Hi everybody,
Is it normal to get static on the data out pin on the receivers when there are no transmissions active? (so I think there is not another transmitter in range that is active.)
When the transmitter is active, all the data goes over just fine, however when the data packets are spaced (100ms between packets) ‘static’ becomes present in between the packets.
It’s not a big deal I can implement a preamble and a CRC but if I don’t have too…
Thanks
Yes, it is common to have random noise on the output. When I first started using these units, I employed a squelch device similar to the one in the link that follows to quiet the output.
Usually the receiver has a data slicer that finds the average noise value, and oscillates around it. Your best bet is to send a couple of characters ahead of the valid data transmission. Keep checking for those characters in order, then you will know the folllowing data is what you want. I find three characters to work quite well at filtering out the switching noise.
RonnyM:
Usually the receiver has a data slicer that finds the average noise value, and oscillates around it. Your best bet is to send a couple of characters ahead of the valid data transmission. Keep checking for those characters in order, then you will know the folllowing data is what you want. I find three characters to work quite well at filtering out the switching noise.
Good tips. My current design uses three characters and then I try to keep the data stream balanced between 1’s and 0’s by sending the data followed by its complement. These receivers have a built in automatic gain control (AGC) which pumps up the receiver sensitivity during dead periods.
Hmmm, sounds good then. I will just use a three byte preamble.
Thx
Taking a brief sample of noise I don’t see any zeros come across. I do see repeating patterns quite frequently. Such as the attached screen shot.
Thinking about numbers… what are the odds of noise triggering a false data packet to be processed during long periods of static? Say the receiving MCU is set up for 4800bps, that’s max 600 bytes per second. With the preamble at 3 bytes : 255255255 = 16581375 possible combinations. Now take that 16million and divide by the number of seconds/minutes/hours it takes to for noise to product the required three byte combination : (16581375 / (600 / 3)) / 60 / 60 = 23.02 hours. So every 23 hours a false packet will be accepted as a true data packet.
So I guess with a five byte preamble it would take 170 years for a false packet to show up at the door step and say ‘all your data’s are belong to us’.
Is my math right?
http://img39.imageshack.us/img39/2218/48085856.jpg
**edit
Neat link.
Manchester code is often used with those low-cost units to get over the problem with unbalanced 1s and 0s.
Leon
Can any post the range they see at open air with no antenna’s attached, please. Anything past 8 feet see’s massive corruption for me.
Thanks again.
Propaganda:
Can any post the range they see at open air with no antenna’s attached, please. Anything past 8 feet see’s massive corruption for me.Thanks again.
wireless requires a careful design, good error detection and correction. As said above, you need a coding scheme such as Manchester to keep the average ratio of 1’s and 0’s close to 50%. And a proper preamble, and a proper error checking code like CRC or at least a half-sum (checksum).
Or, if you don’t want to reinvent all of this, use a radio module that costs more but all this, plus node addressing, is integral. Such as the 802.15.4 modules from Digi/XBee, Jennic, et al.
Adding a five byte preamble of decimal 170 my three byte payload come across with less corruption at a little longer range. I see about 65% packet failure at indoors 30 feet range.
My goal is to see these units max reliable indoor range, hoping to see a radius of 100 feet with obstructions. (this is seaming like a pipe dream) The end project is a simple RGB LED controller which does not require frequent updates or heavy data transfer.
On another RGB LED control project I am using nRF24L01s and they work great. Running at 2000kps I can refresh 100 single RGB LED nodes every 19ms or so. (using 10 packets of 30byte payloads) However the range is going ok with 25% packet loss at 90 feet no direct obstructions inside a domicile. Add a wall and packet loss jumps too high.
So my hope is to see good range at a low cost without using too much hardware. (One pin input is really nice and simple) Well, I guess everyone wants that. =)
Propaganda:
Can any post the range they see at open air with no antenna’s attached, please. Anything past 8 feet see’s massive corruption for me.Thanks again.
Without an antenna I would not expect much range. The antenna is very important for the operating range and the range can be greatly improved with a proper antenna.
At 434, a quarterwave antenna is around 6". Even a little 2" pigtail, would significantly increase range. Try experimenting with the shortest length that gives adequate reliability for your purposes.
Oh, yeah, the difference on using a 17cm 24gauge solid core wire and nothing is working and not working.
I am going to be taking the setup outside to true open air and test the 500ft. claim. I am expected less than 250 feet because my antennas are untuned wires.
I might etch a few PCB boards after I run through the calculators again.
Again, can anyone post what kind of range they see with these units either open air, inside, or otherwise. Along with a simple explanation of the antenna setup they are using. Thanks
Using a 432 MHz link with quarter wave antennas on both ends, I have a 2400 baud link of just under 100 feet from the family room, through two walls, outside to a weather station. I use a simple squelch system on the receiver, 3 preamble bytes, and send a data byte followed by its complement.
Thanks riden.
I would like to see that same performance. What voltage is the transmitter at?
Naturally, I see 3x range with 12v vs. 5v, however my range is not good to start out with. :? But that’s because my test bed is sloppy ATM, and, don’t slap me, I don’t have filtering caps and I am using a switching PSU. I will etch some PCBs soon.
My garage door opener (built into the sun visor), and my key fob for my car work for hundreds of feet. Lots of redundancy in transmission. 315MHz. A “garage door is open” transmitter I have yields 200 ft. or more.
Transmitter is probably less than 1mW using 3VDC. Crummy antennas.
Propaganda:
Thanks riden.I would like to see that same performance. What voltage is the transmitter at?
You’re welcome. I’m powering the transmitter with 5v, so I probably could get more range if I needed it.
Testing my parents garage door opener I see the same ~200ft. range. Just gotta pop it open and poke around and check out the antenna design.
Removing the switching wall adapter from the receiver test board and replacing the discarded power supply with three AA batteries range and reliability increased from 10ft. indoors with clear line of site and 50% packet corruption to 45ft. with heavy obstructions and only 2% packet corruption. The most amazing part is if a line was plotted from RX to TX, obstructions include three interior walls filled with ductwork, conduits, and skylight tubes.
This is a great find. Knowing that this link does fit my range needs, now I can slow down and take the necessary time to implement proper testing procedures for range and board development.
