As mentioned in my last entry I was toying with the idea of frequency hopping on the RF side of things, well looks like someone has implemented something similar here http://www.raccoonrezcats.com/rfmodem.html I’ll like to do something similar to this as it seems doable.
While not able to get some “hands on” development time over the weekend, I did more reading of the HopeRF modules and these will be able to achieve the hopping, as the can programmed to any frequency in the 902-928Mhz Freq range so definately looking achievable.
If what the above links says on the site then we could (at least in theory) have a max output of 1watt if we chose to implement hopping, else we are limited to 1mW output power.
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February 24th, 2010 at 12:33 am
link not working
February 24th, 2010 at 9:56 am
bugger, looks like it’s gone. Will try and track it down.
February 24th, 2010 at 9:58 am
this was the original link to the project from hackaday:
http://hackaday.com/2007/11/17/spread-spectrum-freq-hopping-usb-rf-modem/
February 24th, 2010 at 10:00 am
I did some initial design work for a mesh using frequency hopping here if that’s any help
http://code.google.com/p/strobit/wiki/WidgetMesh
February 25th, 2010 at 1:48 am
how many channels can we achieve in the rfm12b , i have used the rfm12b-915 . i see its frequency ranges from 900 to 929 in 7.5Khz steps but have tested on two frequencies 914 and 922 , but there has to be band gap due to frequency deviation ,how much channels can i achieve ? . i will then move on to 433Mhz for greater range. but i think the channels will reduce. it has 2.5Khz steps.
February 25th, 2010 at 11:57 am
It all depends on the bit rate you select as the deviation and bandwidth are different for each datarate. The main thing is the bandwidth as deviation needs to be less than the bandwidth, so use bandwidth in your calc.
bps channels Dev(khz)BW (khz)
@9600 60 75 134
@19200 52 75 134
@38400 45 90 200
@57600 37 105 270
@115200 29 180 340
Channel 9600 19200 38400 57600 115200
1 900.84 900.84 900.84 900.84 900.84
2 901.32 901.40 901.49 901.62 901.85
3 901.80 901.95 902.13 902.40 902.85
4 902.28 902.51 902.78 903.18 903.86
5 902.76 903.06 903.42 903.96 904.86
6 903.24 903.62 904.07 904.74 905.87
7 903.72 904.17 904.71 905.52 906.87
8 904.20 904.73 905.36 906.30 907.88
9 904.68 905.28 906.00 907.08 908.88
10 905.16 905.84 906.65 907.86 909.89
11 905.64 906.39 907.29 908.64 910.89
12 906.12 906.95 907.94 909.42 911.90
13 906.60 907.50 908.58 910.20 912.90
14 907.08 908.06 909.23 910.98 913.91
15 907.56 908.61 909.87 911.76 914.92
16 908.04 909.17 910.52 912.54 915.92
17 908.52 909.72 911.16 913.32 916.92
18 909.00 910.28 911.81 914.10 917.93
19 909.48 910.83 912.45 914.88 918.93
20 909.96 911.39 913.10 915.66 919.94
21 910.44 911.94 913.74 916.44 920.94
22 910.92 912.50 914.39 917.22 921.95
23 911.40 913.05 915.04 918.00 922.95
24 911.88 913.61 915.68 918.78 923.96
25 912.36 914.16 916.32 919.56 924.96
26 912.84 914.72 916.97 920.34 925.97
27 913.32 915.27 917.61 921.12 926.97
28 913.80 915.83 918.26 921.90 927.98
29 914.28 916.38 918.90 922.68 928.99
30 914.76 916.94 919.55 923.46
31 915.24 917.49 920.19 924.24
32 915.72 918.05 920.84 925.02
33 916.20 918.60 921.48 925.80
34 916.68 919.16 922.13 926.58
35 917.16 919.71 922.77 927.36
36 917.64 920.27 923.42 928.14
37 918.12 920.82 924.06 928.92
38 918.60 921.38 924.71
39 919.08 921.93 925.35
40 919.56 922.49 926.00
41 920.04 923.04 926.64
42 920.52 923.60 927.29
43 921.00 924.15 927.93
44 921.48 924.71 928.58
45 921.96 925.26 929.22
46 922.44 925.82
47 922.92 926.37
48 923.40 926.93
49 923.88 927.48
50 924.36 928.04
51 924.84 928.59
52 925.32 929.15
53 925.80
54 926.28
55 926.76
56 927.24
57 927.72
58 928.20
59 928.68
60 929.16
February 26th, 2010 at 12:05 am
i get it , it depends on the bitrate , deviation and bandwidth , but how they are related , what is the formula for calculating in any frequency band.