[time-nuts] Thunderbolt AMU to dBc conversion
Arnold Tibus
Arnold.Tibus at gmx.de
Sun Feb 14 00:11:16 UTC 2010
Magnus,
that may be fully correct. It does look like that it is not ok to look
for levels below values of 3 AMU.
At least Trimble does mention this:
'The approximation is accurate from 3 to 25 AMUs.'
I found this statement in the TSIP Reference docu
(P/N 34462-00 rev. C, April 1999)
The complete explanation for AMU / dBc/ Hz is:
'Signal Level
The Signal Level parameter (Byte 5-8) (an approximation of C/N0)
is stated in antenna amplitude measurement units (AMUs), a
Trimble devised unit. An approximate correlation of AMU levels to
C/N0 follows:
5 AMUs @ 41 dB Hz
16 AMUs @ 51 dB Hz
26 AMUs @ 55 dB Hz
Formula: C/N0(dBHz) @ 20 log(AMU) + 27
Four basic parameters affect C/N0:
" Signal strength from the GPS satellite
" Receiver/antenna gain
" Pre-amplifier noise figure
" Receiver noise bandwidth
The approximation is accurate from 3 to 25 AMUs.'
It took me a while to find it in the net.
I found that the given offset value of 27 is too high for my
Thunderbolt, fiddling a bit I find the value of 23.5 a good candidate
with perhaps some tenths of variance. I could check it between
AMU 5 to 18... Perhaps there are some 'unlinearities' still in,
but it matches also reasonable with the values I reported in a old
mail ...
Arnold
On Sun, 14 Feb 2010 00:50:57 +0100, Magnus Danielson wrote:
>Arnold,
>Arnold Tibus wrote:
>> Magnus & Mark,
>>
>> for my Thunderbolt system I found a very good correlation
>> between AMU and dBc/ Hz ( observed on my system using
>> the ThunderboltMonitor watching over long time stable satellite signals )
>> when I use the Formula: C/N0(dBHz) = 20 log(AMU) + 23.5.
>It would work well, but for lower AMU values (in particular 0) it would
>not work so well. Something like the above was my initial assumption.
>The availability of AMU=0.0 for a 20 dBc/Hz signal suggests a little
>more happens.
>Cheers,
>Magnus
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