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giorgio

RSSI on RC receiver ?

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Cyber Flyer,

I saw your homemade gen lock overlay on your website, how (where) did you find the RSSI on the RC rx and transmit it down?

What brand of radio are you using?

Also, what software are using for the arrow (vector in)?

Simon

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Both Futaba and JR PCM receivers have RSSI pin on their boards.

I attach a picture for Futaba RSSI pin (in the middle of three pin connection)

Regards,

Val.

post-6-1076807300_thumb.jpg

Edited by cyber-flyer

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I found the pin!

Now How do you send this data (volts) down to the ground station?

Which interface ?

Thank for helping me :D

Simon

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Both Futaba and JR PCM receivers have RSSI pin on their boards.

That's great to know Val. Thanks for pointing it out.

What's the voltage range?

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How do you send this data (volts) down to the ground station?

I use MIM-2 module that reads voltage and sends it as APRS telemetry packet. Another option is to use TL100 and overlay RSSI voltage directly on the screen.

What's the voltage range?

This is one of the things that I never remember. Also it may change from model to model. It's very easy to measure - just switch TX ON and OFF.

Regards,

Val.

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Here is what I get on the RSSI output of a Futaba receiver:

0.2v : Full Strength Signal (Tx on and right next to the receiver)

2.1v: No signal (Tx off)

To simulate the load of an ADC pin, I attached a 2k resistor between the RSSI output and ground. But when I do this, the voltage always remains at zero, even with no signal. This tells me the RSSI output is only capable of driving loads with very high impedance’s on the order of 1M ohm. Have you guys also found this problem. I guess this means I will have to use an opamp buffer between the RSSI output and the ADC input if this is infact true.

Edited by jheissjr

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Incase anyone is interested here is a picture of where the RSSI output can be found on a Futaba R127 receiver.

Question..

For the feedback resistor in a regular inverting opamp configuration (Gain = -Rf/Rin), are there disadvantages to using a high value feedback resistor, somwhere in the range of 1Meg-1.5Meg? I would like have a little gain in the buffer, but to do this I would have to a large value resistor in the feedback.

post-6-1138515960_thumb.jpg

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The problem may be simply the result of a sneaky series resistor on the FM chips' RSSI output. What is the FM demod chip used in the Rx? Have you tried connecting directly to it rather than on the PCB pad? And some source current, where as others sink current (yours appears to be a sinker). So, it is important to observe its requirements.

BTW, why the tough 2K ohm requirement? The AD inputs on most PICS can use up to 10K load source impedance without degradation. Higher values will work if you don't mind a small amount of attenuation (which can be accommodated in software). FWIW, I just do not believe it is worth a lot of effort to condition the signal with robust buffers unless it is absolutely necessary. So, don't give up just yet with a simple unbuffered solution. But, keep the OPAMPS nearby. :)

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I looked into this further and unfortunately I was right about the RSSI not being able to drive low impedance loads. I was wrong though about using a pull down resistor when it should be a pull up resistor. It is a sinker and I have found a 470k pull up works about right. The RF chip used in my Futaba R127 receiver that produces the RSSI output is a TA7761.

I definitely agree that 2.5k is a considerably low input resistance for an A/D converter. The only place I can find in the A/D's datasheet that talks about this is on here on pg. 10. Do you think this value I am reading is not it? I hope I am wrong, this seems too low.

post-6-1138571774_thumb.jpg

Edited by jheissjr

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On a practical note, the <1.5K ohm source impedance is the recommendation for fastest acquisition times. The datasheet mentions that higher impedances will require longer acq clock times. There is a formula on an adjacent page from your excerpt. It does mention that very high impedances will need to be buffered.

It is a sinker and I have found a 470k pull up works about right.

Yes, that value would need to be buffered. But, why use such a huge current source resistor?

Forgot to ask, what chip is used in your receiver.

I am currently using an old Hitec Supreme. It has a TA7761 with a 68K ohm current sourcing resistor. Works perfectly with a PIC microcontroller's 10-bit A/D input. I have also sampled other FM demodulators IC's and they had slightly different requirements (slope, offset, and gain related), but all were compatible with a PIC's A/D. I have no idea how they would have played with your MAX ADC.

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Its hard to see (SMD is so small) but I think on my Futaba receiver circuit board, there is a 100k pull up attached to the RSSI output. Maybe I don't need the additional 470k pull up after all. The reason why I thought adding it would be beneficial is that using it extended the RSSI range from 0-1.5v to 0-2.5v with it attached.

I have been trying to read into A/D’s and I think the built in A/D converters on PIC's are of the Sigma-Delta type. The one from Maxim I have above is a Successive Approximation A/D, or SAR. I am still trying to figure it out, but apparently Sigma-Delta A/D’s have higher impedances then SAR’s. If this is true then it would explain why your PIC is able to read the RSSI so well. Does any of this sound correct?

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I think that in the grand scheme of things, the ADC's input impedance is at the discretion of the silicon designer since they can do whatever they wish to mitigate it. My gut feeling is that you don't need to buffer the signal and it would do no harm to try it without the additional OPAMP circuitry. If you can obtain a ~1V transition from full-off to full-on RF signal levels, then you are good to go. Any data range beyond that is fluff (just my opinion).

General purpose RSSI signal level readouts are not fussy things and do not deserve extreme measures. If you are designing a commercial quality RF power measurement tool, with 80dB of dynamic range, then that is another story. As with any design, it all depends on what your requirements are.

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Two more :)

1) I may remember reading somewhere that on some receivers the servo signal outputs run at 3 volts instead of 5. I checked my receiver and the servo outputs are infact 5 volts but that may not be true on all receivers. Just kind of curious.

2) Is it possible to run two PIC's off of one crystal. If this is possible, would you just branch the leads of the crystal off to each PIC?

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1. True. It is possible to find lower voltages than 5V. The servo pulse levels are usually limited by the internal Vreg that powers the Rx circuitry (typically 3.3V to 4.0V). Some Rx's include large value series resistors (up to a couple K ohms) on the servo outputs too.

2. Yes, you can cascade as many as you want in daisy chain fashion. Be careful of EMI/RFI.

It is probably best to start a new topic since this one is getting a bit OT.

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