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Old 26th Apr 2018, 3:25 am   #1
Bazz4CQJ
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Default Quick-and-Dirty VFO 1kHz-33MHz

Following in the new "Quick-and-Dirty" series, the LTC1799 chip looks interesting. It is described as a "Resistor-Set SOT-23 Oscillator" and can generate 1 kHz to 33 MHz square waves just by changing one external resistor. No coil, just one resistor and one cap.

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File Type: pdf 1799 oscillator data sheet.pdf (196.8 KB, 82 views)
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Old 26th Apr 2018, 3:55 am   #2
Radio Wrangler
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

Take note of the word 'dirty'. The phase noise sideband level of an oscillator is related to the Q of the oscillator's resonator. Super-duper quality coils can give a Q of a few hundred, and the better varactor diodes will be a further limitation, typically halving that figure depending on the fixed/variable capacitive proportions. Mechanical tuning capacitors have much higher Q. At the other end of the Q range, some larger SMT parts can reach 100. small cheap SMT parts can be around 20.

Having set the scene, now for the punch line: R-C oscillators have effective resonator Q's below unity!

Add in a fair amount of drift with temperature.

These sorts of parts have uses in general electronics, but prove to be seriously limited in radio work.

They can be phase locked, and the loop used to rein-in the noise over the loop bandwidth, but they are always worse than L-C based designs.

Sorry for the cold water, but I thought I'd better point out the limitations as they particularly affect radio applications.

One really wild thing in the oscillator world are integrated MEMS oscillators with MHz range mechanical resonators. More like a crystal substitute than a VCO.

David
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Old 26th Apr 2018, 5:42 am   #3
Bazz4CQJ
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

Quote:
Originally Posted by Radio Wrangler View Post
These sorts of parts have uses in general electronics, but prove to be seriously limited in radio work.
Well yes, but just as Tanuki wasn't claiming that his 1MHz chip was anything more than something you could literally throw in your boot...this too? Alternatively, if you really are just getting started in the hobby, and on a really tight budget, then your 'cheap as chips' signal generator to fix your Bush/Echo/Hacker might be really quite handy.

All that said, I'm sure I bought some IC's way back in ~1990 (SN74LS629 ?) that were R-C VCO's running to ~30MHz?
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Old 15th May 2018, 9:12 am   #4
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

One of these has just dropped through the letterbox. Whether it sees any practical service remains to be seen.
Don m5aky
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Old 15th May 2018, 10:06 am   #5
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

I can't think of a use case I wouldn't use something else other than this IC for. The BOM cost is really high as well.

A Si5351 (1), crystal (0.40) and microcontroller (0.50) to drive it are cheaper and more flexible. 0-200MHz, three independent outputs, rock solid frequency stability.

For everything inbetween a couple of CMOS gates are probably good enough with an RC circuit.
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Old 15th May 2018, 10:34 am   #6
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

This one came from China on a board with two preset pots etc for 8.00. Won't break the bank.
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Old 16th May 2018, 2:23 am   #7
Bazz4CQJ
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

Please let us know what you think of it.
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Old 16th May 2018, 12:17 pm   #8
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

I haven't done much yet but so far it does not appear to be very stable on frequency counter at HF. Not easy to set to precise frequency with the preset pots. Square wave looks fine on scope.
Might be improved in an enclosure.
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Old 18th May 2018, 9:00 am   #9
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Default Re: Quick-and-Dirty VFO 1kHz-33MHz

This is what i used mine for http://www.taswegian.com/NBTV/forum/...=2436&start=45
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