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Forum Index : Microcontroller and PC projects : Pre-amble question....
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Radiometrix NTX2/NRX2 pair, narrowband 434.650MHz @ 25mW 10kbps. I have a project using these modules, and they are working fine, but some of the ones at the far edge of the range don't always respond as well as I would like. They retry till reset, so it's not a major, but it does 'Annoy' me. I used 15 "U" character bytes to sync the data-slicer in the receiver, and that works very well for all the units closer to the receiver, but the ones on the outer edge - not so much. Data baud-rate is 2400, so WELL within the 10kbps rating of the modules. I thought I would try adding more pre-amble, so I increased it from 15 bytes to 50 bytes of "U", thinking and hoping this would give the receiver more time to sync, but this does not really appear to be the case. So, the question is: Can you have TOO MUCH pre-amble? I have noted that a couple of units I updated the firmware in that are well within range, now don't always work with 50 sync bytes, and I have had to go back to 15 - which then works. That is odd to me - I figured more would be better when it comes to sync bytes.  One of my other tech chums here also suggested I should be having POST-amble bytes also, but I fail to see the need for that, so what do the members think of the post-amble idea? I was of the opinion, that once you sync the receiver, there should not be any need for post-amble bytes, as you only need the sync so the receiver starts counting bytes at the right place. Once it has the last byte, the system does not care if the rest of the RX output is white-noise - it will simply ignore it till the next sync burst. Thoughts and comments? Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
The more preamble there is, the greater the chance of interference occurring during transmission. You should allow sufficient time for the transmitter to stabilize after enabling then send the short preamble. I can't see what use a post-amble would be. I am assuming that the modules are simple pass-through with no 'smarts' involved. Sometimes, a higher baud rate and sending the data packet multiple times will be more robust. Jim VK7JH MMedit |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
Hi Jim  Yes, they are 'Dumb modules' - no smarts at all in either the TX or the RX. The TX has an enable pin to minimize standby current, but the RX outputs continuous white-noise when there is no valid data. This is one application where the 8-pin PICAXE is a winner - as a data filter. The PICAXE outputs the wanted message, and ignores all the white-noise. So, it would seem that having a longer preamble is not necessarily a good thing then? I'm surprised by that, but it is what it is. I figured more preamble would give the RX longer to sync the data-slicer, but too much preamble obviously can be a bad thing then.  Smoke makes things work. When the smoke gets out, it stops! |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 5091 |
The transmitter and receiver bandwidth are fixed at 5kHz allowing 10kbps modulation. Using lower datarates (2400baud) does not increase the signal to noise as long as the receive bandwidth is fixed at 5kHz. So Jim's proposal to use higher data rate and send the message more often, and compare best out of 3 or 4 may be a solution. Another thing on the website: you may get better performance is you drive the transmitter 2Vpp around 1.2V centre (I guess you are feeding it 3.3V now) and prevent DC levels (RS232 UART ..? you can't.). Maybe use some manchester encoding.... Last: they specify different ranges for different antenna's. You could improve by choosing the best antenna... Success Volhout PicomiteVGA PETSCII ROBOTS |
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