lt5527euf-trpbf Linear Technology Corporation, lt5527euf-trpbf Datasheet - Page 12

• Manufacturer Part Number: lt5527euf-trpbf
• Description: 400mhz To 3.7ghz High Signal Level Downconverting Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT5527
Direct 8:1 IF Transformer Matching
For IF frequencies below 100MHz, the simplest IF match-
ing 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).
Lowpass + 4:1 IF Transformer Matching
The lowest LO-IF leakage and wide IF bandwidth are
realized by using the simple, three element lowpass match-
ing 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 inductor
value and parasitics. Therefore, the 8:1 transformer tech-
nique is recommended for low IF frequencies.
Suggested lowpass matching element values for several
IF frequencies are listed in Table 4. High-Q wire-wound
12
Figure 9. Typical Conversion Gain, IIP3 and
SSB NF Using an 8:1 IF Transformer
25
23
21
19
17
15
13
11
9
7
5
3
1
10
IIP3
SSB NF
G
C
20
IF OUTPUT FREQUENCY (MHz)
U
30
40
U
50
RF = 900MHz
HIGH SIDE LO AT 0dBm
V
T
C4 = 2.7pF, C5 = 3.9pF
CC
A
60
= 25°C
= 5V DC
70
W
80
90 100
5527 F09
U
chip inductors (L1 and L2) improve the mixer’s conver-
sion 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
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
internal 2.5pF capacitance. L3 also supplies bias voltage to
the IF
equate for L1 and L2. A high Q wire-wound chip inductor
is recommended for L3 to maximize conversion gain and
minimize DC voltage drop to the IF
blocking capacitor.
PLOT
1
2
3
4
5
6
+
pin. Low cost multilayer chip inductors are ad-
–10
–15
–20
–25
–30
FREQUENCY
–5
0
1 to 100
Figure 10. IF Output Return Losses
with Lowpass/Transformer Matching
0
(MHz)
140
190
240
380
450
1
IF
50
100
2
150
IF FREQUENCY (MHz)
200
L1, L2
Short
(nH)
120
110
82
56
43
3
250
4
300
350
C3 (pF)
5
2.7
2.7
2.2
2.2
—
—
400
+
6
450
5527 F10
pin. C3 is a DC
500
TRANSFORMER
ETC4-1-2 (4:1)
ETC4-1-2 (4:1)
ETC4-1-2 (4:1)
ETC4-1-2 (4:1)
ETC4-1-2 (4:1)
TC8-1 (8:1)
IF
5527f