LF: Analog oscillators [Vackar]

["Dave G3WCB." <ganymede444@btopenworld.com>]

Hi, Group, Pete,

 

I never bothered with the "Tempatrimmer", although if you come across one at a junk-sale it's probably worth picking it up. You can fine-trim the oscillator temperature response by selecting between positive temperature-coeffecient or NTC capacitors. Shouldn't be necessary for work on MF.

 

The tips about mechanical stability, etc, are still relevant today, even for PLL and varactor-tuned designs.

 

73, Dave G3WCB IO91RM 

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org]On Behalf Of M0FMT
Sent: 06 July 2012 11:28
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: RE: Analog oscillators [Vackar]

Hi Stefan and group

 

Look carefully at the circuit for the VACKAR VFO which has been hawked around in the RSGB Handbook for years. Note in the bottom LH corner of the schematic Oxley "TEMPATRIMMER" without this piece of "Unobtainium" its like any other free running osc.

 

They are not made now but someone is selling old bits for about 35 dollars US at http://www.bmius.com/p-9560-oxley-xxx-tempatrimmer.aspx 

 

That sort of price will get you a modern DDS with all the facilities but IMHE they are susceptible to RF possibly more so at LF because the antenna tuner is big and in my case not shielded, so "RF in the Shack" issues.

 

Beating two xtals works for me but I have never checked the stability seriously. It works OK for 500kc/s because these are standard xtals 60P ea (probably of dubious stability) 6.000 and 6.500 and are pulled to a max of four (4)kc/s. 477 is more difficult these wont be "off the shelf" items.

The Si57x is another solution there are "VFO" applications using it. Or PIC control with say a range of closely set spot QRGs. Look in the archive of the Softrock Yahoo group for the design and software details of the "Xtall" LO.

73 es GL Pete M0FMT IO91UX

From: Dave G3WCB. <ganymede444@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Friday, 6 July 2012, 8:43
Subject: LF: RE: Analog oscillators [Vackar]

Hi, Stefan, LF,

I suggest you consider the Vackar oscillator. There is a very good
FET/transistor design by G3PDM which you can reach by following the link
below.

It's also important to consider the mechanical design of the oscillator and
its enclosure. I think you'll be disappointed with the results of your
construction - the tinplate box will be quite "flimsy", and the arrangement
with the tuning capacitor in the lid means that you will have quite long
unsupported wires connecting the frequency determining components.

I suggest that you start with a more rigid metal box, say an aluminium
die-cast box, and mount all the rf components, including the coil, close
together and as rigidly as possible. In the past I have used ceramic
stand-off pillars, but strips of fibreglass PC board material soldered to M3
screws works just as well. For the coil, use a styrene or ceramic former
with a dust-iron (not ferrite) tuning slug. Avoid toroids for the oscillator
coil, if possible.

The G3PDM article has some useful information on mechanical constuction.
There was a load of work done by hams and professionals on transistor
oscillators in the 1970s and 1980s, but it's been largely forgotten with the
introduction of PLLs and DDSs. It just needs to be re-discovered!

Good luck!

73, Dave G3WCB IO91RM

http://www.qsl.net/va3diw/vackar.html



>
> 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
>