HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Stretchyman on October 31, 2015, 1600 UTC
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Anyone used one as a VXO?
I'm thinking of offering them as an option on my LuLu's.
Seem stable enough?
???
Stretchy.
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I absolutely doubt this chip could be used reliably.
For one it has far too much noise so you can expect that clean output to be non-existent and for two if it is controlled by a resistor anything outside of a high quality <%1 tolerance will give you a drift that will cover kHz with temperature change.
I would take a close look at the datasheet. These look quite useful for there intended purpose but as a HF VCO they appear useless from what I have read.
Not to damper your inquiry but I would look into other alternatives.
Nothing beats a real VFO using the proper selection of temperature compensated capacitors with an FET front end and buffer. Problem is that requires some serious engineering to deal with drift compensation which most people don't want to mess with. Alternatives often are the easier choice like a DDS, or PLL, but those also share their own set of problems with spectrum output.
Sadly there is no easy alternative to vibrating a rock, and when you do it requires its own special circuitry outside of a transmitter if you want to do it properly.
Here is a VFO I have been working on with some good success... http://darkliferadio.proboards.com/thread/833/shortwave-vfo-project
I need to update the post but so far I have added proper low pass filtering and a final PA stage to get it up to 100mw output with harmonics down to the floor and no spurs. Harder to achieve that sometimes with a DDS.
Just sharing to give you an idea of the work involved in literally reinventing the wheel. If you have the money and source it is stupidly easy to just go with a quarts crystal.
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Ah.. There is life out there!
Thanks for the reply BTW.
Sure I've got DDS's and prefer the Si570 (still waiting for my 'Friend' to program them) but was looking for something really simple to offer with my kits.
Will check out your schematic when I get home (blocked at work and my phone wont download the link?)
Looks like your're busy over there in .nl.
Nice!
Cheers from T' Norvern UK.
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To re-use an old musing for simplicity V performance, "you get what you pay for."
+-RH
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Looks like your're busy over there in .nl.
Actually in the US and is a US based forum that is more radio tech orientated. No idea why the site would be blocked. Images are hosted on imgur so maybe that is why.
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The company I work for blocks a lot of sites as do most employers these days.
They actually expect you to do some work!
When I checked the links via my phone they took me to the correct page but the links to the schematic was blank.
Have checked via the web at home and all is well!
OK, I see now, a Colpitts, I think I'll stick to the Si570.
The LTC1799 are a bit jittery with wide skirts in the spectrum but for experimenting I'm really pleased with such a simple device.
Temp wise once the TX has been left on a while the temperature and the frequency are fairly stable.
I figure any drift under 100Hz or so will be barely noticeable anyway.
:P