Concepts and features
R&S
®
ZNA
140
User Manual 1178.6462.02 ─ 20
Stability factors are calculated as functions of the frequency or another stimulus
parameter. They provide criteria for linear stability of two-ports such as amplifiers. A lin-
ear circuit is said to be unconditionally stable if no combination of passive source or
load can cause the circuit to oscillate.
●
The K-factor provides a necessary condition for unconditional stability: A circuit is
unconditionally stable if K>1 and an additional condition is met. The additional con-
dition can be tested with the stability factors μ
1
and μ
2
.
●
The μ
1
and μ
2
factors both provide a necessary and sufficient condition for uncondi-
tional stability: The conditions μ
1
>1 or μ
2
>1 are both equivalent to unconditional
stability. This means that μ
1
and μ
2
provide direct insight into the degree of stability
or potential instability of linear circuits.
References: Marion Lee Edwards and Jeffrey H. Sinsky, "A New Criterion for Linear 2-
Port Stability Using a Single Geometrically Derived Parameter", IEEE Trans. MTT, vol.
40, No. 12, pp. 2303-2311, Dec. 1992.
4.3.8
Delay, aperture, electrical length
The group delay τ
g
represents the propagation time of wave through a device. τ
g
is a
real quantity and is calculated as the negative of the derivative of its phase response.
A non-dispersive DUT shows a linear phase response, which produces a constant
delay (a constant ratio of phase difference to frequency difference).
The group delay is defined as:
df
d
d
d
deg
rad
g
360
where
Φ
rad
, Φ
deg
= phase response in radians or degrees
ω = angular velocity in radians/s
f = frequency in Hz
In practice, the analyzer calculates an approximation to the derivative of the phase
response, taking a small frequency interval Δf and determining the corresponding
phase change ΔΦ. The delay is thus computed as:
f
deg
meas
g
360
,
The aperture Δf must be adjusted to the conditions of the measurement.
If the delay is constant over the considered frequency range (non-dispersive DUT, e.g.
a cable), then τ
g
and τ
g,meas
are identical and:
c
L
t
f
d
t
f
d
mech
g
360
)
360
(
Measurement results
