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From: <jrusgrove@comcast.net>
To: <rsgb_lf_group@blacksheep.org>
References: <4EF3689B.1030902@iup.uni-heidelberg.de>
Date: Thu, 22 Dec 2011 13:42:57 -0500
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Subject: LF: Re: Automatic Variometer adjustment at DK7FC
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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" <Stefan.Schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
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!!!!!
>
>
>