# Frequently Asked Questions

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Noise is a random signal inherent in all physical components. It directly limits the detection and processing of all information. The common form of noise is white Gaussian due to the many random processes that make up electric currents or thermal agitation of conductive elements

If you are attempting to make a noise figure measurement typically you should choose a calibrated coaxial noise source, with either 6, 15 or 30 dB ENR . This will allow you to measure Noise Figure using the cal points provided with a noise figure meter or a spectrum analyzer. 15 dB is the most common as it can comfortably measure high and low noise figures. Very low noise figures can use a 6 dB source which will have reduced VSWR uncertainty and reduced Y factors. 30 dB sources are used in high noise figure applications or when the noise maybe injected via a coupler, or high loss device.

If you are looking to make BER measurements typically you would want to choose a higher power noise source like an amplified module or an instrument. This will allow you to set Carrier to Noise Ratios easier. Although it can be done with a low power noise source the measurement is difficult at these low powers. Since the BER depends primarily on the ratio of the Carrier to Noise, typically the CNR is set at higher powers with a power meter and a calibrated filter or by measuring on a spectrum analyzer.

If you are looking to make BER measurements typically you would want to choose a higher power noise source like an amplified module or an instrument. This will allow you to set Carrier to Noise Ratios easier. Although it can be done with a low power noise source the measurement is difficult at these low powers. Since the BER depends primarily on the ratio of the Carrier to Noise, typically the CNR is set at higher powers with a power meter and a calibrated filter or by measuring on a spectrum analyzer.

The term Gaussian refers to the voltage distribution of the source of noise. Due to its random nature the noise voltage of a component is usually a Gaussian distribution. This is characterized by its mean value and random voltage excursions that follow a bell shaped Gaussian curve

White refers to the noise source power spectral density, which is ideally flat with frequency. In reality at some point often due to mismatch there is a reduction in the measurable noise level.

Noise Figure is defined as the ratio of the signal to noise power at the input to the signal to noise power at the output of a device, in other words the degradation of the signal to noise ratio as the signal passes through the device. Since the input noise level is usually thermal noise from the source the convention is to adopt a reference temperature of 290deg K. The noise figure becomes the ratio of the total noise power output to that portion of the noise power output due to noise at input when the source is 290 deg K. Hewlett Packard App Note 57-1 describes these definitions and calculations.

Noise powers add as incoherent signals which means that their powers must be added. For example if your inject a noise source into a spectrum analyzer and see that the noise floor increases 3 dB, then the actual noise source power is at the original noise floor level.
This relationship allows you to calculate the noise power of signals below the measurement noise floor:

10 log [{Inverse log (diff/10)} -1)]

Where diff is the dB difference in measured powers. Of course small changes in power occur as the unknown noise is far below the known and this results in increasing inaccuracy as the power goes much lower.

10 log [{Inverse log (diff/10)} -1)]

Where diff is the dB difference in measured powers. Of course small changes in power occur as the unknown noise is far below the known and this results in increasing inaccuracy as the power goes much lower.

This is a convenient number to use, it represents the amount of power in a one hertz bandwidth that a thermal noise source has at the reference temperature of 290K, which is close to room temperature. This results from the equation P= kTB where k= Boltzmann’s constant, T is temp in degrees K and B is the bandwidth. For example the available thermal noise power in a resistor in a 1 MHz bandwidth would be -114 dBm because 10 log (1MHz) =60 dB is added to the -174 dBm/ Hz.

Output Power (dBm) = No + 10 Log (BW)

No = -174 + ENR for ENRs > 15 dB

No= Output Power(dBm) – 10 Log (BW)

No = -174 + ENR for ENRs > 15 dB

No= Output Power(dBm) – 10 Log (BW)

No is the noise density of the Noise Source. It is the output power per hertz that the source provides. To calculate the power that the source will have in a BW the No is added to the dB (BW). For example a -80 dBm/Hz amplified noise module with 1 GHz BW will have a minimum of -80 dBm/Hz + 10 log (1 GHz) = -80 dBm/Hz + 90 dB = +10 dBm. If this source is measured on a spectrum analyzer with the Resolution BW set to 1 MHz then -80 dBm/Hz + 10 log (1 MHz) = -20 dBm will be displayed. In actuality the noise source will have some out of band noise and the resolution BW has a noise equivalent BW greater than its setting so some adjustment of these numbers will be needed for a more accurate number. For many applications this first order approximation will suffice. Aggregate output power should be measured on a power meter, although it could be approximated by adding 10 log (BWns/RBW) to the number on the spectral analyzer. Also when performing power calculations on Noise Sources if the ENR is known the output power density can be approximately calculated by adding the ENR to -174 dBm/Hz. This is accurate to less than 0.2 dB at 15 dB ENR and less than .01 dB for ENRs greater than 30 dB. For example a 34 dB ENR noise source would have a noise spectral density of -174 dBm/Hz + 34 dB = -140 dBm/Hz. In a 10 MHz BW this would result in -140 dBm/Hz + 70 dB = -70 dBm. For lower ENRs the Th has to be solved for directly from the definition of ENR then the noise density 10 log kTh would be computed.

If you are attempting to measure a lower power noise source, < 30 dB ENR, in all probability the spectrum analyzer Noise Figure, which usually is at a minimum of 25 dB and many times is 35 dB, is above the noise level of noise source. At these levels we can approximate Noise Figure and ENR and compare directly to see if the noise source will be detectable. This source could be measured with an LNA in front of the Spectrum Analyzer although to get an exact ENR we would need to know the NF of the LNA and its gain but we can see if the approximate deflection occurs. For example a 15 dB ENR noise source should change the noise level about 10 dB if the noise figure of the LNA is about 5 dB, as long as the LNA gain is sufficient to overcome the Noise figure of the analyzer.
Higher power noise sources can be measured on a spectrum analyzer for flatness and on a power meter for output power.

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