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Date: Sat, 21 Jul 2012 19:32:06 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: RE: Analog oscillators
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MF,

Another episode about my analog oscillator for MF story.

After the hints from the LF group i have now collected a Amidon T50-6=20
iron dust toroid to wind the coil for a free running analog oscillator.=20
The oscillator runs on 1900 kHz and is intended to drive my 472...479=20
kHz PA. This PA has an internal divide by 4 IC inside, so it can=20
alternatively be driven from a conventional HF TRX on the 160m band range.

The idea to have a very small analog oscillator covering the 630m band=20
comes from the need for low power consumtion for /p MF test and volume=20
limitations in a backpack. My PA was designed to run on 12V DC directly=20
(https://dl.dropbox.com/u/19882028/MF/100W%20475kHz%20PA.png) and=20
especially to be very COMPACT. The efficiency is arround 85%, including=20
the driver power.

For normal CW operation, a drift of say 20 Hz in 5 minutes can be accepte=
d.

First i took a FT37-77 ferrite core to build the resonsnce coil. All the=20
capacitors are "Styroflex types". The drift of the oscillator was not=20
acceptable. See https://dl.dropbox.com/u/19882028/MF/Drift%20FT37-77.png

The new coil which uses about 60 turns of 0.22 mm diameter enameled Cu=20
wire produces this drift (same scroll rate and displayed range in the=20
spectrogram): https://dl.dropbox.com/u/19882028/MF/Drift%20T50-6.png

That looks much better :-)

After running the VFO for 15 minutes, the drift is stabilized to 2 Hz in=20
10 minutes! See=20
https://dl.dropbox.com/u/19882028/MF/Drift%20T50-6_Langzeitdrift.png

That is very satisfying and so the VFO which uses a 33pF variable=20
capacitor to cover 472...479 kHz is well suitable for CW!

Of course there will be a higher drift if the supply voltage is changing=20
or the ambient temperature. And i hope the RF will not affect the=20
transmit frequency significantly..

This evening i want to test the VFO on the band. Maybe there will be=20
some QSO partners? :-)

Thanks for the explanations about the temp drifting components. If you=20
have further useful suggestions, don't hesitate to tell them.

73, Stefan/DK7FC





Am 07.07.2012 23:11, schrieb Stefan Sch=E4fer:
> Hello Jim,
>
> Am 07.07.2012 20:55, schrieb James Moritz:
>> When you replace the existing inductor with a high-Q one, you will=20
>> find that the loop gain of the circuit is increased due to the higher=20
>> Q, and you will be able to use a lower gain/reduced transistor bias=20
>> current.
> Give me some days. Soon i will report about the progress. A T50-6 os=20
> at my parents QTH where i will be in some days.
>
>>
>> For CW enthusiasts, I am suprised more use is not made of simple LC=20
>> VFOs. A drift of +/- 100Hz during the time taken for a QSO is usually=20
>> not noticeable,=20
> Oh i would say 100 Hz is a bit hard but 20 Hz should be OK in my opinio=
n.
>
>> and it is easy to achieve this in the LF/MF range. Also, the narrow=20
>> tuning range means an elaborate tuning mechanism with reduction=20
>> gearing is not needed - just a knob with a simple hand-marked scale=20
>> is adequate.
> EXACTLY!!!!
>
>
>
> 73, Stefan/DK7FC
>
>> It is much simpler than the schemes needed to obtain an LF/MF signal=20
>> by dividing/mixing down the output of an HF rig, etc. A VFO is also a=20
>> much better practical choice than a crystal oscillator, which leaves=20
>> you stuck on a fixed frequency.
>>
>> Cheers, Jim Moritz
>> 73 de M0BMU