Author Topic: Constant noise when receiving  (Read 1177 times)

MH6

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Constant noise when receiving
« on: September 23, 2017, 04:21:26 AM »
I am trying to use a RFM69HCW to receive a 915MHz FSK signal from a motion detector.

I can see the signal being received when the sensor transmits, but I also get a ton of noise when the sensor is not transmitting. I would have thought changing the RSSI Threshold would eliminate this noise, but it seems to have no effect. I see this noise both with my logic analyzer and with my oscilloscope.

The noise is around -95dBm whereas the signal is around -40dBm. Yet no matter what I set the RSSI threshold to, I get constant noise. I would expect that setting the threshold high/low enough would eventually make it so I stopped receiving any data on the DIO2 pin, but this never happens.

Here is an example of the data I am getting on the DIO2 pin. It is fairly scaled down but you can clearly see the constant noise when the detector is not actively transmitting.




I have set the RFM49 to receive in Continuous mode, and I have tried with bit syncing and without. I have also tried messing with AGC and turning it off as I thought maybe that was affecting it, but that did not help.

Any ideas as to how to eliminate the noise? I have read the datasheet probably a dozen times but am at a loss.

Felix

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Re: Constant noise when receiving
« Reply #1 on: September 25, 2017, 10:57:47 AM »
1) Wiring or schematic and/or code?
2) Are you using a Moteino or RFM69 library?
3) What antenna do you use?
4) What's the motion sensor and how does it work / packet structure / details ?
« Last Edit: September 25, 2017, 10:59:26 AM by Felix »

MH6

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Re: Constant noise when receiving
« Reply #2 on: September 25, 2017, 02:45:38 PM »
1) Wiring or schematic and/or code?
2) Are you using a Moteino or RFM69 library?
3) What antenna do you use?
4) What's the motion sensor and how does it work / packet structure / details ?


1) I have basically just hooked up the SPI lines from the RFM69 module to an Arduino Pro Mini 3.3V (for control), and have attached my logic analyzer to the DIO2 and GND pins on the module (for reading the raw data). My SPI hookups are the same as in this guide. I can post a schematic or upload a picture when I get home if preferred, but it is a very straight forward hookup.

My code right now just constantly prints the RSSI value. Around -95dBm when idle, -40dBm when a sensor is transmitting.

What's probably more useful is the RFM69 library configuration data. Mind you, I've tweaked about every parameter I could in attempts to eliminate the random noise. Having tried about every permutation I could think of for the config values, I may have mucked up the config a bit. This is just the last version I tried, after deciding to remove anything that seemed irrelevant to continuous mode (i.e. all packet/sync configs). It does the same even with all default values. I've changed bit rate, frequency deviation, RSSI threshold, I've tried turning off AGC, messing with RXBW, etc., but I always get noise (except when the sensor is actual transmitting...which makes me think its some automatic gain thing?). Bit rate should technically be 20Kbps but that should only matter when I go to actually parse the data, not when just trying to eliminate noise I expect?

2) I am using this board (915Mhz RFM69) with the RFM69 library.

3) Initially I used a 78mm length of wire, and in attempts to reduce the noise I desoldered the actual antenna from a receiver unit I had and put it on the RFM69 board, but saw no appreciable difference.

4) The motion sensor is a Guardline Driveway Sensor. You can see specifics here. It is a very simple protocol, basically transmits a 24bit ID repeatedly 30 or so times. I've already successfully read this ID by grabbing the data from the DATA_OUT pin on the Amiccom A7201B chip the receiver units use to read the transmission, now I am trying to do away with the receiver unit entirely by using my own receiver module. Since I cannot source A7201B chips I decided to try this RFM69 module first.

The data is sent at frequency 914.8MHz @ 20Kbps using FSK modulation.

I had assumed this module would be able to demodulate the signal and pass me the raw data with low noise so my MCU could parse the signal. Perhaps there is another module that would be better suited for what I am trying to do? Or if this module should be able to handle it fine, maybe there is some setting I have not configured properly.
« Last Edit: September 25, 2017, 02:48:01 PM by MH6 »

Felix

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Re: Constant noise when receiving
« Reply #3 on: September 26, 2017, 09:16:33 AM »
Noise floor at -95dBm is pretty common. Not really sure how you define noise.
To attempt to receive your sensor signal, you will have to match its FSK settings.

MH6

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Re: Constant noise when receiving
« Reply #4 on: September 26, 2017, 09:53:32 AM »
My issue isn't that the noise is around -95dBm, it's that even when setting a RSSI threshold ABOVE -95dBm (i.e. -45dBm), I still get noise on the line. Just a constant stream of garbage.

Correct me if I'm wrong, but wouldn't setting a RSSI threshold that is above the noise floor mean I get no data out on DIO2? Because it should be below the threshold, and thus ignored. Otherwise what is the point of setting a threshold?


To be more specific, this line right here:
Code: [Select]
    /* 0x29 */ { REG_RSSITHRESH, 80 }, // must be set to dBm = (-Sensitivity / 2), default is 0xE4 = 228 so -114dBm

Should this not, when set to a proper value (i.e. anything above -100dBm in my case) eliminate all the noise that is at -95dBm? Because it seems to have no affect whatsoever, regardless of what value I set.
« Last Edit: September 26, 2017, 10:03:37 AM by MH6 »

Felix

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Re: Constant noise when receiving
« Reply #5 on: September 26, 2017, 10:30:31 AM »
I think you're too stuck on the noise issue. Leave the defaults and focus on your settings. Receive signal should be way stronger than "the noise".
Just ensure your signal can be received. The "garbage" is probably because the receiver doesn't really receive anything it can "understand".