Fig. 1.
Definition of effective noise bandwidth.
Table 1.
Relation between cut-off frequency and ENBW for RC low-pass filters of different orders.
| Filter order | Slope (dB/dec) | ENBW/ω−3 dB |
|---|---|---|
| 1 | 20 | 1.57 |
| 2 | 40 | 1.22 |
| 3 | 60 | 1.15 |
| 4 | 80 | 1.13 |
| 5 | 100 | 1.11 |
Fig. 2.
Measurement noise floor obtained by measuring a short circuit with two simultaneously sampling nanovoltmeters (NVM1, NVM2).
Fig. 3.
Measurement in high gain using two simultaneously sampling digitizing voltmeters.
Fig. 4.
Noise measurement in G=1 using two simultaneously sampling nanovoltmeters.
Fig. 5.
Noise measurement in G = 1 using two ultra-low noise amplifiers and DVMs or an oscilloscope.
Fig. 6.
Providing robust equipotentiality and minimization of loop area to reduce magnetic field pick-up: (a) two LNAs interconnected in a shielding box; (b) detail of output coaxial cables routed to an oscilloscope.
Fig. 7.
Conceptual diagram of the low noise amplifier.
Fig. 8.
Ultra low-noise amplifier. Low pass filters occupy the majority of the left half. High-stability C0G/NP0 capacitors provide only low capacity, so a large number must be connected in parallel.
Fig. 9.
Measured LNA input-referred voltage noise density. The violet curve is measured with a DVM; all others with an oscilloscope.
Fig. 10.
LNA performance check by resistor thermal voltage noise. Dashed lines are the theoretical values; LNA bandwidth is 18 Hz.
Fig. 11.
Noise floor of evaluated instruments. Ranges are as follows: NVM 1 and 10 mV, DVM 100 (200) mV, oscilloscope 4 (5) mV/div.
Fig. 12.
Measurements in high gain as per Section 2.B.
Fig. 13.
Direct measurement by nanovoltmeters as per Section 2.C. The contribution of the low-pass filter was subtracted.
Fig. 14.
Measurements using an LNA as per Section 2.D.