Portsdown NF Meter

The Portsdown 4 includes a Noise Figure Meter (STILL UNDER DEVELOPMENT) that uses the LimeSDR Mini and a user-provided noise source and custom switching power supply to provide an indication of the performance of preamps and transverters. It is hoped to describe this emerging capability in CQ-TV 274 (Winter 2021). This Wiki page is purely to inform the early beta testers before the capability goes on general release.

General Principle

The limiting factor on the performance of many microwave receivers and some VHF receivers is the amount of noise generated by the input stage of the receiver. G8GTZ discussed this in his excellent CAT 21 Video which is essential viewing before trying to use the Portsdown 4 Noise Figure Meter.

The input stage noise can be estimated by making comparisons between the receiver output with no input, and its output when connected a calibrated low-level noise source. This is the method used by the popular HP Noise Figure meters and the method used in the Portsdown. The noise source is permanently connected and switched on and off at about 10 Hz. The noise level at the output if the preamp/transverter is measured during the on and the off periods, averaged, and then the noise contribution of the receiver itself can be calculated.

The noise contribution from amateur preamps and transverters is normally stated as a logarithmic "Noise Figure" measured in dB. Levels around 3 dB are acceptable for VHF receivers (where there is a lot of atmospheric noise) whereas good microwave receivers typically achieve between 1 and 2 dB. Lower noise figures are desirable for EME and satellite use where the background noise is much lower. Lower noise figures may also be stated as a noise temperature. There is a direct logarithmic relationship betwen the two. This Wiki page provides a full explanation: https://en.wikipedia.org/wiki/Noise_temperature .

Equipment Required

To measure noise figure using a Portsdown 4, a calibrated noise source and matching switched power supply are required.

Many professional noise sources (as used by HP) need a 28v supply in their on state. They have a stated "Excess Noise Ratio" (ENR) which is a measure of the noise that they produce, and they may well have a calibration chart or table stating how this ENR varies with frequency. Typical ENR values are 5 or 15 dB.

The accuracy of the noise figure meter is limited by the accuracy of the noise source ENR calibration. However, an uncalibrated noise source can still provide a very valuable alignment aid.

If you do not have a calibrated HP or similar noise source, G8FEK sells some suitable substitutes: https://g8fek.com/precision-noise-sources.html . Note that these need

The switched power supply needs to turn the noise source on when commanded to by the Portsdown 4 and then turn it off again. The drive for the noise source switching comes from pin 26 of the Portsdown GPIO port, and is at 3.3v when the noise source is on, and 0v when the noise source is off. It is hoped to publish some circuits for possible switched power supplies in future, but there is an example here: https://www.qsl.net/yo4hfu/Files/CANFI/SWITCH_28V.pdf .