LT5527
12
5527fa
Direct 8:1 IF Transformer Matching
For IF frequencies below 100MHz, the simplest IF matching
technique is an 8:1 transformer connected across the IF
pins. The transformer will perform impedance transfor-
mation and provide a single-ended 50Ω output. No other
matching is required. Measured performance using this
technique is shown in Figure 9. This matching is easily
implemented on the standard evaluation board by short-
ing across the pads for L1 and L2 and replacing the 4:1
transformer with an 8:1 (C3 not installed).
APPLICATIONS INFORMATION
frequencies are listed in Table 4. 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.
Measured output return losses for each case are plotted
in Figure 10 for the simple 8:1 transformer method and
for the lowpass/4:1 transformer method.
Table 4. IF Matching Element Values
PLOT
IF FREQUENCY
(MHz)
L1, L2
(nH)
C3
(pF)
IF
TRANSFORMER
1 1 to 100 Short — TC8-1 (8:1)
2 140 120 — ETC4-1-2 (4:1)
3 190 110 2.7 ETC4-1-2 (4:1)
4 240 82 2.7 ETC4-1-2 (4:1)
5 380 56 2.2 ETC4-1-2 (4:1)
6 450 43 2.2 ETC4-1-2 (4:1)
Figure 9. Typical Conversion Gain, IIP3 and
SSB NF Using an 8:1 IF Transformer
Figure 10. IF Output Return Losses
with Lowpass/Transformer Matching
Lowpass + 4:1 IF Transformer Matching
The lowest LO-IF leakage and wide IF bandwidth are real-
ized by using the simple, three element lowpass matching
network shown in Figure 7. Matching elements C3, L1 and
L2, in conjunction with the internal 2.5pF capacitance,
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 a 50Ω
single-ended output.
This matching network is most suitable for IF frequencies
above 40MHz or so. Below 40MHz, the value of the series
inductors (L1 and L2) becomes unreasonably high, and
could cause stability problems, depending on the induc-
tor value and parasitics. Therefore, the 8:1 transformer
technique is recommended for low IF frequencies.
Suggested lowpass matching element values for several IF
Discrete IF Balun Matching
For many applications, it is possible to replace the IF
transformer with the discrete IF balun shown in Figure 2.
The values of L1, L2, C6 and C7 are calculated to realize
a 180 degree phase shift at the desired IF frequency and
provide a 50Ω single-ended output, using the equations
listed below. Inductor L3 is calculated to cancel the in-
ternal 2.5pF 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 induc-
tor is recommended for L3 to maximize conversion gain
and minimize DC voltage drop to the IF
+
pin. C3 is a DC
blocking capacitor.
IF OUTPUT FREQUENCY (MHz)
10
G
C
(dB), IIP3 (dBm), SSB NF (dB)
13
17
21
25
50
5527 F09
9
5
11
15
19
23
7
3
1
20
30
40
60 70 80 90 100
RF = 900MHz
HIGH SIDE LO AT 0dBm
V
CC
= 5V DC
T
A
= 25°C
C4 = 2.7pF, C5 = 3.9pF
IIP3
SSB NF
G
C
IF FREQUENCY (MHz)
–30
IF PORT RETURN LOSS (dB)
–20
–10
0
–25
–15
–5
100 200 300 400
5527 F10
50050
1
2
3
45
6
0 150 250 350 450
