LTC559X View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC559X Datasheet PDF : 20 Pages
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LTC5567
Applications Information
Highpass IF Matching
By simply changing component values, the bandpass IF
output matching network can be changed to a highpass
impedance transforming network. This matching network
will drive a lower impedance differential load (or trans-
former), like the 200Ω wideband bandpass matching
previously described, while delivering higher conversion
gain, similar to the 400Ω bandpass matching. The high-
pass matching network will have less IF bandwidth than
the bandpass matching. It also uses smaller inductance
values; an advantage when designing for IF center frequen-
cies well below 100MHz.
Referring to the small-signal output network schematic in
Figure 13, the reactive matching element values (L1, L2,
C7 and C8) are calculated using the following equations.
The source resistance (RS) is the parallel combination of
external resistors R1 + R2 and the internal IF resistance,
RIF taken from Table 4. The differential load resistance
(RL) is typically 200Ω, but can be less. CIF, the IF output
capacitance, is taken from Table 4. Choosing RS in the
380Ω to 450Ω range will yield power conversion gains
around 2dB.
RS = RIF || 2·R1
Q = √(RS/RL–1)
YL = Q/RS + (ωIF • CIF)
L1, L2 = 1/(2 • YL • ωIF)
C7, C8 = 2/(Q • RL • ωIF)
(R1 = R2)
(RS > RL)
LTC5567
IF+
11
C7
R1 L1
RIF
CIF
10
IF–
VCC
RL
R2 L2 C8
5567 F13
Figure 13. IF Output Circuit for Highpass Matching Element
Value Calculations
To demonstrate the highpass impedance transformer
output matching, these equations were used to calculate
the element values for a 153MHz IF frequency and 200Ω
differential load resistance. The output matching on the
16
wideband test circuit, shown in Figure 11, was modified
with the following new element values, and re-tested.
L1, L2 = 150nH
C7, C8 = 10pF
R1, R2 = 1.1k
Measured voltage conversion gain for the highpass and
wideband bandpass methods are shown in Figure 14, for
comparison. Both circuits are driving a 200Ω differential
load, but the highpass version delivers 2.3dB of additional
gain at 153MHz. Measured performance for both circuits
is summarized in Table 6. As shown, the highpass method
has less than half the IF bandwidth, and 3dB lower P1dB.
Table 6. Measured Performance Comparison for Highpass
and Wideband IF Matching (RF = 1950MHz, IF = 153MHz,
Low Side LO).
GV IIP3 P1dB 1dB (CONVERSION GAIN)
IF MATCHING (dB) (dBm) (dBm) IF FREQUENCY RANGE
Highpass 8.5 26.9 10.0
110MHz to 320MHz
Wideband 6.2 26.9 13.0
45MHz to 590MHz
9
8
153MHz
7
HIGHPASS
6
5
WIDEBAND
4
BANDPASS
3
2
1
0
RF = 1.7GHz TO 2.2GHz
LO = 1.65GHz AT 0dBm
–1
ZRF = 50Ω
–2
ZIF = 200Ω DIFFERENTIAL
–3
TC = 25°C
–4
–5
50 100 150 200 250 300 350 400 450 500 550
IF FREQUENCY (MHz)
5567 F14
Figure 14. Voltage Conversion Gain versus IF Frequency
for 153MHz Highpass and Wideband Bandpass IF Matching
Networks
Mixer Bias Current Reduction
The IADJ pin (Pin 8) is available for reducing the mixer
core DC current consumption at the expense of linearity
and P1dB. For the highest performance, this pin should
be left open circuit. As shown in Figure 15, an internal
bias circuit produces a 3mA reference current for the
mixer core. If a resistor is connected to Pin 8, as shown
5567f
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