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Aug 30, 2008, 06:19 AM
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Jack Crossfire's Avatar
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72mhz mojo

So the answer is still no. U can't sample analog levels near a 72Mhz transmitter. The inductor method on Goog didn't work. The PPM wire was still tempermental. After a day of trying everything, ended up converting the PPM wire to an open drain inside the shielded part of the transmitter. There was only enough room in the shielded area for a transistor. It was a one component bullseye & the PPM became 72Mhz immune.

So use open drains with direct grounding & U don't need to worry about 72Mhz energy. Still no idea how precise the PPM is. It seems a bit noisy.
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Aug 30, 2008, 01:18 PM
Private Pilot, S.E.L.S.
DonQ's Avatar
Headed-for-Fame Test Pilot Jack Crossfire,

I'm not aware of your background, so forgive me if I'm repeating things you already know.

Reading an analog signal in the presence of 72MHz should be fairly easy. It's done all the time. You will have to filter out the 72MHz and not let it back in before the signal gets to the input pin. The attached drawing shows a 'pi' filter, that may be more than you need. You could probably leave out C2 but caps are cheap. Other variation of this filter include using a resistor instead of the inductor.

Important to have the ground of the caps connect to the same ground as the analog input, in other words, a ground pin on the chip. In the case of RF, ground is not necessarily ground. It is another antenna wire. The wiring path is important, not just the signal it hooks to. Ideally, it would be parallel and close to the signal wire that connects to the analog input. Both of these wires also need to be short!

All components are imperfect, especially for RF. The capacitance of the inductor you use could largely negate the effect of the filter. Likewise, the inductance of the capacitor can have the same effect. Ceramic disk capacitors are fine, and single layer inductors (those that look like axial resistors). Don't just assume that 'more' inductance is 'better'. Using a resistor here would be better than a poor quality inductor.

The information of the analog or PPM signal would be in the mS range, and the 72MHz is in the nS range, so the filter should be set for something in-between. There's lots of room so it's not very critical. Just pick a time-constant in the high microseconds for starters. Let me know if you want more info on that.

The twisted pair would help a lot too. That is assuming that you don't want to use co-ax cable which would be even better. In both cases, the ground path at either end is just as important as the signal path.

There are some other pointers but following these should give you better results.


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