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Eugen_E
Joined: 29 Nov 2004
Posts: 317
Helped: 30
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 08 May 2008 13:01 RC oscillator - your opinion on choosing component values |
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Hello,
I need to use the 4046 PLL, and i don't know how to choose RC values that are optimum to minimize jitter&noise of the VCO?
In general, should I use a high C value, thus charging/discharging the cap with higher current, or use a lower C ?
Because I need a narrow VCO domain, offset should be used so the VCO range doesn't start from very low frequency?
Thanks
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Eugen_E
Joined: 29 Nov 2004
Posts: 317
Helped: 30
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| 09 May 2008 18:20 Re: RC oscillator - your opinion on choosing component valu |
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If 4046 is too bad regarding jitter, i want to ask about another type.
A ring VCO based on inverters (74HC04) would give a lower jitter than 4046?
Or in general, ring VCOs are cleaner than RC VCOs?
It's something for hobby, and i thought of using 3 or 5 inverters from 74HC04, and controlling frequency from the 74HC04 supply. The datasheet says propagation delay varies a lot with supply voltage.
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Miguel Gaspar
Joined: 06 Mar 2006
Posts: 385
Helped: 36
Location: Culiacán, Sinaloa, Mexico
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| 09 May 2008 19:24 RC oscillator - your opinion on choosing component values |
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Well on the other wayyou can use R's metal film resistors and the C's NPO capacitors.
Good luck.
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FvM
Joined: 22 Jan 2008
Posts: 2240
Helped: 374
Location: Bochum, Germany
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| 09 May 2008 22:13 Re: RC oscillator - your opinion on choosing component valu |
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| What are your jitter and frequency range specifications?
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Eugen_E
Joined: 29 Nov 2004
Posts: 317
Helped: 30
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| 09 May 2008 22:43 Re: RC oscillator - your opinion on choosing component valu |
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Thanks for reply.
Frequency 14-16MHz. Jitter should be minimized, as it is for a ham radio receiver - I know it will be bad anyway, but at least minimized.
I need to get quadrature outputs, so the 14-16MHz signal is divided by 4. Maybe the division will improve jitter a bit.
I use the PLL with a reference of 1-10kHz, not decided already.
Regarding 74HC4046
This document says increasing current reduces jitter
www.eecg.toronto.edu/~kphang/papers/2001/wilkin_vco.pdf , page 8.
I thought to use a large capacitor, use offset resistor, to increase the current, and reduce VCO range to the minimum needed.
Also I would try to decouple to ground R1 and R2 pins with 100n caps - these pins drive the internal current source. Is this good or bad?
I know it's not right, but I try to use this rather than an LC osc in an HF receiver. I've seen this http://groups.yahoo.com/group/softrock40/files/fracturedrock/fr80.pdf and now i have a bit of "courage" 
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