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-   -   Quick-and-Dirty VFO 1kHz-33MHz (https://www.vintage-radio.net/forum/showthread.php?t=145965)

Bazz4CQJ 26th Apr 2018 3:25 am

Quick-and-Dirty VFO 1kHz-33MHz
 
1 Attachment(s)
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.

B

Radio Wrangler 26th Apr 2018 3:55 am

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

Bazz4CQJ 26th Apr 2018 5:42 am

Re: Quick-and-Dirty VFO 1kHz-33MHz
 
Quote:

Originally Posted by Radio Wrangler (Post 1038340)
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?
B

vosperd 15th May 2018 9:12 am

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

MrBungle 15th May 2018 10:06 am

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.

vosperd 15th May 2018 10:34 am

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.
Don

Bazz4CQJ 16th May 2018 2:23 am

Re: Quick-and-Dirty VFO 1kHz-33MHz
 
Please let us know what you think of it.
B

vosperd 16th May 2018 12:17 pm

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.
Don

dalekmoore2007 18th May 2018 9:00 am

Re: Quick-and-Dirty VFO 1kHz-33MHz
 
This is what i used mine for http://www.taswegian.com/NBTV/forum/...=2436&start=45

M3VUV51 5th Nov 2018 10:01 pm

Re: Quick-and-Dirty VFO 1kHz-33MHz
 
just think build a transmitter with that wow you could cover all the bands dc to light all at the same time !! m3vuv.


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