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Noise Figure inferences of spectrum analysis of weak signals

This article reports some observations using two common PC based audio spectrum analysis tools to analyse the strength of weak signals.

In all of these measurements, signal level was below the AGC threshold, so the receiver gain was fixed at maximum. If signals cause AGC gain compression, the analysis below does not necessarily apply.

Stage 1 - SSG

In Stage 1 the receiver is connected to a standard signal generator. The signal generator was adjusted for 10dB SINAD at the receiver output using a Motorola 1012 SINAD meter. The Effective Noise Bandwidth of the receiver has been measured at 1450Hz (Measuring receiver bandwidth). Table 1 shows the calculated Noise Figure.

Table 1: Sensitivity measurement with SSG and SINAD meter.
Quantity Value Comment
SSG power -124.5dBm  
SINAD 10dB  
ENB 1450Hz  
NF 8.3dB  
S/N 9.5dB  

Fig 1 shows the display in Spectrogram.

Fig 1: Spectrogram analysis of SSG.

The Spectrogram analysis indicates signal level of -30dB, noise in 2.7Hz bandwidth adjacent to the signal of -62dB, so a S/N of 32dB.

Table 2: Analysis of Spectrogram display.
Quantity Value Comment
SSG power -124dBm  
S/N 32dB  
ENB 2.7Hz  
NF 13.2dB  

The Noise Figure for the narrowband measurement around the carrier is quite different to the Noise Figure using the SINAD meter. The reason for this is the influence of the receiver audio response on the total audio noise power. Although the input noise power is broadband, and essentially flat across the receiver channel, the effect of the filtering and audio shaping is to reduce the contribution of higher audio frequencies to the total noise output power, having the effect of reducing the total noise output power for an improvement in SINAD and NF, albeit somewhat deceptively.

Calculation of SINAD from an export of the spectrum log from Spectrogram accounts for the amplitude / frequency response of the noise output, and reconciles with the measured SINAD.

Another factor that may contribute is the FFT windowing used in the software, but since neither package describes its windowing, nothing more can be said.

Fig 2 shows the display in Spectran.

Fig 2: Spectran analysis of SSG.

The Spectrogram analysis indicates signal level of -22.7dB, noise in 2.7Hz bandwidth adjacent to the signal of -57.0dB, so a S/N of 34.3dB.

Table 3: Analysis of Spectran.
Quantity Value Comment
SSG power -124dBm  
S/N 34.3dB  
ENB 2.7Hz  
NF 10.9dB  

Again, the indicated Noise Figure is quite an amount better than measured with the SINAD meter. See the comments re Spectrogram, the same issues apply.

Stage 2 - Antenna

In Stage 2, the receiver is connected to an antenna pointing at the beacon at Nimmitabel on 144.413MHz.

Fig 3 shows the display in Spectrogram.

Fig 3: Spectrogram analysis of beacon.

The Spectrogram analysis indicates signal level of -26dB, noise in 2.7Hz bandwidth adjacent to the signal of -53dB.

Table 4: Analysis of Spectrogram.
Quantity Value Comment
S/N 34.3dB  
ENB 2.7Hz  
NF 10.9dB  

Fig 4 shows the display in Spectran.

Fig 4: Spectran analysis of beacon.

The Spectran analysis indicates signal level of -22.6dB, noise in 2.7Hz bandwidth adjacent to the signal of -56.9dB.

Table 1: Analysis of Spectran.
Quantity Value Comment
S/N 34.3dB  
ENB 2.7Hz  
NF 10.9dB  

Determining ambient noise level

The ambient (or external) noise level can be determined by measuring the change in noise between a known noise source and the antenna. The noise level measured in the first stage is from a known source, it is the noise in a resistor which can be calculated.

The change in noise from stage 1 to stage two can be assessed from the spectrum displays, additionally, it can be measured with a true RMS voltmeter.

Table 1: 
Quantity Voltmeter Spectrogram Spectran
Tcold (K) 290 290 290
Noise on SSG (dB) -16.2 -62 -57.0
Noise on Antenna (dB) -7.9 -53 -48.6
Y (dB) 8.3 9.0 8.4
Ta ( K) 14,510 17,430 14,900
Fa (dB) 17.0 17.8 17.1

Links

Many calculations in the above tables were performed with the following tools:

Changes

Version Date Description
1.01 15/10/2006 Initial.
1.02    
1.03    
1.04    
1.05    

V1.01 20/02/09 09:57:11 -0700 .


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