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Hi!
I fully agree with "for modulated signals, power estimates get
more complicated"; I have the uncontrollable urge to be a
smartass, however, about this:
A sine wave only has one bin.
Nope. That's true only for oscillations that fit exactly in
the number of samples observed by the DFT you're doing. For example:
Assume you do a 1024-FFT¹.
When your sine has a period of let's say 64, everything is fine. You
get an DFT that's all zeros but for one bin, because the oscillation
that repeats 16 times within one DFT correlates perfectly with that
(so you get one peak for the negative- and one peak for the
positive-frequency complex sinusoid).
But suppose you do a sine of periodicity 65! Now, there's no single
bin that correlates "perfectly", because 65 is not a factor of 1024.
But: Parseval's Theorem states that the energy can't disappear. In
fact, that energy ends up in neighboring bins.
Which, by the way, is the reason why it's dangerous to trust a
DFT-based spectrum estimate, unless you know that the signal
bandwidth is significantly larger than the bin spacing. You can only
achieve that through increasing the DFT (FFT) length, and that in
turn increases the observation time.
Best regards,
Marcus
¹NB: DFTs /FFTs do not have to be powers of two in length.
There's plenty of FFT implementations for length that can be
factorized to "small" prime factors, and you can do any (non-FFT)
DFT length, albeit at higher computational cost. So, a 1023-DFT is
perfectly possible to have.
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
Subject: Re: [Discuss-gnuradio] dB or dBm
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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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