Noise floor measurement with Pico 4262

Thread Starter


Joined Apr 15, 2021
Hi all,

I would like to measure the noise floor of several amplifiers in the 10Hz-10 MHz band.
I would like to use a pico 4262 ( Dual channel oscilloscope / spectrum analyzer, 16-bit resolution, Low distortion (96 dB SFDR), Low noise (8.5 µV RMS), 5 MHz bandwidth)

In FFT mode, the noise displayed by the pico scope is the orange curve (that is the FFT Noise Floor).

* To get the RMS noise level ( in uV), I add 10log(M/2) (Process gain, more info ). M is the number of FFT Bin points. To get Vrms value, I use the calculator

* To get the noise floor ( blue curve) ( per Hz), I subtract 10log(fs/2)- 10log(M/2) = 10log(fs/M).

I measure the noise floor of a certain amplifier in two ways ( time mode and fft mode):
BW = 5 MHz
Sample rate ( Fs) : 10 MHz
Range = 10 mV
M = 1048576 bin points
Couplage : AC

My goal is to obtain the RTI Voltage Noise spectral density ( in nV/sqrt(Hz) )

Time mode :


The peak to peak voltage is about Vpp = 1.1 mV (measured by hand, I don't trust the values displayed).
So Vrms noise = Vpp/6.
So we have Vrms = 180 uV. We want to get the RTI Voltage Noise spectral density , so we divide by the square root of the band:

VRTI = Vrms / sqrt( (pi/2)*5MHz) = 6.4E-8 = 64 nV/sqrt(Hz).

FFT mode ( with M = 1048576 points bin)

I use Average mode ( Blackman windows) :


We measure -136 dBV approximately ( orange curve of the first graph).

* To get the Noise Floor ( blue curve of the first graph)., I subtract by 10log(Fs/M) = 10log(10E6/1048576) = 10 Hz so 10 dB.
The result is -146dBV and therefore about 45nV/sqrt(Hz) with the calculator.

Another way to do it:
* To get the RMS value, we add the process gain ( 10log(1048576/2) = 57 dB.
The result is -79 dBV RMS and therefore 112 uV RMS with the calculator.
We want to get the RTI Voltage, so we divide by the square root of the band:
VRTI = Vrms / sqrt( (pi/2)*5MHz) = 112E-6 / sqrt( (3.14/2)*5MHz) = 40 nV/sqrt(Hz)

This is what I get, does the measurement method seem correct to you?
Any commentary ? Indeed, noise measurement is a rather complex measurement, and it is easy to measure anything!

Thank you very much.

Deleted member 115935

Joined Dec 31, 1969
pico have an active forum and tech support site,
why not ask them how they test / define the numbers ?