GW Instek GRF-1300A - Manual (Page 102)

GRF-1300A User Manual and Teaching Materials
10 lg
I
c
R
P
G
P
=
In the formula, both the RF input power
R
P
and IF output
power
I
P use dBm as the unit.
There exists conversion loss when the conversion gain is
<1, therefore, for passive diodes, which is expressed by Lc.
While for active frequency mixers such as transistors,
FETs or integrated analog multipliers, the conversion gain
is frequency gain> 1.
In measuring the conversion gain Gc, the LO drive power
must be a fixed power level. For example, for a diode ring
mixer with a 50Ω input/output impedance, the LO
standard power level is 7dBm. For the integrated analog
multiplier MC1596, the LO standard power level is
20dBm.
When measuring and applying the frequency mixer, it
should be noted that impedance matching should be used
with the three ports of the frequency mixer. When the RF
port of the frequency mixer passes through image
rejection filter and is connected to LNA, the input
impedance in the RF port of frequency mixer must match
the output impedance of its filter to ensure the
performance of the filter. Generally the output impedance
of the filter is 50Ω. Similarly, the output impedance of the
IF port of the frequency mixer should match the input
impedance of the IF filter. Generally the impedance of an
IF filter with a frequency lower than 100MHz is greater
than 50Ω.
(2) 1dB compression point
The Compression point is used to denote the non-linear
distortion of a frequency mixer. For a signal with small
RF, the frequency mixer is a linear network. When the RF
input level is much smaller than the LO level, the
frequency mixer operates linearly. In other words, the RF
output increases linearly with an increase in the RF input
level. However, when the RF input increases to a certain
extent, such as when the difference between the RF input
level and the LO level is more than 10dB, the rate of the IF
output slows, and the relationship between the RF input
and RF output is no longer linear. At this point, the
frequency mixer begins to enter saturation and exhibits
100