Re: calculating a link budget

On Fri, 30 May 2008 12:33:30 -0700 (PDT), Michael <nleahcim@xxxxxxxxx>
wrote:

The page says you need 21db gain (i'm assuming per antenna) so somehow
our numbers aren't matching. Any idea why?

I'll decode your calculations later tonite. Gotta run and rescue a
customer from the mistakes I made yesterday. I hate it when that
happens.

SOM dB Reliability % Downtime per year
8 90 876 hours
18 99 88 hours
28 99.9 8.8 hours
38 99.99 53 minutes
48 99.999 5.3 minutes
58 99.9999 32 seconds

Does this table only apply for 802.11 or can it be used for anything?

Any RF link. Going from reliability to downtime is easy enough. Just
multiply the % reliability by the number of minutes in a year. I
forgot how going from fade margin to reliabily was calculated. I
gotta dig that out of a book somewhere. Sorry.

I get 249.625 meters. Our numbers are close enough for government
work, methinks :)

Read a little on signifigant figures. Generating a 6 digit accurate
number from two digit data, is rediculous. You can maintain such
level of accuracy for intermediate calculations, but your final result
will only be accurate to two digits. Try 250 meters.

Wrong. Pins 1 and 15 are TX enable and RX enable. These form the
parts of the TX/RX switch.

Sure - but what about on the Nordic part? What would drive the rx/tx
enable lines on the RF amp? Doesn't it need to be told whether the
Nordic part is sending or receiving

Dunno. Acrobat refuses to resurrect itself until I reboot. I'm in a
rush and don't to wait forever for the reboot. I'll look at it later.
If there's no tx/rx output from the Nordic chip, you get to build an
RF detector, that will sense input RF power, and use that to key the
power amplifier.

So when a power amplifier says that it is a 22dBm amplifier, it means
that it's actually adding +22dBm to the strength of the signal, not
bringing the signal to 22dBm?

The power amp does NOT add the input power to the output.
Amplification converts the input power to output power. The output
power is +22dBm. The input power is whatever drive level the data
*** says is required to get +22dBm output.

So if the NF of the amp was 5dB worse than the NF of the receiver, the
receiver sensitivity would go from -94dBm to -89dBm?

No. The equation includes the gain of the amplifier. The input
device amplifies both the signal AND the noise. See calculations for
cascading RF amplifier stages:
<http://en.wikipedia.org/wiki/Friis_formula>

If there is a good book on the topic, I would be interested! Bear in
mind I have a degree in EE, so I'm not a *complete* dolt. (just 80% of
one)

RF is magic. A sheepskin doesn't help with RF.

I think you need something basic:
"Complete Wireless Design" by Cotter W. Sayre.
A bit old (2001), but that's what fell off the bookshelf.

--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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