LTC5540 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5540' PDF : 16 Pages View PDF
LTC5540
Applications Information
Bandpass IF Matching
The IF output can be matched for IF frequencies as low
as 70MHz or as high as 500MHz using the bandpass
IF matching shown in Figure 1 and Figure 7. L1 and L2
resonate with the internal IF output capacitance at the
desired IF frequency. The value of L1, L2 is calculated
as follows:
L1 = L2 = 1/[(2 π fIF)2 • 2 • CIF]
where CIF is the internal IF capacitance (listed in Table 4).
Values of L1 and L2 are tabulated in Figure 1 for various IF
frequencies. For IF frequencies below 70MHz, the values
of L1, L2 become unreasonably high and the lowpass
topology shown in Figure 9 is preferred. Measured IF
output return loss for bandpass IF matching is plotted
in Figure 10.
VCCIF
3.1-5.3V
C9
1µF
T1
L1
82nH
4:1
C8
22pF
C13
1.5pF
IFOUT
50Ω
30MHz TO 150MHz
L2
82nH
R2
1k
19 IF+
LTC5540
18 IF–
5540 F09
Figure 9. IF Output with Lowpass Matching
0
–5
Table 4. IF Output Impedance vs Frequency
FREQUENCY (MHz)
70
DIFFERENTIAL OUTPUT
IMPEDANCE (RIF || XIF (CIF))
674 || -j1137 (2pF)
140
628 || -j569 (2pF)
190
606 || -j419 (2pF)
240
584 || -j316 (2.1pF)
300
561 || -j253 (2.1pF)
380
532 || -j182 (2.3pF)
450
511 || -j154 (2.3pF)
Lowpass IF Matching
An alternative IF matching network shown in Figure 9 uses
a lowpass topology, which provides excellent RF to IF
and LO to IF isolation. VCCIF is supplied through the
center tap of the 4:1 transformer. A lowpass impedance
transformation is realized by shunt elements R2 and
C13 (in parallel with the internal RIF and CIF), and series
inductors L1 and L2. Resistor R2 is used to reduce the
IF output resistance, or it can be deleted for the highest
conversion gain. The final impedance transformation to
50Ω is realized by transformer T1. The matching element
values shown in Figure 9 are optimized for a wideband
30MHz-150MHz IF match. The demo board (see Figure 2)
has been laid out to accommodate this matching topology
with very few modifications.
–10
–15 270nH
150nH
100nH
33nH
–20
50 100 150 200 250 300 350 400 450 500
FREQUENCY (MHz)
5540 F10
Figure 10. IF Output Return Loss - Bandpass Matching
IF Amplifier Bias
The IF amplifier delivers excellent performance with
VCCIF = 3.3V, which allows the VCC and VCCIF supplies
to be common. With VCCIF increased to 5V, the RF input
P1dB increases by almost 3dB, at the expense of higher
power consumption. Mixer performance at 900MHz is
shown in Table 5 with VCCIF = 3.3V and 5V. For the highest
conversion gain, high-Q wire-wound chip inductors are
recommended for L1 and L2, especially when using
VCCIF = 3.3V. Low-cost multilayer chip inductors may be
substituted, with a slight reduction in conversion gain.
Table 5. Performance Comparison with VCCIF = 3.3V and 5V
(RF = 900MHz, High-Side LO, IF = 190MHz)
VCCIF
ICCIF
GC
P1dB
IIP3
NF
3.3V
96
7.9
11
25.9
9.9
5V
99
7.9
14.5
25.9
10.0
5540f
13
Share Link: 
