EMRFD Message Archive 12561

Message Date From Subject
12561 2016-03-31 20:41:16 Rod KM6SN vfo stability
Hi Nick and others,

Some tricks to vfo stability

1. a type 6 core is good
2. wind the inductor under tension with the biggest wire you can manage
3. boil it in water for a minute or two
4. when it cools and dries, coat it with the XYLs "Harder than Nails" stuff
5. use polystyrene caps wherever possible in the frequency determining area

Tthe attached ltspice file is an oscillator which
is designed for immunity from active device parameters, from the
Design Handbook", a collection from RFdesign magazine, page 76 (out of
I will upload copy of that article to emrfd FOR EDUCATIONAL PURPOSES
(it is copyrighted).
It is 4 jpegs about 6mb each.

Using the above techniques, and ugly construction, and a high quality
cap, a 10.9 MHz vfo holds within 1Hz for hours.

For other frequencies:
C4 XC= 10k
C1 XC=300 ohms
C6 XC=10 ohms
L1 XC=300 ohms

Be advised, that oscillator does NOT produce a sine wave- rather it
a square wave with about a 70% high duty cycle. This oscillator topology
be used at 2x the desired frequency, divided by two with a 74hc74, and then
LPFed into a beautiful rock-steady sine wave. (or, don't use an LPF, and
your mixer with a square wave-- be careful on signal levels.)

That oscillator circuit is amazingly stable up into VHF.

In summary,
type 6 is good
pay special atention to the inductor
use polystyrene caps
select an oscillator circuit that is inherently immune to active device
changes (Seiler, Vakar, or the one mentioned above).

Remember, homebrewing is about learning, in addition to the joy of using
HB gear.


[Non-text portions of this message have been removed]
12565 2016-04-01 00:26:25 Dana Myers Re: vfo stability
12567 2016-04-01 09:05:29 markwdunning Re: vfo stability


Actually my LT spice simulation shows that it can produce a very good sine wave if you take an output off the junction of C2/C1 and L1 and lightly couple it to a J310 source follower.

I am tempted to build a version for 160M but I have so many other projects in the way.

Thanks for sharing this circuit.

73 Mark VK6WV

12569 2016-04-01 09:56:52 Alberto I2PHD Re: vfo stability
12571 2016-04-01 11:13:02 Rod KM6SN Re: vfo stability

I agree about the waveform.

The purpose of this oscillator is stability; I wonder if the light coupling to the J310 would
risk that stability. I should test that some day.....



12572 2016-04-01 13:59:42 Dana Myers Re: vfo stability
12573 2016-04-01 14:31:26 Rod KM6SN Re: vfo stability

Yes, that circuit has some similarities to a Vackar.
Here is a very good article on a Vackar:


The circuit I posted probably is more stable, but I cannot attest to the phase noise.

I suspect the posted circuit is more stable because:

1. The capacitance swamping the base-emitter is much larger, and

2. the coupling to the collector is much lighter,
and those both reduce the vulnerability to changes in the active
device parameters.

Have fun with both, and in bocca al lupo!