Hi Stefan I may be shot down here but I believe you must avoid toroids as
the tuning element in a stable oscillator. It is difficilt to be sure of the
mechanical stability of the winding and ferrites have a high tempco, iron
dust are better but not as good as an air core. My old fashioned idea is
that ceramic tube formers are the thing, I am not sure about styroform for
RF, I would use NPO ceramic. You will probably need to trim the temp
coefficints to get drift under control.
Lookinf at your requirements of 4times I doubt you would get enough swing
mixing 2 oscillators to 2MHz. If you do you will have to start in the
overtone range say 35MHz I think. I've not tried it so high.
Alan.
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Thursday, July 05, 2012 11:24 PM
Subject: Re: LF: RE: Analog oscillators
Hello again,
It is a bit disapointing. The oscillator is drifting several 100 Hz when
looking to the divide by 4 side, i.e. the PA output i.e. close to 475 kHz.
Does someone have the circuit of the 137 kHz oscillator of DJ2EY? It must
have been published in the CQDL in 2003 i think. This oscillator was very
simple and reasonable stable.
I used 9 turns on a FT37-43. No Amidon cores of the T50 or T37 series are
available in the moment. For the JFET is used the BF862 which may be not
the best choice since the drain currents are quite high due to the high
gain. I also used styroflex Cs but don't know if it is a good choice.
Varactor diodes, yes, another idea. But the drift problems may be the
same. I can accept a drift of < 20 Hz after switching the VFO on (1
minute).
So maybe i have to find a suitable pair of xtals and build a similar
design than Ha-Jo does.
Ah BTW, what is the name of that popular DDS VFO IC. Not the SI570. It has
to work without any programming and USB interface and all that PC stuff.
However a DDS IC that operates on a frequency defined by external parts
(potentiometer) will have the same problems, or not?
Any suggestions.
73, Stefan/DK7FC
Am 05.07.2012 22:42, schrieb Roelof Bakker:
Hello Stefan,
The NE602 works very well as oscillator and delivers a clean and low
noise signal.
One of the main problem in building VFO's is the mechanical design.
It can be done, but it is not trivial when you use a tuning capacitor and
gearbox.
Mechanical rigidity and freedom from micropfonics requires a solid
enclosure which with the gearbox need to be mounted on the frontpanel,
rather than on the chassis.
As the frequency range is small, it will be mechanically much easier to
use varactor tuning in conjunction with a ten turn potmeter. I have been
there and done it all.
I have build a 40 m receiver with varactor tuning and it is rock stable.
Building VFO's is fun!
73,
Roelof, pa0rdt
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