Oscillator without a crystal?

[K5UOS]

I can't add anything to the other fine comments but suggest an avenue for experimentation.

You can build a heterodyne oscillator using two crystals which can provide much of the stability of a VXO with a bit of the range of a VFO.

For example a fixed frequency 28060KHz crystal can be "mixed" with a 21060KHz crystal VXO to yield a tuning range of approx 7000 to 7040 KHz.

This is one of my experimental heterodyne oscillators. I built it for a tranceiver LO. I used a 50 ohm feedback amp after the filter followed by a 7 pole low pass filter.

[IMG][/IMG]

I experimented with taking the output from the gate of the 28MHz xtal and from the source. These were typically cleaner outputs as the drain is rich in harmonics. Two cascode connected J310's can be used for a DGM.

The same concept can be used with simple passive double balanced diode mixers that from experience require less output filtering. Using two high frequency xtals to create a lower frequency puts the xtal harmonics way above the desired one.

Good news is that it can be very stable compared to a L/C VFO. The bad news is that it can be a fairly high parts count and require some form of measurement at the final tune up stage. I could tune the VXO approx 40 KHz with an output that varied only about 2 dB over the tuning range.

I got my crystals from this place:

http://www.expandedspectrumsystems.com/prod4.html

The worst you get for experimenting is two QRP crystals.

I hope you find a circuit that satisfies your needs. Oscillators of any type are really amazing. It is usually my favorite part of a project. Making a handful of parts oscillate at 7,000,000 cycles per second is still exciting. I have total respect for the UHF and VHF homebrewers.

Don K5UOS

[VK2TIL]

That is a clever design!!

[W4HAY]

Double-sided printed circuit board works well for enclosures. It's easy to cut with a small hand saw, and the pieces can be soldered together with a small iron to make a very rigid structure.

[WA4OTD]

Looks like a standard colpitts oscillator. http://www.kenneke.com/~jon/pixie/pixie2.gif

Use the frequency formula and get a tuning capacitor and stable inductor.

Examples: http://www.electronics-tutorials.com...scillators.htm

This is same circuit as my first patent where I used a colpitts oscillator for TV tuner design.

http://www.google.com/patents?id=vCc...age&q=&f=false

[K5UOS]

That is a clever design!!

Thanks Kerry,

It is just a different way of providing a reasonably stable frequency with wider range than a single fundamental crystal without too much additional circuitry. Idea came from SPRAT. GQRP printed some designs of different single active device mixing VXO's sent to me by Ian K3IMW.

The prototype below shows that it is a pretty high parts count with the buffer/amp (it needs an external variable cap). But as OTD said the Pixie uses a textbook Colpitts circuit. My circuit might be substituted directly. The only caveat would be problems with harmonics or fundamental xtal frequencies or problems with keying.

This circuit just a trivial something that might spark an idea for the original poster or someone else with a Pixie.

K5UOS

[WA9SVD]

Murphy's Law: Homebrew amplifier circuits oscillate. Oscillator circuits do not.

[W8HDU]

This circuit just a trivial something that might spark an idea for the original poster or someone else with a Pixie.

Thank for posting that. I've been looking for a design for some highly stable crystal oscillator for bench calibration work. I've been looking around trying to find some ideas for something that could be designed onto a small PCB. The part I'm struggling with is when trying to order crystals for some of the articles I've seen posted on the web, note of the articles seen to give a crystal formulas.

[VK2TIL]

Just today KK7B posted a very interesting comment about PTO VFOs on the Yahoo EMRFD Group;

http://groups.yahoo.com/group/emrfd/message/3493

He indicates that these PTOs are relatively easily-available in the US; wish that were true in VK.

[K5UOS]

Thank for posting that. I've been looking for a design for some highly stable crystal oscillator for bench calibration work. I've been looking around trying to find some ideas for something that could be designed onto a small PCB. The part I'm struggling with is when trying to order crystals for some of the articles I've seen posted on the web, note of the articles seen to give a crystal formulas.

Fred,

I would not consider my circuit adequate for calibration work. Its not because it is simple or a novelty circuit but because there are better alternatives.

Kerry gave an alternative suggested by Rick KK7B, one of the authors of the ARRL publication "Experimental Methods in RF Design". Rick is one of my favorite homebrewers. I have used his DC receiver designs many times particularly the diplexer and audio filter ideas.

If I was looking for a stable, clean oscillator using two crystals as in the DGM circuit I would use a diode mixer. I would probably use a higher level mixer like level 10. I would not be worrying about parts count. You would have to build two oscillators and develop output levels and impedances consistant with the mixer requirements. It sounds harder than it is.

http://www.minicircuits.com/products..._level_10.html

The mosfet circuit is not balanced. The outputs require considerable filtering. The diode mixer is balanced and cleaning up the output is not trivial but straight forward, more predictable and the methods well documented.

I have never needed the precision or lab quality stability. In so many cases over the years I have built limited range oscillators as needed. If you wanted to build a precise temperature controlled oscillator the VHF or UHF guys like Glen, K9STH, make have some ideas.

What frequencies did you have in mind?

K5UOS

[W8HDU]

Just today KK7B posted a very interesting comment about PTO VFOs on the Yahoo EMRFD Group;

http://groups.yahoo.com/group/emrfd/message/3493

He indicates that these PTOs are relatively easily-available in the US; wish that were true in VK.

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