Re: [Discuss-gnuradio] dB or dBm

[lists]

When a signal is modulated, you need to use a FFT technique ‎to get the power. My experience is all based on outmoded analog modem design, but the idea is the same. The hardware FFT based spectrum analyzer sums up the power in all the bins and gives you a number.  

A sine wave only has one bin. So a modulated signal will always have lower peaks than a sine wave for the same power.

I have a HackRF and appreciate  this work.

From: Ralph A. Schmid, dk5ras

Sent: Sunday, June 25, 2017 11:44 PM

To: 'Murat Aksu'; address@hidden

Subject: Re: [Discuss-gnuradio] dB or dBm

Well, dBm is an absolute power, based on 0dBm = 1mW. The dB figures of your receiver are only relative values, they have no meaning. However those get interesting when something changes. You just need to understand that the input of -20dBm power has nothing to do with the resulting reading of -58dB. When you reduce the input power for another 10 dB (no matter if you turn it down or if you insert an additional 10dB attenuation, makes no difference), then your reading should change from -58dB to -68dB.

 

Then you have found a part of the linear range, where a direct relationship exists between the input power and the power reading.

 

AGCs may influence this, also nobody knows how accurate the peak power measuring works. As others already had mentioned, it could be useful to play with unmodulated carriers, just to get a feeling for this stuff.

Ralph.

 

From: Murat Aksu [mailto:address@hidden
Sent: Thursday, June 22, 2017 4:58 PM
To: Ralph A. Schmid, dk5ras
Subject: Re: [Discuss-gnuradio] dB or dBm

 

Dear Ralph,

 

Thank you so much for your support. I really do not understand these dB values. When I inject 802.11g signal with -20 dBm power level and 20 dB attenuator which means -40 dBm going in the HackRF One, after running gr-scan command line or QSpectrumAnalyzer GUI, I am observing peak power values around -58 dB.

 

As you suggest, if I use 30 dB attenuator instead of 20 dB attenuator, I will still get some dB values and really don't know how they are related to power levels.

 

I would appreciate your guidance about this confusion.

 

Best,

Murat

 

---

From: Ralph A. Schmid, dk5ras <address@hidden>
Sent: Thursday, June 22, 2017 7:38 AM
To: 'GNUBeginner'; address@hidden
Subject: RE: [Discuss-gnuradio] dB or dBm

 

But still the dynamic range usually ends already quite below the maximum
allowed level what is more a kind of hardware protection rule.

Add some more attenuation and see if at least a 5dB change on your
transmitter creates a 5dB change on your SDR. Then you are in the linear
range and can start trying to calibrate the thingy for your frequency. And
turn off AGC if this option is available.

Ralph.

> -----Original Message-----
> From: Discuss-gnuradio [mailto:discuss-gnuradio-
> address@hidden] On Behalf Of GNUBeginner
> Sent: Wednesday, June 21, 2017 10:48 PM
> To: address@hidden
> Subject: Re: [Discuss-gnuradio] dB or dBm
>
> I am already aware of what the maximum allowable receiver power which is
-5
> dBm. That is why I am starting from 0 dBm with 20 dB attenuator before
> injecting it to the HackRF One.
>
>
>
> --
> View this message in context: http://gnuradio.4.n7.nabble.com/dB-or-dBm-
> tp64323p64335.html
> Sent from the GnuRadio mailing list archive at Nabble.com.
>
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