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 talk to CAT 21 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 .