Noise factor is the ratio of noise power to the irreducible power of thermal noise. It is a measure of excess noise in a system or component and reveals how close the noise performance is to ideal. A component that adds no noise has a noise factor of 1. One that doubles thermal noise power has a noise factor of 2. Noise figure is noise factor expressed in dB.
I wanted to measure the noise figure of a CATV preamp in the FM broadcast band. Normally you need a calibrated noise source designed for the purpose, but I didn't have one. I found a way to use a calibrated signal generator and spectrum analyzer instead.
To lower the noise figure of my HP 141T/8553B/8552B spectrum analyzer, I precede it with an HP 461A amplifier with 40 dB of gain. With the combination I measure the output noise level of the test preamp with its input terminated. I compare that power with the power expected from thermal noise alone. The difference in dB is the system noise figure, which includes the noise of the preamp, 461A, and 141T. Then I measure the noise figure of the 461A + 141T alone and use it to calculate the preamp noise figure. I use the following procedure for both measurements.
My absolute level reference is a calibrated HP 8640B signal generator. I use an HP 3336C level generator to calibrate the relative levels of the 8640B signal and the noise. Correction factors account for the response of the 8552B logarithmic envelope detector to Gaussian noise and the noise bandwidth of its IF filter.
First I tune the 3336C and spectrum analyzer to 1 MHz. I set nonscanning mode and 2 dB/div. Then using the 3336C attenuator, I choose two screen levels 10 dB apart. I connect an HP 3456A digital voltmeter to the 8552B vertical output and record the corresponding DC voltages. This method provides much better accuracy and repeatability than using the screen calibration. I use maximum video filtering in the 8552B and engage the 3336C noise filter.
Next I tune the spectrum analyzer to the middle of the FM broadcast band and terminate the system input. I set the 8552B IF bandwidth to 100 kHz and adjust the log reference level so that the vertical output voltage corresponds to the −10-dB level. I replace the termination with the 8640B and tune it to peak the spectrum analyzer response. Then I adjust its output level until the vertical output voltage corresponds to the 0-dB level. Call this signal level S.
As described in HP Application Note 1303, a spectrum analyzer's logarithmic envelope detector undermeasures Gaussian noise by 2.51 dB. Its IF filter overmeasures because its noise bandwidth is greater than the −3-dB bandwidth. I measured the response of my 100-kHz filter and calculated its noise bandwidth as 107.9 kHz, a 0.33 dB overmeasurement. Thus a 10-dB difference for two CW signals becomes a difference of 10 − 2.51 + 0.33 = 7.8 dB for a CW signal and noise. Therefore the absolute noise level N is S − 7.8 dB. (S includes noise, but as the app note shows in Fig. G, the error is vanishingly small for a log detector when the noise is more than a few dB down.)
Thermal noise power at room temperature is −174 dBm/Hz. In a 100-kHz bandwidth it becomes −174 + 10log(100000) = −124 dBm. Therefore the noise figure is N − −124 = N + 124 dB.
From HP Application Note 57-1, the noise factor FS of a cascaded system is
FS = F + (FR − 1) ⁄ G
where F is the noise factor of the first stage, G is its gain, and FR is the noise factor of the rest of the system.
Preamp noise figure NF therefore is
NF = 10log(10logNFS − (10logNFR − 1) ⁄ 10logGdB)
where NFS is the system noise figure, NFR is the 461A + 141T noise figure, and GdB is preamp gain in dB.
Using the 8640B and a Wavetek SAM signal meter, I measured the gain of the CATV preamp as 13.7 dB in the FM broadcast band. Two 3336C signal levels 10 dB apart yielded 8552B vertical output voltages of −0.481 V (0 dB) and 0 V (−10 dB) on the 3456A voltmeter. At 96.9 MHz, after adjusting the 8552B reference level so that the vertical output was 0 V with the 461A input terminated in 50Ω, the 8640B signal required for −0.481 V was −103.3 dBm. This implies a noise level of −103.3 − 7.8 = −111.1 dBm and a 461A + 141T noise figure of −111.1 + 124 = 12.9 dB. Adding the preamp and readjusting the reference level with the preamp input terminated in 75Ω, the signal required was −110.0 dbm, or −117.8 dBm for the noise, and a system noise figure of 6.2 dB. Plugging these numbers into the second equation yields a preamp noise figure of 5.3 dB. I neglected the losses of the 50:75Ω matching networks I used with the preamp, whose sum I had previously measured as 0.1 dB.
88–108 MHz