LINRAD AS A NF METER. S/N DIFFERENCES AT 50 OHMS.
(Aug 13 2012)
This page is a sub-page to LOW NOISE AMPLIFIERS. HOW TO OPTIMIZE AND MEASURE PERFORMANCE.

S/N at room temperature is the same as the NF.

The definition of NF, noise figure, is that it is the loss of S/N compared to the S/N of a totally noise free receiver when listening to a signal that comes from a room temperature source (290 K).

Linrad can be set to display S/N in the S-meter graph. By adding a scale shift one can arrange for the S-meter graph to show i.e. -0.64 dB for an amplifier with a known NF of 0.64 dB when a signal is sent into Linrad through a room temperature attenuator.

Linrad evaluates S at a narrow bandwidth which is always OK, but the noise is evaluated at the full bandwidth supplied by the hardware and that may cause errors.

As an example the amplifiers evaluated here: A study of several low noise amplifiers with Linrad-03.41 give S/N readings at 273K as listed in table 1.

   Unit           NF      S/N at 273K   S/N at 273K     Diff
  (name)         (dB)          (dB)        (dB-.2105)     (dB)
PSA4-5043        0.64        -0.4295         -0.64         0
MGF1425old       0.62        -0.0642         -0.2747      0.3453
MGF1801          0.31         0.0190         -0.1915      0.1185
MGF1425          0.28         0.0675         -0.1430      0.1370
ATF33143         0.24         0.0084         -0.2021      0.0379
2xAFT33143       0.24        -0.0373         -0.2478     -0.0078
ATF33143negimp   0.25         0.0652         -0.1453      0.1047
Table 1.. The S/N value for a noise bandwidth of 1.8 MHz reflects the NF to within about 0.1 dB except for the MGF1425old amplifier which seems too good by 0.3 dB. That is because of the selectivity of this amplifier that reduces the effective noise bandwidth.


Measurement system.

Linrad was running with the SDR-IP at a bandwidth of 145 kHz. An SBL1 Schottky diode mixer was connected to the input with a HP8657A as the LO at 134 MHz. In front of the mixer a filter with more than 60 dB attenuation on the mirror frequency 124 MHz was inserted to ensure that all the signal reaching the SDR-IP on 10 MHz originated in 144 MHz. Low noise amplifiers in front of the filter provide a system NF of 3.4 dB. Another HP8657A signal generator was connected to precision attenuators to provide an accurate match to 50 ohms as measured on the connector used to connect to the different amplifiers under test.

Stability can be an issue in this kind of measurement. Figure 1 shows S/N vs time (yellow) and signal level (white.) with the FHX05FA/LG connected.


Fig. 1. Measurement of S/N (yellow) and signal level (white) over nearly 10 hours. The start is 0350 with a temperature of 23.4 C at the precision attenuator. At 1210 when the temperature was 24.1 C the windows were opened. At 13.25 the measurement was stopped at a temperature of 22.4 C.



From figure 1 one can conclude that a temperature change from 24.1 to 22.4 causes a S/N loss of about 0.035 dB. The temperature ratio 297.25/295.55 should give an Y-factor of 0.025 dB (neglecting the noise from the FHX05FA/LG amplifier.) The signal level from the HP8657 generator is also affected by the temperature.

Amplifiers were compared in groups of three. Table 1 shows the results. screen dumps are here.


No    Time    FHX05FA/LG  ATF33143negimp  2xATF33143   ATF33143   MGF1425   MGF1801  MGF1425old   PSA4-5043
1       0       0.2383          -             -            -      0.2680       -          -         0.6428
2      11          -            -          0.3358          -         -         -        0.5512      0.6429
3      23       0.2463       0.2066           -            -         -      0.3146        -            -          
4      34          -         0.1988           -         0.2790    0.2646       -          -            -    
5      46          -            -          0.3300       0.2780       -         -        0.5528         -
6      58       0.2343          -          0.3346          -         -      0.3056        -            -  
7      70          -            -             -         0.2786       -      0.3107      0.5485         - 
8      82          -         0.1977           -            -         -         -        0.5470      0.6403   
Average         0.2396       0.2010        0.3335       0.2785    0.2663    0.3103      0.5499      0.6420 
Table 1.. S/N values for the different amplifiers at T=297 K


The system NF in these measurements was 3.4 dB. The corrections for the contribution of subsequent stages and for measuring at 297 K rather than at K are computed in table 2.


___1______________2_________3_______4________5_______6________7_______8____
   Unit           NF       S/N     Gain    T2nd       T      Tcorr      NF    Diff
  (name)         (dB)     (dB)     (dB)    (K)     (K)     (K)    (dB)    (dB)
ATF33143negimp   0.25    -0.201   30.5    0.30    14.07    14.04   0.205   0.045  
FHX05FA/LG        ?      -0.240   29.5    0.38    16.88    16.50   0.240     -
MGF1425          0.28    -0.266   28.7    0.45    18.76    18.31   0.266   0.014
ATF33143         0.24    -0.278   25.4    0.97    19.63    18.66   0.271  -0.031
MGF1801          0.31    -0.310   25.5    0.95    21.98    21.03   0.304   0.006
2xATF33143       0.24    -0.334   25.0    1.06    23.74    22.68   0.327  -0.087
MGF1425old       0.62    -0.550   29.5    0.38    40.09    39.71   0.557   0.063
PSA4-5043        0.64    -0.642   22.4    1.93    47.31    45.38   0.631   0.009
None             0.00    -3.40     0.00    336      336  
Table 2.. Evaluation of table 1.
Col. Explanation
1 Amplifier name. (None is a BNC female to BNC female.)
2 NF as evaluated here.
3 S/N values from table 1.
4 Amplifier gain.
5 Contribution to input noise from the test system (NF=3.4 dB.)
6 Noise temperature of amplifer assuming column 3 is the correct NF at 297 K.
7 Corrected noise temperature.
8 NF of amplifier (referenced to 290 K.


By connecting a reactance, an LC resonator, 1/8 wavelengths from the test object on the input side one can tune the resistive part of the source impedance up or down. Table 3 shows S/N at 33 and 75 ohms (SWR=1.5) as a reference the S/N values of table 1 are included. The numbers are extracted from screen dumps here. The losses of the impedance changer are not included in the table.



   Unit         S/N(33)    S/N(50)    S/N(75)
  (name)         (dB)       (dB)        (dB)
ATF33143negimp  -0.32      -0.20       -0.21  
FHX05FA/LG      -0.33      -0.24       -0.27
MGF1425         -0.38      -0.27       -0.28
ATF33143        -0.41      -0.28       -0.28
MGF1801         -0.42      -0.31       -0.35
2xATF33143      -0.43      -0.33       -0.40
MGF1425old      -0.74      -0.55       -0.54
PSA4-5043       -1.08      -0.64       -0.84
Table 3. The NF at different source impedances. The losses of the impedance changer are not included.


It is obvious from table 2 that the amplifiers are not well matched to 50 ohms. Only the FHX05FA/LG amplifier has its best NF for an impedance close to 50 ohms. That is an amplifier tweaked by use of Linrad as an NF meter. All the other amplifiers were tuned long ago. The purpose of this page is to provide a consistent set of NF values to compare with results to be obtained at the 2012 meeting in Cambridge Several amplifiers could have been tuned for a lower NF, but I will bring them as they were tuned long ago before I had access to Linrad in S/N mode. Hopefully the NF meters will rank the amplifiers in the same order and give similar differences for the NF values.