Signal strength conversion to dB

Hi, I'm trying to get a formula for converting signal strength in counts into dB?

I have seen the graph in the bottom of these notes:

http://www.nortekusa.com/principles/Doppler.html

but wondering what the exact conversion is?

any help would be great

 thanks

 parach

Hi Rachel

The scaling for the conversion from counts to dB varies a little from one instrument to another.  For the Continental, I believe the correct number is about 0.4 dB/counts.  You can also see this technical note on sediment measurements for more details:

http://www.nortek-as.com/library/technical-notes/seditments

Best regards, Atle Lohrmann

Thanks Atle,

We are indeed trying to determine the SSC from backscatter (from a 4 mHz Aquadopp profiler), but the battery power reduces quite a bit over the deployment time (starting at 13.2V and finishing at 10.8V over 30 days). This must affect the signal strength, potentially affecting the results if a linear conversion is used?

Is there any way of knowing the conversion taking this into account?

we only want to look at the top bin, so depth is not a great issue this time.

thanks in advance

Rachel

It is correct that the transmission power is a function of the battery voltage.  To first order, you can simply think of the output reduction as

20log10 (Vactual/13.2V).  You can then adjust the number of SNR so you have a time-indpendent measure of the echo strength.

Best regards, Atle Lohrmann

Thanks again Atle,

I have to apologise as I am new to all this. I have calculated the conversion formula, but don't quite understand what you mean by adjusting the number of SNR? Could you possibly give me an example?

thanks in advance

Rachel

Dear Rachel, the idea is that the data should not depend on the battery voltage.  This can be achieved by adjusting for the battery voltage V(t) at time t so that the SNR in cell number is:  SNR(t) = Counts*0.4 + 20*log10 (V(t)/13.2V).   This gives you a SNR value that allows you to see how the backscatter varies over time.   The absolute value of SNR does not mean anything but SNR(t1)-SNR(t2) is quite meaningful as a measure of time variability.

- Atle Lohrmann

thanks Atle,

I now have the SNR, and have looked at the variability over time. How do I then apply this to the original data so I go from signal strength in counts to backscatter in db, normalised for the effects of reduced battery voltage over time?

thanks in advance

Rachel

Dear Rachel, I must admit that I am at the end of my ability to explain.  Maybe there is someone else in the office where you work that can help out?  We are just  humble instrument manufacturers

- Atle Lohrmann

Hi, I`m trying to use an AWAC - ADCP (600 Khz) for estimate suspended sediments concentration. In your technical note No 3 you mention something about a (linear) dynamic range of 70 db, It`s that means that any counts value bigger than 70 db can´t be use for sediment analyses? And, also, what is the minimum value that can be used?

The 70 dB linear range means that it is 70 dB between the weakest and strongest signal we can properly measure.

The minimum signal level is limited by the instrument electronic/acoustic noise level.  In my experience this corresponds to concentration levels somewhere in the range 0.1 to 1 mg/l.

- Atle Lohrmann

.

Dear Daniel León

Between 10% and 15% of the water depth (=distance from instrument to surface).  For areas with strong scattering it is closer to 10% and for clear water it is around 15%.

Best regards, Atle Lohrmann