Category: Receivers

The article Can we find the noise power captured by a 50Ω antenna and ambient noise figure using a SDR or spectrum analyser? gave a brief explanation of Quasi Peak (QP) as applied to noise measurement.

I say “noise measurement” loosely, because since noise is a random phenomena, what we do is to sample the phenomena over some period of time, and if we were to sample it again, whilst it might seem to be the ‘same’ noise, the random events  are new, they are a new set of samples, and summary of them is new. This contributes to the large variance observed in noise ‘measurement’.

QP is commonly used to express the magnitude of noise in the context of emissions and interference potential. QP derives from a very old CCIR recommendation describing the response of a noise measurement instrument. The recommendation included a specified meter response to short tone bursts. The QP detector has a slowed rise time, and slower decay time, so it under captures isolated very fast impulses, but accumulates high repetition impulses. The QP response reads higher than an average power detector, and lower than a peak power detector. My experience is that QP is about 4-5dB higher than average power on white noise, and typically around 6-7dB higher on off air noise with little impulse noise, but high impulse noise can result in a much higher QP reading. For more information, see (ITU-R. 1986).

Most EMC receivers and Spectrum Analysers with EMC feature have a QP ‘detector’. Communications receiver S meters often have a nearly QP response.

Above is a plot of sampled noise and QP calculation (the QP ‘detector’) made during development of FSM (Field Strength Meter) software. Observe that impulses have effect but the QP is closer to average power than repetitive peaks, much closer than infrequent impulses.

Note that Peak/Avg power for a sine wave is 3dB, QP/AVG power for a sine wave is almost 3dB. Continue reading Noise and Quasi Peak

Ambient noise

Ambient noise usually takes the form of random noise with uniformly distributed power density spectrum, ie the power per Hz of bandwidth is approximately uniform over a wide frequency range.

Ambient noise is often expressed as an ambient noise figure Fam or ambient noise temperature Tam, see ITU-R P.372.

Captured power

There is a direct relationship between ambient noise level and power captured by an antenna system in a given bandwidth.

SDRs and SAs are often calibrated in absolute power units, commonly dBm. SAs often have useful filters to slow the response and help to find the average of a varying signal (usually termed Video Bandwidth VBW).

Any features in an SDR to reduce or cancel noise will probably compromise its use as a measuring instrument.

Bandwidth

SAs as measuring instruments usually have calibrated filter bandwidths (usually termed Resolution Bandwidth RBW). SDRs often have selectable filter bandwidths, but they are often nominal bandwidths rather than Equivalent Noise Bandwidth (ENB), some work may be required to find the actual ENB.

SAs often have useful filters to slow the response and help to find the average of a varying signal (usually termed Video Bandwidth VBW).

Total, internal and external noise power

Both SDRs and SAs have internal noise, and their display is usually calibrated to display the equivalent total power at the input terminals, ie internal and external power, so to arrive at the external power, the internal power must be deducted. That said, if the internal noise is relatively low, it has little influence on the result.

The graph above shows the error in reading the level of two combined noise sources as the level of the higher one. The error is very small when the measurement is more than 20dB above the noise floor, below that the result to be calculated for best accuracy. If for example the noise power displayed on the SDR or SA with 50Ω termination on its input is xdBm, and the antenna noise reads x+10dBm, the true antenna power is x+9.5dBm. Continue reading Can we find the noise power captured by a 50Ω antenna and ambient noise figure using a SDR or spectrum analyser?

The article Is a receiver test with different resistors connected it its input terminals meaningful? ended with the question Can we find the noise power contributed by a 50Ω antenna using a receiver of known sensitivity?

The technique depends on the fact that the audio output power of a traditional SSB communications receiver is linearly related to the RF input power (including the equivalent internal noise power) up to the onset of AGC action, which is typically more than 20dB above the equivalent  input noise power. Be warned that disabling the AGC does not usually provide a significant extension to the linear range, some stage will overload but you have no warning you have reached that threshold.

For this article, I will use Equivalent Noise Temperature as the sensitivity metric, the reason will become obvious. You can convert various receiver ‘sensitivity’ metrics using the handy Receiver sensitivity metric converter.

Example

Let’s say that a certain receiver has sensitivity specified as 50Ω input, 0.1µV for 10dB S/N. The specifications probably do not give the Equivalent Noise Bandwidth, let us assume it is 2000Hz for this example (appropriate to an SSB telephony receiver but the actual bandwidth is critically important, it can be measured).

Let’s convert that specification to Tr.

Above, Tr is 434K. Continue reading Can we find the noise power contributed by a 50Ω antenna using a receiver of known sensitivity?

The question arises from time to time, is a receiver test with different resistors connected it its input terminals meaningful?

Scope

This discussion applies to linear receivers. A receiver using a diode AM detector, with or without BFO injection is NOT a linear receiver for this purpose, nor is an FM receiver. A good traditional superheterodyne SSB Communications Receiver is a linear receiver for the purpose of this discussion, but for example any techniques designed to reduce / cancel noise will render it non-linear

Noise in resistors

Thermal agitation within a resistor gives rise to broadband noise (Johnson-Nyquist noise, thermal noise), the noise power that can be captured from a resistor in a given bandwidth is given by \(P=k_0 T B\) where:

• k0 is Boltzman’s constant;
• T is the absolute temperature; and
• B is the bandwidth.

Receiver sensitivity

If:

• a receiver is designed for a 50+j0Ω source; and
• has noise figure specifications or specifications that imply a noise figure; and
• and is tested to specification

the measurement is done with a 50+j0Ω source that contributes thermal noise (50Ω @ 290K) and the equivalent internal noise contribution of the receiver can be calculated. Continue reading Is a receiver test with different resistors connected it its input terminals meaningful?

Signal to Noise Degradation, SND is a measure the extent of how the off-air or ultimate S/N ratio is degraded by a receive system.

We can calculate \(SND=10 \log (1+\frac{T_{sys}}{T_{amb}})\) where:

• Tsys is the equivalent system noise temperature at the space interface; and
• Tamb is the equivalent ambient noise temperature at the space interface.

For more explanation of the metric Signal to Noise Degradation (SND), see Signal to noise degradation (SND) concept.

The Noise Figure (NF) of a system or system component is often defined as the extent of how the system (or component) degrades S/N. The unstated assumption is that the source has an equivalent noise temperature of 290K.

We can calculate system \(NF=10 \log (1+\frac{T_{sys}}{T_{0}})\) where:

• Tsys is the equivalent system noise temperature at the system interface; and
• T0 is the assumed reference temperature, 290K.

System NF can be thought of as a measure of system degradation of S/N for a specific source noise temperature of 290K.

The two expressions above might be similar in form, but are different and give very different results.

NF does not bring to book the external or ambient noise as it applies to a specific scenario, so it does not provide a complete picture of S/N degradation. Continue reading How is SND different to NF?