TEST - Fluke 8921A, 20MHz true RMS voltmeter: some performance measurements

[CarlG]

[NOTE: this post is moved from EEVBlog #406 thread]

Upon request, I have made some measurements on a vintage True RMS voltmeter.

The 8921A is 4 1/2 digits, has up to 20 MHz bandwidth, 700V max input, and resolution down to 0.1 uV on lowest range, and also selectable dB scale. Great stuff! Very useful for measuring residual noise on (switching) power supplies for instance.

The 8921A is 4 1/2 digits, has up to 20 MHz bandwidth, 700V max input, and resolution down to 0.1uV on lowest range, and also selectable dB scale. Great stuff! Very useful for measuring residual noise on (switching) power supplies for instance.

CarlG,  the manual I looked at showed 180µV as the lower functional limit independent of resolution. Is that what you find in practice. If you use it below 180µV how low does it go and what did you use to verify its accuracy below the 180µV. I am not disputing your info, I just may be interested in one if it works well in the single digit µV.

So, here's some measurement on the Fluke 8921A. The noise floor is about 20-22 uV for the lo range which limits the bandwidth to 2 MHz.

Fluke 8921A noise measurements

Noise floor:
Condition         Displayed level        Note
Inputs open     22            [mV]        Inputs floating (Zin = 1 Mohm)
                        1.55         [mV]        LO = GND
                        1.68         [mV]        LO=GND; Low range en

term=50 ohm   
HI to LO           0.091       [mV]         Inputs floating
                        0.105       [mV]         LO=GND
                        0.022       [mV]         LO=GND; Low range en
                        0.021       [mV]         Inputs floating; Low range en

Inputs shorted   0.022    [mV]         Inputs floating

Accuracy measurements

Used equipment:
1) Signal generator: Agilent 33522A
2) RF step attenuator: hp 355D
3) 50 ohm BNC feed through termination
4) BNC to banana plug adapter

50 ohm coax from generator to attentuator
50 ohm coax from attenuator to termination
termination plugged into adapter which is inserted in the DUT
----------------------------------------------------------------------

Attenuation        0         -20           -40        -60        -80       -100       -120       [dB]

Generator output
Sine 2Vrms        1.97    0.197      --          --             --         --      --           [V]

Noise 200 mV    187.5   18.79     1.849     0.186     0.031   0.0244    0.0243    [mV]
BW=2MHz

Noise 1 Vrms     901      89.2      8.86       0.901     --         --              --          [mV]
BW= 30MHz

Noise 1 Vrms     942      94.3      9.32       0.936     --         --           --           [mV]
BW=500 kHz


A rough calculation for the measured value for Noise, BW=2MHz , -100dB (based on the -60 dB value)
e_n = sqrt[e_n1² + e_n2² = sqrt[22² + (1.846/100)²] = 29 uV which I think is reasonable close to the measured 31 uV.

So my conclusion is that even though the 8921A accuray is not specifed for reading < 180 counts, it's still usable for basic masurements down in to around 30-40 uV.

Note also that the instrument isn't calibrated during the last 5 years. I don't have the calibration manual

Hope it's good enough for you (robrenz).

Comments on the picture. Display reading 18.8 uV; "mV" and "2 MHz max" LEDs are lit but hard to see
//C