Harald Friis (Friis 1944) gave guidance on measuring the noise figure of receivers, and explains the concept of Effective Bandwidth.
The contribution to the available output noise by the Johnson-noise sources in the signal generator is readily calculated for and ideal or square-top band-pass characteristic and it is GKTB where B is the bandwidth in cycles per second. In practice, however, the band is not flat; ie, the gain over the band is not constant but varies with frequency. In this case the total contribution is ∫GfKTdf where Gf is the gain at frequency f. The effective bandwidth B of the network is defined as the bandwidth of an ideal band-pass network with gain G that gives this contribution to the noise output.
Continue reading Noise Figure – Equivalent Noise Bandwidth
A reader of my articles commented on them and some of my calculators regarding the use of 290K as the reference temperature (T0) for Noise Figures.
(Friis 1944) suggested that temperature as reference temperature and it has been widely used since. One may also see 293K (eg in certain ITU-R recommendations), but in my experience, 290K is most commonly used and is for instance the basis for calibration of Keysight noise sources in Excess Noise Ratio (ENR).
The assumption in measurement of Noise Figure or of sensitivity is that the ‘cold’ source has a known source resistance with Johnson noise equivalent to 290K (16.85° C). That noise producing resistance is commonly achieved using a large attenuator at the generator output.
References / links
- Friis, HT. Noise figures of radio receivers. Proceedings of the IRE, Jul 1944 p420.
- Keysight. Jul 2018. Keysight 346A/B/C noise source operating and service manual.
NFM has been updated to v1.19.0.
The update corrects an error in conversion between ENR and temperature where Tcold<>290K.
- Duffy, O. 2007. Noise Figure Meter software (NFM). https://owenduffy.net/software/nfm/index.htm (accessed 01/04/2014).
Minimum ambient noise level – ITU-R P.372-13 guidance discussed S/N degradation in a receive system with given noise figure (NF) based on ITU-R P.372-13. This article uses the same data to determine the maximum acceptable receiver noise figure for a given S/N degradation.
The analysis assumes linear systems (eg no signficant intermodulation distortion).
What is the minimum ambient noise level?
Above is Fig 2 from ITU-R P.372-13 which shows some key components of total ambient noise. The solid line is entitled “minimum noise level expected”, and it is a combination of curves B, C and D. Above 0.7MHz, only curves C and D are at play. Continue reading Maximum acceptable receiver noise figure – derived from ITU-R P.372-13 guidance
I recently came across an article Signal level measurement with PowerSDR and external transverters in which Carol (KP4MD) details a set of measurements of a Flex 1500 transceiver and Electraft XV144 transverter.
Carol gives the following table of measurements and calculated results.
|Table 1. Transverter Measurements
|50 Ω expected
Lets focus on the 144MHz measurements. Continue reading Noise Figure measurement of a converter / transverter
This article is a follow up to Optimum receive system noise figure for given ambient noise – Flex 6600 using data published at (Farson 2014) to make similar estimates for the Flex 6700.
How to determine the amount of RF Preamp gain to apply for band conditions suggests that the 6700 figures might also apply to the 6500 and 6400(M).
Farson gives a table of MDS in 500Hz bandwidth figures for the 6700 on certain bandws, including MDS for 4 RF Gain configurations, 0, 10, 20, and 30dB.
Above is Farson’s data with my chosen RF Gain option (selected for SND<3dB) and calculated values in yellow and orange for: Continue reading Optimum receive system noise figure for given ambient noise – Flex 6700
Gerald Youngblood (K5SDR) of FlexRadio wrote of
optimal receiver noise figure relationship to antenna noise in a blog posting about SDR receivers.
This article discusses that posting in the context of linear receivers, ie effects of intermodulation distortion are not included.
His gives the following advice:
For optimal weak signal performance near the atmospheric (antenna) noise floor you want your receiver noise floor (sensitivity/MDS) to be 8 to 10 dB below the noise coming from the antenna. For strong signal reception, less sensitivity is almost always better.
The terminology is not industry standard, but that is quite usual for hams who have a need to redefine well known terms, and this is really loose with implied equivalence (eg sensitivity/MDS).
ITU-R P.372-14 speaks of natural noise as including
atmospheric noise due to lightning, and also speaks of man made noise.
It is likely Youngblood is actually talking about man made noise since he uses man made noise figures from an earlier revision of P.372.
Optimal is a compromise between weak signal performance (ie S/N degradation due to internal receiver noise) and handling of strong signals that might clip in the ADC of an SDR receiver.
He gives a table of measured MDS (minimum discernable signal, which actually is synonymous with Noisefloor) for recommended configurations of a Flex 6600 radio on several bands.
Above is Youngblood’s data with my calculated values in yellow and orange for: Continue reading Optimum receive system noise figure for given ambient noise – Flex 6600
Comments were received from some readers of the article S/N degradation is related to external noise level and receive system internal noise.
Essentially, two questions were asked:
- what is the minimum HF ambient noise level; and
- explain observation of lower HF ambient noise level.
What is the minimum ambient noise level?
Above is Fig 2 from ITU-R P.372-13 which shows some key components of total ambient noise. The solid line is entitled “minimum noise level expected”, and it is a combination of curves B, C and D. Above 0.7MHz, only curves C and D are at play. Continue reading Minimum ambient noise level – ITU-R P.372-13 guidance
A question that arises from time to time is what is the minimum receiver noise figure for a given application.
This discussion considers the question applied to linear receivers, ie receivers with zero intermodulation distortion (IMD) and other non ideal characteristics, other than their internal noise which can be described by their Noise Figure (NF).
By definition, NF is the amount by which the component or system degrades the NF, so in dB it is the difference in the S/N in to S/N out. Implicit in that definition is that it is based on source internal noise of 290K equivalent.
So for example lets say a receiver with quivalent noise bandwidth 2000Hz measures sensitivity of -125dBm for 10dB S/N out. We can calculate the noise in 2000Hz bandwidth from a 290K source to be -141dBm, and therefore the input S/N is -125 – -141 = 16dB. The ratio of the input S/N to output S/N is the difference in those in dB, 16-10=6dB. The NF is 6dB. We can also calculate an equivalent internal noise temperature of (10^(6/10)-1)*290=865K.
By convention, ambient noise (or external noise) is expressed in Kelvins, or dB wrt 290K. That does not imply that an antenna contributes exactly 290K. Continue reading S/N degradation is related to external noise level and receive system internal noise
For a lot of experiments, knowledge of the Equivalent Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency.
Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.
In brief, a white noise source is connected to the receiver input, Filter2 (nominal 500Hz bandwidth sharp response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.
Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.
The method is explained in detail at Measure IF Bandwidth.
Above is the spectrum plots, as receivers go this is relatively flat.
Continue reading Equivalent noise bandwidth – IC-7300 CW Rx Filter2 – (500Hz sharp)