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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I want to do it with least number of easily available components. I'd also like to vary the frequency for at least some +-100..200kHz while maintaining exact duty cycle.
common NE555 has upper limit of 500kHz and it seems impossible to achieve stable duty cycle there. I tried several NE555 circuits, also one with external transistor which is supposed to keep 50% duty cycle, which worked to a point but loses as frequency approaches maximum.
CMOS 555 works for higher frequencies but I don't have it here and I don't think it would have stable duty cycle either.
Only verified solution I know would be 74HC14 + 74HC74 flip-flop multivibrator, which would divide the oscillator output by 2 and always keep 50% duty cycle.
But this would take me 2 additional big IC's so I'm just wondering if I could do it simpler way.
Wouldn't like to have to use 4046 either, as this is a SSTC driver circuit I'm actually trying to get rid of it
I have lots of opamps, comparators and various common logic IC's around here, I'd love to see some ideas.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
After some research, it looks like I'm condemned to use a bistable and some simple non-50% duty cycle oscillator, or an IC containing all that (I don't have 4047 in my stash, though).
Have a look at this.
Shawn, I don't have a negative supply in my circuit and circuit expectedly doesn't work without it with LM393, since positive input can never get lower than negative.
Also, duty cycle turns pretty uneven even at about 100kHz. LM311 is faster, but I'm not sure if it would work well enough at 500kHz.
I have one faster comparator but it's quite expensive and I wouldn't like to use it for that.
It just looks like analog approach for 50% d.c. is not a way to go.
I'm going to try some more complex comparator circuits but I'm not sure if they are worth.
Can anyone by the way tell, how does the high-frequency VCO (first circuit on page 6) create linear sawtooth only with comparators and passive components, without any current sources in sight? Or is it just approximate?
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400kHz 50%duty cycle oscillator
