you are looking for is here:http://physics.ucsd.edu/was-sdphul/labs/120b/exp4.pdf74LS124 VCO set at 10 KHZ and adjustable.Looks like the geniuses have let you down so far... ;)Have fun!MP Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 21, 2004 Report Share Posted August 21, 2004 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.). Quote Link to comment Share on other sites More sharing options...
MP Posted August 23, 2004 Report Share Posted August 23, 2004 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 23, 2004 Report Share Posted August 23, 2004 Hi Luzell,You can use just the VCO part of the complicated phase-locked-loop tutorial that MP posted. Instead of the additional phase-locked-loop parts, you can use a simple variable DC control voltage from a pot that I recommended to set its frequency to 10KHz. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.