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
iron dust toroid to wind the coil for a free running analog oscillator.
The oscillator runs on 1900 kHz and is intended to drive my 472...479
kHz PA. This PA has an internal divide by 4 IC inside, so it can
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
comes from the need for low power consumtion for /p MF test and volume
limitations in a backpack. My PA was designed to run on 12V DC directly
(https://dl.dropbox.com/u/19882028/MF/100W%20475kHz%20PA.png) and
especially to be very COMPACT. The efficiency is arround 85%, including
the driver power.
For normal CW operation, a drift of say 20 Hz in 5 minutes can be accepted.
First i took a FT37-77 ferrite core to build the resonsnce coil. All the
capacitors are "Styroflex types". The drift of the oscillator was not
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
wire produces this drift (same scroll rate and displayed range in the
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
10 minutes! See
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
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
or the ambient temperature. And i hope the RF will not affect the
transmit frequency significantly..
This evening i want to test the VFO on the band. Maybe there will be
some QSO partners? :-)
Thanks for the explanations about the temp drifting components. If you
have further useful suggestions, don't hesitate to tell them.
73, Stefan/DK7FC
Am 07.07.2012 23:11, schrieb Stefan Schäfer:
Hello Jim,
Am 07.07.2012 20:55, schrieb James Moritz:
When you replace the existing inductor with a high-Q one, you will
find that the loop gain of the circuit is increased due to the higher
Q, and you will be able to use a lower gain/reduced transistor bias
current.
Give me some days. Soon i will report about the progress. A T50-6 os
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
VFOs. A drift of +/- 100Hz during the time taken for a QSO is usually
not noticeable,
Oh i would say 100 Hz is a bit hard but 20 Hz should be OK in my opinion.
and it is easy to achieve this in the LF/MF range. Also, the narrow
tuning range means an elaborate tuning mechanism with reduction
gearing is not needed - just a knob with a simple hand-marked scale
is adequate.
EXACTLY!!!!
73, Stefan/DK7FC
It is much simpler than the schemes needed to obtain an LF/MF signal
by dividing/mixing down the output of an HF rig, etc. A VFO is also a
much better practical choice than a crystal oscillator, which leaves
you stuck on a fixed frequency.
Cheers, Jim Moritz
73 de M0BMU
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