# Estimating signal strength from PCM-encoded signal

I am processing rtl_fm output (in time domain), extracting Manchester encoded bits. I can detect preamble and, therefore, decode the message which follows the preamble. This is how my signal looks like (here I have 2 messages, the rest is noise):

And this one of the messages zoomed in:

Now I want to know how strong was the signal that I've just decoded. I know what is the average amplitude for my symbols: "1" is 2900 and "0" is 1500 (for the amplitude range of -32768 .. 32767). Is it possible to know the signal strength in dB in this case?

## 1 Answer

In general, it is very hard to estimate the power of a signal after it has gone through an analog front-end and sampled. The front-end filters and mixes the signal, which can amplify or attenuate it. It's also common to have an automatic gain control (AGC) stage, which adjusts the signal's amplitude to match the ADC's input range and maximize dynamic range. In many cases, the digital back-end will also normalize the signal amplitude to a pre-defined range, such as -1 to 1.

If you understand every piece in the system very well, you may be able to estimate the signal's power from its samples. Another alternative is to use a scope or a spectrum analyzer to measure the signal as close as possible to the receiver's input.

• baz is right. you need to know what gains the pre-amp to the ADC have and you need to know the $V_\text{ref}$ of the ADC. and the way to sorta skip over all of that is to measure a few sine waves at different frequencies, both at the input with a scope and with the digital display you have shown in the question. that should give you a pretty good absolute reference. – robert bristow-johnson Jan 24 '16 at 0:23
• just now discovered how old and stale the question (and answer) is. i wish "Community" would not bother us with old and stale questions that may be left without being marked as "answered" by the OP. – robert bristow-johnson Jan 24 '16 at 0:27
• @robertbristow-johnson The comment is still appreciated, at least by me. – MBaz Jan 24 '16 at 1:15
• yeah @MBaz, but i still wish "Community" would not be forever resurrecting these stale questions. i tire seeing them. – robert bristow-johnson Jan 24 '16 at 5:31