Noise floor measurement with Pico 4262 - Page 1
Forum section: electronics / rf-microwave

author: Integrol
author_rank: Newbie
timestamp: April 25, 2021, 01:41:20 pm
content: Hi all,I would like to measure the noise floor of several amplifiers in the 10Hz-10 MHz band.I m using 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)I would like to know if my measurement method and conclusions are correct ....My amplifier has a gain of 1 and with a short-circuit input ( Total Voltage Noise = 60.5 nV/√Hz for Gain = 1 in the datasheet ).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 infohttps://www.picotech.com/support/viewtopic.php?t=39442&start=15). M is the number of FFT Bin points. To get Vrms value, I use the calculatorhttp://www.sengpielaudio.com/calculator-db-volt.htm* To get the noise floor ( blue curve) ( per Hz), I subtract 10log(fs/2)- 10log(M/2) = 10log(fs/M).I'm measuring the noise floor of amplifiers with time mode and fft mode:BW = 5 MHzSample rate ( Fs) : 10 MHzRange = 10 mVM = 1048576 bin pointsCouplage : ACMy 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 Vnoise_RMS = Vnoise_pp/6.6 (https://www.allaboutcircuits.com/te...istribution-noise-rms-peak-to-peak-value-PSD/)Vrms = 166 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)*3MHz) = 76 nV/sqrt(Hz). We have chosen a sensitivity of 10mV so the frequency band is 3 MHz (see datasheet of the pico).This value is close to 60 nV/sqrt(Hz) but the measurement of the peak to peak value does not seem accurate in time mode.FFT mode ( with M = 1048576 points bin)I use Average mode ( with rectangular windows) :We measure -138 dBV approximately at any frequency (orange curve of the first graph).To get the Noise Floor (blue curve of the first graph). We add 10log(M/2) = 10log(1048576/2) = 57 dBThe result is -138+57 = -81dBV and therefore about 89 uV RMS with the calculatorhttp://www.sengpielaudio.com/calculator-db-volt.htm.VRTI = Vrms / sqrt( (pi/2)*5MHz) = 31 nV/sqrt(Hz).This value seems very low compared to the theoretical noise of the amplifier ( 60 nV/sqrt(Hz)).This is what I get, does the measurement method seem correct to you?Any commentary ?Thank you very much.

author: jwet
author_rank: Frequent Contributor
timestamp: April 26, 2021, 02:47:35 am
content: Your overall approach and understanding is correct. Your Pico scope sounds like it should be able to give you decent data. If you can be get within a factor of two of factory data, that really isn't bad- its a tough and almost subjective measurment. Here are a few comments to consider.1. There is a good app note on measuring noise in op-amps by Renesas- AN1560. They acquired Intersil and this is likely from them. They use a low frequency DSA, dynamic signal anaylser and some filters. Your Pico Scope is serving mostly the same purpose.2. The Art of Electronics has some very good stuff on Op-Amp noise.3. The measurement bandwidth is important and is probably poorly controlled for an instrument like the Pico. If you just plug 5 Mhz into your formulas, this probably isn't very precise. You might want to add a filter to nail this down a bit more. Of course, you'll have to de-embed the filter's noise from the measurement.