Using an attenuator for NoiseLo/NoiseHi in NFM

A common method of making Noise Figure measurements of a receiver is to use a noise generator of known noise power. The output power of the DUT is measured with the generator off (NoiseLo) and on (NoiseHi), a Y factor calculated, and from that Noise Figure is calculated.

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G5RV woes – W6OGC

G5RV antenna figure prominently in many online forums.

At (eHam 2014) W6OGC describes his G5RV antenna system.

102′ center feed point at 32′ drooping off on each side to 12-15′, 31′ of 450 ohm twin lead, fed with ~130′ or so of RG-8X through a 1:1 current balun at the base of the mast, aligned roughly N-S. On no band is it resonant nor has SWRs less than 3 or so to 1…

Though a G5RV is commonly thought as a ‘no tune’ antenna, just make it to the dimensions in the book, the above shows a possible outcome.

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Simple Morse beacon keyer updated 2014/03/01

W4HBKgrab001

Above is a clip from W4HBK’s 40m grabber today, the signal is VK2OMD running 5W QRSS6 over a 14,700km path. We can infer (Duffy 2012b) from the 15dB S/N in that capture in 0.25Hz noise bandwidth, that in an 800Hz CW filter for say -5dB S/N (threshold of copy) we need 15dB more signal, or 160W for reliable copy. (Less power may be adequate for very short QSOs at the peak of fade cycles.)

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Designing high performance VHF/UHF receive systems – Part 7

Summary

This series of seven articles has:

  • explained the meaning and value of G/T as a single metric for receive system performance;
  • defined and explained the G and T terms;
  • explained the relationship between Teq and Noise Figure;
  • explained how to analyse simple cascaded stages and hence more complex networks;
  • described how to estimate transceiver Noise Figure and Teq;
  • demonstrated application of the analysis techniques to a set of practical configuration options to provided quantitative comparison of the S/N performance of the options; and
  • discussed measurement of G/T as a means of validating system performance.

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Designing high performance VHF/UHF receive systems – Part 6

Measurement of G/T

G/T can be measured using celestial noise sources provided the antenna can be pointed to them. The noise source that is most appropriate will depend on expected G/T, frequency, time etc.

Sun

Continue reading Designing high performance VHF/UHF receive systems – Part 6

Designing high performance VHF/UHF receive systems – Part 5

Bringing it all together

This part explains how to build a model of the entire receive system to calculate G/T.

Firstly, make an inventory of all of the system elements that you intend to model.

A model needs to be no more detailed than is necessary to provide adequate accuracy for the purpose at hand.

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ARRL Test Procedures Manual (Rev L) – Noise Figure calculation

(Allison et al 2011) detail the method used by the ARRL in their test reports on equipment.

Effectively they calculate NF=-174+27-MDS where MDS is measured in the CW mode using the 500 Hz, or closest available IF filter (or audio filters where IF filters are not available).!–more–>

One flaw in this method is that the factor 27dB in the NF formula implies that the Equivalent Rectangular Bandwidth (ERB) of the receiver when measuring MDS is exactly 500Hz. More correctly, the formula should be NF=-174+10*log(ERB)-MDS. The error could be significant, especially with the closest available…rovision in the test requirement.

Clip 159

For example, above is the measured IF response of a TS-2000 set to 500Hz bandwidth. As filter responses go, it is very good, having quite a flat region, better than most crystal filters used for CW, but the ERB is actually 430Hz, that is 0.66dB less noise than a 500Hz ERB filter and any NF based on 500Hz is low by 0.66dB.

The red plot is that of an idealised filter of the same ERB.

Clip 157The plot above is for an R-5000 with 500Hz crystal filter. ERB is 446Hz, 0.5dB lower than the nominal 500Hz bandwidth which would lead to an error of 0.50dB using the ARRL’s method for estimating NF.

A more fundamental flaw is  uncertainty in measured MDS leads to a relatively large uncertainty in NF when NF is small. The test reports do not specify the uncertainty of MDS, it is unlikely (Duffy 2007), (Duffy 2007b) that with receiver bandwidth of 500Hz (as specified for the MDS test) and the HP339A instrument used, that uncertainty to 95% confidence level is as low as ±0.5dB probably closer to ±1dB. The method used is just not suitable to low noise receivers.

Giving NF rounded to 1dB is not very informative for receivers with NF below 5dB, and fairly useless at 2dB as in (Wilson 2012).

References

  • Allison, B; Tracy, , M; Gruber, M. 2011. Test Procedures Manual Rev L. ARRL Newington.
  • Duffy, O. 2007. Uncertainty of the noise sampling process. https://www.owenduffy.net/files/NoiseMeasurementUncertainty.pdf .
  • ———. 2007b. Noise measurement uncertainty calculator. VK1OD.net (offline).
  • ———. 2014. ARRL Test Procedures Manual (Rev L) – Noise Floor test. https://owenduffy.net/blog/?p=1165 (accessed 15/03/2014).
  • Wilson, M. 2012 ICOM IC-9100
    MF/HF/VHF/UHF Transceiver In QST Apr 2012.