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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