Stefan
Good new on the variometer ... guess this means you're one step closer to 0 dBfs! Another 3 - 6 dB
and I'll need welding goggles to safetly view the signal. Looking forward to receiving your signal
in CW mode.
Congratulations on your continued improvements
Jay W1VD WD2XNS WE2XGR/2
---- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Thursday, December 22, 2011 12:27 PM
Subject: LF: Automatic Variometer adjustment at DK7FC
LF,
I'm totally thrilled.
Yesterday in the night i designed a circuit to automatically resonate my LF TX system by tuning
the variometer. This allows to exactly hold the phase and hold the antenna in resonance in
changing sourrounding conditions, like upcoming rain. But the most useful feature is: It
automatically adjusts the variometer if i do QSY (remote operation!),e.g. if i go from 137.7 to
136.172 it re-adjusts the system in less than 5 seconds!
A small drive engine is used to turn the small coil into the main LF loading coil, see:
http://dl.dropbox.com/u/19882028/LF/LF_variometer_drive_engine_arrangement.jpg
The drive engine works from 3...24V DC and takes 0.15A. It is steered automatically by the circuit
described below.
The basic phase measurement circuit is taken from the M0BMU tuning meter which one can find
somewhere in the web and here: http://dl.dropbox.com/u/19882028/LF/LFtunemeter.pdf BTW this tuning
meter is THE most useful device for antenna measurements at all, in my opinion. The phase meter
can directly drive an analog meter showing if there is a positive or negative or no phase
(=resonance) between voltage and current. I took a very nostalgic and high quality analog meter to
nicely display the phase, see http://dl.dropbox.com/u/19882028/LF/phase_meter.jpg. The circuit
works very nice and accurate. Resonance showing and the maximum of the antenna current exactly
coincides. The circuit need no external power supply.
The automatic motor drive circuit is here:
http://dl.dropbox.com/u/19882028/LF/DK7FC%20Automatic_variometer_tuner%20.jpg
It uses a quad OP amp, TL084. The first stage low pass filters and amplifies the input voltage
(taken from the analog phase meter, i.e. switched in parallel), which has a DC part and a 137 kHz
ripple. Another OP amp generates the ground level which is in the middle of the 12 supply voltage
(e.g. a battery or usual power supply). Then there are 2 comparators and a ICL7667 MOSFET driver.
The 7667 works in a H bridge configuration and inverse switches the outputs. The outputs can be
opposite in both directions, turning the engine CW or CCW or they can have equal potential to stop
the engine. 2 BAT42 shottky diodes are used to generate a +- 0.3V threshold voltage to prevent
self oscillation arround the resonance point. The sensitivity of this circuit and the phase
threshold (hysterisis) can be aligned by adjusting the gain of the input OP amp stage. The 7667
provides a low impedance output switching and easy circuit design (no discrete push pull stages or
so). A resistor is put in series to the engine to reduce its speed a bit. Also a 0.1A poly switch
type fuse is put in series. This fuse is under-dimensioned for the engine but will work for a few
seconds before it interrupts. This a some fail save protection in case the circuit may be damaged
by what ever and wants to turn the engine endless. Two anti parallel switched LEDs indicate a
running engine and its direction. They will be labeled with L+ and L- later on the PA front panel.
As a test i was transmitting a key down carrier at 137.7 kHz and activated the circuit. A short
impulse, indicated by one LED, set the variometer EXACTLY to resonance. Then i QSYed to 137.0 kHz
without interrupting the TX. Since the frequency is lower, the L is to low. Thus the circuit acts
resistive-capacitive and the phase angle is negative. L must be increased. The phase meter first
jumped into the negative half of the meter but it took about 2 seconds until it came quickly back
to zero where it stopped immediately.
I took the SpecLab frequency generator for that test (have an IQ mixer that adds 125 kHz to the
soundcard output, so i have to generate 12 kHz to come out at 137 kHz). Then i added the FM
modulator within the signal generator and choosed a frequency deviation of 300 Hz and a period
time of 20 seconds. This is how that transmission looks on the DF6NM grabber:
http://dl.dropbox.com/u/19882028/LF/137kHz_%2B_FM0.3kHz.jpg
Without the automatic tuner beeing active the phase meter moves like a windscreen wiper of course.
When the circuit is active the needle of the meter does very fine twitch but does not leave the
resonance significantly at all! :-)
The experts know how the LED blinking pattern must look: Faster blinking on one side, then slower,
then the other LED starts to blink slow, then faster, slower again and so on :-))
There are still no mechanical switches in series to the engine supply, which would be another fail
save circuit, to be added later, hopefully ;-)
This may be one way for other LF transmitting stations to increase the comfort of running their TX
system by automatically adjusting their system to resonance, even when QSYing by > 1 kHz :-)
73, Stefan/DK7FC
PS: A photo of the board is here:
http://dl.dropbox.com/u/19882028/LF/automatic_variometer_board.jpg
PPS: Caution: This is not NEW and there are other circuits with similar functionality
existing!!!!!
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