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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