Keysight Technologies CXA N9000B, Руководство пользователя

Страница: 51 / 88

N9069C Noise Figure Measurement Guide 51
Measurement Related Tasks and Concepts
Setting the Measurement System
It is conventional to call the higher frequency band of an image pair the
Upper-Sideband, USB, and the lower frequency band of an image pair the
Lower-Sideband, LSB.
Non-ideal mixers exhibit some unwanted behaviors:
— Some of the input signal leaks directly to the output.
— Some of the LO signal, and its harmonics, leak directly to the output.
— Mixing products are created between the input signal and the harmonics of
the LO.
There are other unwanted products involving input signal harmonics, but these
tend to be less troublesome than those above, provided the mixer is operated
at a level within its linear range.
Signal Leakage
Direct signal leakage of an input signal through to a mixer's output can occur,
because the noise sources cover a broad frequency range. Signal leakage is
not normally a problem unless the noise source has a large variation in ENR, or
the mixer's RF-to-IF leakage is high.
LO Leakage
The LO power is normally greater than the largest input signal that a mixer is
intended to operate with. The LO power leaking from the mixer's output is at a
high level compared to the signal levels involved in the noise figure
measurement. Hence, LO leakage needs to be considered when measuring
noise figure of a frequency converting DUT.
When working in low band < 3.6 GHz, if the LO frequency is low enough to be
passed by the input filter of the analyzer's RF section (a 3.6 GHz Low Pass
Filter), the LO leakage can prevent successful measurement of the DUT noise
figure. Desensitization by LO leakage can be avoided by adding a filter
between the DUT and the analyzer to remove the LO frequency component.
Low pass filters with cutoffs at low frequencies, may exhibit spurious
resonances and leakage at low microwave frequencies. It may be necessary to
use a pair of lowpass filters, one microwave, one RF, to assure a stopband
attenuation over a wide frequency range.
LO Harmonics
Many mixers are operated by sinusoidal LO signals. LO harmonics can be
formed in the mixer at significantly high levels. It is common for the specified
LO input level for a diode mixer to be chosen to operate the diodes between
saturation and off conditions, thereby making the mixer act as a switch. LO
harmonic derived products from industry standard double-balanced mixers
may be similar in level to what they would have been with a square-wave LO
signal. Instead of just being sensitive at one pair of frequencies , the
mixer input is sensitive at a series of pairs:
F
LO
F
IF
± [ ]