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Author Topic: Comparing the relative percentage change of two voltage outputs.  (Read 6566 times)

KH6AQ

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RE: Comparing the relative percentage change of two voltage outputs.
« Reply #15 on: December 15, 2016, 12:29:43 PM »

Chris,

that looks like a fun project, 1 kW at 136 kHz.

The 47k connecting to the 20nF and the diode appears to have two functions: as an RC filter and a current limiting resistor into the D3. D3 isn't really needed as the ESD clamp diode in IC1b will do that function. It looks like the circuit looks at reflected power only and I've seen this in other solid state amplifiers. As the power is decreased the allowable SWR increases. I'll put the directional coupler into SPICE and see how it looks. For the directional coupler investigation I used a Stockton circuit in SPICE.

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2E0ILY

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RE: Comparing the relative percentage change of two voltage outputs.
« Reply #16 on: December 16, 2016, 06:51:32 AM »

Thanks Dave, that's very good of you and I will be very interested in how it looks. The comparators came today and hopefully over the weekend I can breadboard your schematic. Should i tap the output of the bridge before that 47K resistor, for reflected power, or after? Should I do what I have with the other IC's and decouple the comparator by the vcc pin to ground with a 220nF cap? You have been a great help!
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Best regards, Chris Wilson.

KH6AQ

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RE: Comparing the relative percentage change of two voltage outputs.
« Reply #17 on: December 16, 2016, 07:11:57 AM »

I would take voltages A and B directly from the bridge outputs. Both A and B should have the same filtering time constant so the voltages rise and fall in tandem. If B were to fall more slowly then A, the comparator could falsely trip. We want this circuit to always trip quickly when needed and to never give false trips.

Yes, it seems to be the style to always place a decoupling capacitor at each IC, even if there is another device nearby that is already decoupled. Just this week at work I commented that because everyone does it I feel forced to place caps on everything even if they are not needed and yet I can provide it by calculation or simulation.  
« Last Edit: December 16, 2016, 07:14:28 AM by WX7G »
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2E0ILY

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RE: Comparing the relative percentage change of two voltage outputs.
« Reply #18 on: December 16, 2016, 09:12:29 AM »

Thanks Dave, I'll be sure to let you know how it goes, it's much appreciated, have a good weekend!
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Best regards, Chris Wilson.

2E0ILY

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RE: Comparing the relative percentage change of two voltage outputs.
« Reply #19 on: December 21, 2016, 02:47:36 AM »

Two things need correcting. The first is that for an excessive signal on B (reflected power) to cause the comparator output to go positive, it should be connected to the non-inverting input.

The second thing is what happens when there is no RF and A and B are zero volts. The comparator input offset voltages will cause the output to go LOW or HIGH, depending on which input has more voltage offset. We need a way to make the output go LOW in the absence of signals. To do this we can introduce some positive offset voltage at the inverting input.

The link below shows the schematic. It is set up to trip if the VSWR exceeds 2:1. The reflection coefficient is 0.33 and the B signal will be 1/3 of the A signal.

The B signal is divided by 2 while the A signal is divided by 6. R5 and R4 introduce 18mV of offset to the inverting input to ensure that with no signals the comparator output goes LOW.

http://s942.photobucket.com/user/telegrapher9/media/VSWR_1A_zpshpy1nfj9.jpg.html?sort=3&o=0


I did not get chance to start building this until now, earlier you suggested that the LM339 was a suitable device, not easily prone to oscillation, yet in your schematic that you linked to you show an LT1012. Is this what you intend me to use, or the LM339? I know some people use design software and pull up a generic device with the correct appearance and leave in the part designation. Finally you show R1 pull up to *5*V, can I use the 12V rail and change the R1 resistor value? Thanks Dave
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Best regards, Chris Wilson.
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