Measurement of noise figure of HF103 rc1

    

“Fairly accurate comparative measurements may be made with HDSDR.  Please remember the method used, without reference to a suitably calibrated source measurements will not be absolute. But this method might provide an experimenter with a useful way of comparing equipment. “  

Source:   Signal Measurement with HDSDR  Thanks to the HDSDR Authors and to G4ZFQ.

My RF reference is a 14 pin metallic dip crystal oscillator at 20MHz with sine wave output. It’s output is 400 mVpp sine wave on 50 Ohm resistor. I placed a  20dB attenuator at the output so that the reference on a 50 Ohm load is  40mVpp,  -24dBm.  I measured the levels with an Hantek Oscilloscope.  

The procedure to measure NF is as follow:

- Connect the reference generator to HF103 antenna input and run the HDSDR application.

- Set RFgain to 0dB attenuation.

- Select the Smeter to rms mode (click with the mouse on peak/rms indication to toggle mode). This is important as peek/rms ratio is different for sinewave or noise. We need rms to measure NF.

- Select USB demodulator. I used a 3 kHz bandwidth filter.

- Tune to generator carrier and measure the tone level with the S-meter; verify that it is the expected value. In case adjust calibration gain to read  the generator level ( mine is -24 dBm ),

- Move to a clean frequency one MHz up or disconnect the generator and terminate the antenna on 50 Ohm.

- Measure the S-meter level of the noise signal with 3kHz bandwidth.   My measure is -125 dBm

- Now we can compute the thermal noise component on 3kHz

at 20°c of a 50 Ohm

K = Boltzmann's constant = 1.381 × 10-23W/Hz/K,

T = 290 K at room temperature 

B = Bandwith

KTB for B 1Hz  =  -174 dBm at room temperature. 

KTB for B 3000Hz =  -174 + 10 log( 3000 )  = - 139 dBm

The measured noise figure is:  NFmeasured =  - 125 - (- 139) = 14 dB 
The expected measurement error should be in the range 1 - 2 dB.

The estimated NF value based on HF103 scheme is:

( I use https://www.everythingrf.com/rf-calculators/cascaded-noise-figure-gain-calculator)

Stage    NF(dB)     Gain(dB)        Note /source

1            0.1            -0,1                Insertion loss 1st filter estimated value

2            1.3            -1.3                DAT-31A-SP+ datasheet typ value

3            0.2            -0.2                Insertion loss 2nd filter estimated value

4            6.2            20                  LTC6400-20 datasheet typ value

5            30,7         Soft. value      LTC2208 see  High speed ADC: one that no one had ever told you  

( see: ADC Noise Figure—An Often Misunderstood and Misinterpreted Specification by Walt Kester  )

NF        13.63                               Preview NF based on estimated components noise figure

The estimated and the measured are within the error margin.  

Here the noise level with no antenna connected.





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