# [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 AMUs.'

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 (AMUs), 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
" Pre-amplifier noise figure
The approximation is accurate from 3 to 25 AMUs.'

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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