LTC5541 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5541' PDF : 16 Pages View PDF
LTC5541
APPLICATIONS INFORMATION
Bandpass IF Matching
The IF output can be matched for IF frequencies as low
as 90MHz 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 90MHz, 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.
Table 4. IF Output Impedance vs Frequency
FREQUENCY (MHz)
90
DIFFERENTIAL OUTPUT
IMPEDANCE (RIF || XIF (CIF))
329 || –j769 (2.3pF)
140
314 || –j494 (2.3pF)
190
305 || –j364 (2.3pF)
240
310 || –j288 (2.3pF)
300
303 || –j226 (2.35pF)
380
289 || –j175 (2.4pF)
500
273 || –j118 (2.7pF)
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. Similar to the bandpass
topology, L1 and L2 cancel out the reactive part of the
internal capacitance and the impedance transformation is
realized by the 4:1 transformer. This topology is preferred
for low IF frequencies since L1 and L2 may be replaced
with shorts. The LTC5541 demo board (see Figure 2) has
been laid out to accommodate this matching topology
with very few modifications.
VCCIF
3.1-5.3V
4:1
T1
IFOUT
50Ω
C9
L1
C8
L2
19 IF+
18 IF–
LTC5541
5541 F09
Figure 9. IF Output with Lowpass Matching
0
–5
–10
–15
L1, L2 = 270nH
L1, L2 = 100nH
–20
50
L1, L2 = 150nH
100 150 200 250 300 350 400 450
IF FREQUENCY (MHz)
5541 F10
Figure 10. IF Output Return Loss
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 approximately 3dB, at the expense of
higher power consumption. Mixer performance at 1950MHz
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 = 1950MHz, Low-Side LO, IF = 190MHz)
VCCIF
ICCIF
(mA)
GC
P1dB
IIP3
NF
(dB)
(dBm) (dBm)
(dB)
3.3V
100
7.8
11.3
26.4
9.6
5V
102
7.7
14.6
27.3
9.7
5541f
13
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