In fact, in real world and with real measurement devices, the units are
not related to "instantaneous voltages" and even lesser "voltages seen
at the antenna". Any form of antenna may deliver different voltages. Is
it some Yagi antenna? Or is the active element a closed loop? Both forms
would differ totally in "antenna voltage" and both forms will transform
the "antenna signal" over impedance transformation to for example some
75 Ohms impedance cable for consumer products or some 50 Ohms impedance
cable when you have some TRX system. Real RF _measurements_ are all
related to RF-power. Even the often used db/µV targets at a given power,
as it only gives valid information when you also provide the impedance
of your RF system. This means: talking about a signal of 40 dB/µV could
be both, enough for FM receiption or to less for it, depending on what
impedance (50 Ohms/ 75 Ohms/ 240 Ohms?) you are refering to!
Never the less you do not have any SDR-Sticks out there with given
sensitivity nummbers. Even the more expensive devices like HackRF and
all the Ettus Research devices are not "measuring" some RF field
strength. All you get is somehow a resulting numeric factor of how good
the SDR can detect some signal. The sensitivity does vary widely over
the frequency ranges of this devices and it is by no means really
directly proportional to some RF power or even a voltage at the antenna
port or your antenna. You may to some degree use SDR-Sticks or devices
like HackRF, USRP's and so on for qualitative informations about a radio
signal, but it won't tell you anything about the real power of a signal,
as opposite to RF measurement systems! With all my devices (ranging from
RTL2832 based sticks over HackRF and others) you get jumps in signal
strength when you do a full band power scan. This happens for example,
when the SDR hardware switches to some other oscillator setting. All of
my devices can do a full band scan of at least 1 GHz or even more, but
none of them can do it by just stepping up the Synthesizer PLL without
for example using harmonics of the base PLL Frequency for mixing down to
the IF band or Baseband. And for each time switching to another harmonic
you get signal strength jumps. And with each other PLL frequency you
might get other mixing products, ghosting from other frequencies and so
on. So, the kind of SDR we refer to here can not give any numeric
factors to RF power nor RF voltages. It is just a numeric value, somehow
more or less proportional to the quality the SDR can receive the signal.
No units, no absolute values at all.
Regards
Am 08.03.23 um 22:35 schrieb Marcus D. Leech:
On 08/03/2023 15:43, DİREN ERDEM AYDIN via GNU Radio, the Free &
Open-Source Toolkit for Software Radio wrote:
Dear All,
Changing only FFT size of the freq sink block during simulation drops
signal power drastically, screenshots are given in the attachments. I
am providing the pulse burst from the vector source as 6144 points (6
ones and the rest of the points are zero). Since FFT sink blocks show
the average power of the points on freq domain, increasing FFT size
would increase the number of zeros in the buffer so power is reduced.
What do you think about this approach, is it ok? and there are
fluctuations in the 32k example, that's why it is thicker than 4k
plot, what is the reason for this?
Moreover, no unit is written in vector or signal sources amplitude
sections. Are units assumed as volts?
Regards,
dea
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Units are *UTTERLY UNSCALED*, but Gnu Radio normally operates within
{-1.0,+1.0} for floating-point values.
Most hardware drivers honor this system and scale their (complex,
usually) samples appropriately--so those samples will
be *linearly related* to the instantaneous *voltage* as seen at the
antenna, but turning that into real-world values
is up to the user, typically. Gnu Radio itself has no idea what
these samples *mean* in terms of the real world.
Any actual hardware device will perform considerable analog (and
then digital) signal processing on the antenna signals.
So all that you know when you get those samples into Gnu Radio is
that they're (mostly) linearly-related to an antenna voltage.
