Automatic Noise Spectrum Analyser

gevv · Tarih: 13.12.2006, 16:42 Mesaj konusu: **Automatic Noise Spectrum Analyser**

Automatic Noise Figure and Gain Measurements
with a Spectrum Analyser.
Luis Cupido - CT1DMK
Abstract
A relatively simple automatic Noise Figure (NF) meter was
developed which is using a spectrum analyser as the receiver and
logarithmic power detector. The presented circuit makes use of the
X and Y output signals available on most spectrum analysers. It
processes these signals to control a noise head and obtain Y, Gain
and NF values. These values are then displayed on digital or
analogue meters.
This simple instrument will enable easy real time alignment and
optimisation of preamplifiers and converters for best
performance.
Introduction
To perform a NF measurement without an automatic PANFI /1/ the operator needs to
make a few power measurements and calculate the corresponding NF. This, can be
accomplished, with good accuracy, using a Spectrum Analyser (SA) equipped with an
additional low noise preamplifier and calibrated noise head, However one cannot
tune a device for best performance while looking at the SA display.
The circuit presented employs the SA as receiver for the automatic measurement and
then displays NF, Y factor and gain simultaneously thus enabling a real time tuning of
preamplifiers and converters for best performance. With this scheme any frequency
covered by the spectrum analyser can be used for the measurement regardless if it is
the operating frequency or the intermediate frequency coming out from a converter.
Principle of operation
All (to my knowledge) commercial SAs have accessible the X and Y output signals to
be used by external displays, recorders and other devices.
The Xout signal is the horizontal scan of the display and indicates the received
frequency. The Yout output contains a voltage signal proportional to the power of the
incoming signal in dB, depending on the vertical scale selected at the instrument.