Re: FFT Size and Signal & Vector Sources Amplitude Unit

[Wolfgang Wilde]

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.