Atec Agilent-8901B - Manual (Page 2)

Direct-Spectrum Carrier Noise
Measurements (cont’d)
Phase noise usually dominates the carrier-noise
measurement at most offsets of interest, so
direct-spectrum noise measurements provide a con-
venient and simple way to measure phase noise of
many sources. This measurement technique is rela-
tively insensitive to many common pitfalls encoun-
tered in making phase noise measurements. There is
no need to phase-lock signals. Testing can be fully
automated using an external controller. Most impor-
tant, selective signal and noise measurements are
fast (five readings/second). The analyzer’s measure-
ment accuracy is better than ±0.5 dB to –129 dBc/Hz,
typically better than ±1 dB to –140 dBc/Hz. The noise
floor is –150 dBc/Hz.
Many applications require spectral purity to be ex-
pressed as residual AM, FM or phase modulation.
With the HP 8901B’s convenient modulation capabili-
ties you make these measurements with one key-
stroke, in a variety of common bandwidths.
To extend the measurement range to 26.5 GHz, add
the HP 11793A Microwave Converter and a low noise
microwave source, such as the HP 8673B Synthesized
Signal Generator. With this system you can test high-
performance sources from 10 MHz to 26.5 GHz.
Block Diagram
The high-selectivity option for the HP 8901B Modu-
lation Analyzer adds an RF switch, a buffer ampli-
fier, variable gain IF amplifiers, narrowband filters
and a rms detector. To make selective power meas-
urements, the analyzer mixes down the RF input sig-
nal using either the internal LO or an external LO.
The IF signal is buffered and sent through one of two
narrowband bandpass filters which provide over 90
dB of rejection. The signal is amplified by precision
IF amplifiers, filtered again and measured by the rms
detector. The IF amplifiers provide as much as 95 dB
of gain in 5 dB increments; the rms detector is used
over a 20 dB dynamic range.