Antenna phase measuring circuit question please?

Discussion in 'General Technical Questions and Answers' started by 2E0ILY, May 16, 2018.

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  1. 2E0ILY

    2E0ILY Ham Member QRZ Page

    I got some great, patient advice re this project about a year ago. It's a phase meter circuit that looks at the phase on an antenna with 1 kW on it at 137khz and the circuity drives a variometer outside to bring the antenna back into phase as the weather or whatever changes. With advice variometer-driver2.jpg phase.jpg I got it working quite well, but what I have found is it's overly sensitive. A snowflake circuit [​IMG] By this I mean it hunts about as the wind blows the antenna. This is annoying and wearing out the variometer and motor drive. It is stable without minor outside environment changes though. Is there a way to make it less sensitive to small phase changes please? The circuit for the phase meter has no meter installed, but a 150 Ohm resistor in its place, still retaining the series 180 Ohm resistor as well. I also, on advice, added a 0.1uF across the output socket of the (separately, metal boxed) phase circuit, and floated its ground to match that of the motor drive circuit's floating ground. I did this with a 0.1uF and 470pF in parallel in breaking the ground where the ground symbol is shown on phase.jpg. I don't think this is the issue, it seems more likely I need to make the circuit less "accurate" so it takes more of a phase change to actuate the motor. I do not know if it's easily done, nor how to do it though. Thanks.
  2. VK2AKG

    VK2AKG Ham Member QRZ Page

    With the disclaimer that it is nearly midnight here (2351K) and I have an 0500 start hence I have NOT fully read the schematics here is an off the cuff response:

    Sensitivity seems to be adjusted via the 1M pot at the op amp. Try lowering its value and maybe also increasing the 1K (between that pot and Vm).

    73 Frank vk2akg
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    You need to do one or both of two things;

    The first is to lower the loop gain of the servo loop by decreasing the resistance of the 1M resistor.

    Second, some more "dead-band" may have to be to introduced in the loop. The two diodes contribute the existing "dead-band", and it may be increased by connecting one more diode in each branch.

  4. 2E0ILY

    2E0ILY Ham Member QRZ Page

    The 1 megohm pot seems to centre the circuit, not adjust gain, is that not how it's expected function? I will try adding another BAT42 to each diode leg. Curious as to the 1 meg pot though, adjusting it with a balanced Vm seems to definitely centre it.... Would changing the 1uF across the 1 meg pot alter the sensitivity or just slow changes down? I was just given the circuit to try, I have never read just how it should work, so I am having to do a lot of guesswork at my level of electronics knowledge. Thanks for the replies!
  5. AI3V

    AI3V Ham Member QRZ Page

    Your op amp with the 1 Meg resistor is running as a simple "open loop" comparator.

    That is, the tiniest difference in voltage between the + and - pins causes the op amp to output full plus or minus (saturated)

    All the pot does is set the switch voltage.

    You want to search " setting gain op amp comparator" , there are bunches of on line lectures :)

    Also search "adding hysteresis to a op amp comparator"

  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    That would be my first stab. :) Anything to reduce the overall loop gain will help, as well....even some mechanical damping!
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    As drawn, the first op-amp runs as a non-inverting amplifier with the gain set by the ratio of the 1M and 1 k resistors. The capacitor will introduce some phase lag and a roll-off at frequencies above DC (a "pole at infinity").

    However, this circuit has several drawbacks.

    One is that this high-frequency pole cannot be independently set by changing the capacitor value alone, the other is that there is almost no proportional component in the servo loop transfer function, as the ICL7667 drivers will conduct heavily as soon as the input voltages come above their threshold values.

    This effectively makes this loop into a non-linear limiting or "bang-bang" servo which will be dependent on dead-band, mechanical inertia and damping to control its movements.
    A "bang-bang" servo is inherently instable.

    I wonder if the original designer had realised the very non-linear property of the ICL7667 driver circuit, which is intended for pulse-width modulation motor control purposes.

  8. 2E0ILY

    2E0ILY Ham Member QRZ Page

    Thanks for the replies, but may I ask a few more questions please that have cropped up since looking at some YouTube videos on comparators and hysteresis?

    Why would reducing the value of a trimmer pot change anything that adjustment of it would not? I am talking of the 1 megohm multi turn pot across the first opamp. Why have BAT42's been specced rather than other diodes, do they need some special characteristics? Finally, what would be the effect of making the 1k resistor in the feed to the circuit from the phase meter a 10K adjustable pot, would that make a bigger phase swing be needed to make anything happen at all, or not? Thanks again everyone.
  9. 2E0ILY

    2E0ILY Ham Member QRZ Page

    Hi, your reply came in as I was typing my last reply, thanks for the explanation. Would you know of a ready produced circuit I could use to overcome these limitations? The phase "meter" circuit seems fine and is now boxed and finished, I will be building the rest in a separate box, so starting afresh is no big deal, I am beginning to see the limitations now! However I am certainly not capable of designing anything more suitable from scratch :(

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