from the table of Reflection
Coefficients. Capacitor C7 is a DC
block for the LO port.
DC bias is applied to the
IAM-91563 through the RFC at the
IF Output pin. The power supply
is bypassed to ground with
capacitor C5 to keep RF, IF, and
LO signals off of the DC bias lines
and to prevent gain dips or peaks
in the response of the mixer. C4 is
a DC blocking capacitor for the
output.
The values of the RF bypass
capacitors and DC blocking
capacitors that are not part of a
impedance matching structure
(i.e., C3 - C7) should be chosen to
provide a small reactance (typi-
cally < 5 ohms) at the lowest
frequency at the port for which
they are used. The reactance of
the RF choke (RFC) should be
high (e.g., several hundred ohms)
at the lowest IF.
The completed 1.9 GHz mixer
from the design example above
with all components and SMA
connectors in place is shown in
Figure 32. Again, L1 is not used
and is replaced by a metal tab.
The length of the shunt transmis-
sion line, MLIN, is adjustable by
moving the position of the short-
ing tab between the line and the
ground pad. Provision is made for
an additional bypass capacitor,
C6, to be added to the bias line
near the V
d
connection to elimi-
nate unwanted RF feedback
through bias lines.
When multiple bypass capacitors
are used, consideration should be
given to potential resonances. It is
important to ensure that the
capacitors, when combined with
additional parasitic L’s and C’s on
the circuit board, do not form
resonant circuits. The addition of
a small value resistor in the bias
supply line between bypass
capacitors will often “de-Q” the
bias circuit and eliminate reso-
nance effects.
Table 1 below summarizes the
component values for the 1.9 GHz
design.
Component
C1
C2
C3, C5, C7
C4
L1
L2
L3
MLIN
RFC
Value
0.5 pF
9 pF
100 pF
500 pF
(not used)
100 nH
8.2 nH
Z
o
=90
Ω
l = 0.41 in.
320 nH
The values shown in Table 1 may
vary from those used above to
describe the basic impedance
matching approach. The final
component values take into
consideration additional effects
such as, the various line lengths
between components, parasitics
in components (e.g., the series
inductance in C1), as well as
other circuit parasitics. A CAD
program such as HP Touchstone
®
may be used to fully analyze and
account for these circuit
variables.
Table 1. Component Values for
1.9 GHz Downconverter.
LO
C7
RF
L3
C1
C3
L1
MUN 1
C2
H
L2
C4
RFC
C5
IF
+V
C6
IAM-91
Figure 32. Complete 1.9 GHz Mixer.
The following performance was measured for a 1.9 GHz circuit:
Measured results:
Conversion Gain = 9.0 dB
SSB Noise Figure = 8.5 dB
P
1dB
(output) = -8.1 dB
IP
3
(Input) = -7 dBm
Operating conditions:
RF Frequency = 1.89 GHz
LO Frequency = 1.78 GHz
IF Frequency = 110 MHz
7-145
LO-RF Isolation = 17 dB
LO-IF Isolation = 34 dB
RF-IF Isolation = 23 dB
LO Drive Level = -5 dBm
DC Power = 3.0V @ 9 mA
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
