[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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