LTC5542IUH-TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5542IUH-TRPBF' PDF : 16 Pages View PDF
LTC5542
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))
320 || –j842 (2.1pF)
140
312 || –j541 (2.1pF)
190
300 || –j419 (2.0pF)
240
294 || –j301 (2.2pF)
300
287 || –j221 (2.4pF)
380
280 || –j161 (2.6pF)
500
269 || –j120 (2.65pF)
VCCIF
3.1-5.3V
C9
1μF
4:1
T1
C8
L1
22pF
82nH
IFOUT
50Ω
L2
82nH
C13
1.5pF
R2
1k
19 IF+
18 IF–
LTC5542
5542 F09
Figure 9. IF Output with Lowpass Matching
0
–5
–10
–15
270nH
–20
150nH
100nH 33nH
–25
50 100 150 200 250 300 350 400 450 500 550
FREQUENCY (MHz)
5542 F10
Figure 10. IF Output Return Loss - Bandpass Matching
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 250Ω to 200Ω 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 selected
to reduce the IF output resistance to 250Ω, 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 to 150MHz IF match.The demo
board (see Figure 2) has been laid out to accommodate
this matching topology with very few modifications.
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 more than 3dB, at the expense of higher
power consumption. Mixer performance at 2400MHz 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.
5542f
13
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