LT5522
13
5522fa
Higher linearity and lower LO-IF leakage can be realized by
using the simple, three element lowpass matching net-
work shown in Figure 10. Matching elements C4, L1 and
L2 form a 400Ω to 200Ω lowpass matching network
which is tuned to the desired IF frequency. The 4:1
transformer then transforms the 200Ω differential output
to 50Ω single-ended. The value of C4 is reduced by 1pF to
account for the equivalent internal capacitance.
For optimum linearity, C4 must be located close to the IF
pins. Excessive trace length or inductance between the IF
pins and C4 will increase the amplitude of the image output
and reduce voltage swing headroom for the desired IF
frequency. High Q wire-wound chip inductors (L1 and L2)
improve the mixer’s conversion gain by a few tenths of a
dB, but have little effect on linearity.
This matching network is most suitable for IF frequencies
of 40MHz or above. Below 40MHz, the value of the series
inductors (L1 and L2) is high, and could cause stability
problems, depending on the inductor value and parasitics.
Therefore, the 8:1 transformer technique is recommended
for low IF frequencies.
Suggested matching network values for several IF fre-
quencies are listed in Table 3. Measured output return
losses for the 140MHz match and the wideband CATV
match are plotted in Figure 13.
Table 3. IF Matching Element Values (See Figure 10)
IF FREQUENCY L1, L2 C4
(MHz) (nH) (pF) IF TRANSFORMER
2-140 Short — TC8-1 (8:1)
70 220 4.7 ETC4-1-2 (4:1)
140 82 1.5
240 56 0.5
380 39 —
50-1000 (CATV) 18 — MABAES0054 (4:1)
For fully differential IF architectures, the IF transformer
can be eliminated. As shown in Figure 14, supply voltage
to the mixer’s IF pins is applied through matching induc-
tors in a bandpass IF matching network. The values of L1,
L2 and C4 are calculated to resonate at the desired IF
frequency with a quality factor that satisfies the required IF
bandwidth. The L and C values are then adjusted to
account for the mixer’s internal 1pF capacitance and the
SAW filter’s input capacitance. In this case, the differential
IF output impedance is 400Ω, since the bandpass network
does not transform the impedance.
For low cost applications, it is possible to replace the IF
transformer with a lumped-element network which pro-
duces a single-ended 50Ω output. One approach is shown
in Figure 15, where L1, L2, C4 and C6 form a narrowband
bridge balun. The L and C values are calculated to realize
a 180 degree phase shift at the desired IF frequency using
the equations listed below. Inductor L4 is calculated to
cancel the internal 1pF capacitance. L3 also supplies bias
voltage to the IF
+
pin. Low cost multilayer chip inductors
are adequate for L1 and L2. A high Q wire-wound chip
APPLICATIO S I FOR ATIO
WUUU
Figure 13. Typical IF Output Return Losses
for Various Matching Techniques
Figure 14. Bandpass IF Matching for Differential IF Architectures
IF FREQUENCY (Hz)
PORT RETURN LOSS (dB)
0
–5
–10
5522 F13
–25
–15
–20
1E9
1E7 1E8
240MHz MATCH
LUMPED ELEMENT
BRIDGE BALUN
LOW FREQ MATCH
(NO IF MATCHING)
8:1 BALUN
140MHz MATCH
(82nH/1.5pF)
4:1 BALUN
50MHz TO 1000MHz
(18nH/0pF)
4:1 CATV BALUN
SAW
FILTER
C3
V
CC
C4
IF
+
IF
–
L2
5522 F14
L1
IF
AMP
