MC13158FTB View Datasheet(PDF) - LANSDALE Semiconductor Inc.
Part Name
Description
Manufacturer
MC13158FTB Datasheet PDF : 23 Pages
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ML13158
LANSDALE Semiconductor, Inc.
Legacy Applications Information
INPUT MATCHING/COMPONENTS
It is desirable to use a SAW filter before the mixer to provide addi-
tional selectivity and adjacent channel rejection. In a wideband sys-
tem the primary sensitivity of the receiver backend may be
achieved before the last mixer. Bandpass filtering in the limiting IF
is costly and difficult to achieve for bandwidths greater than 280
kHz.
The SAW filter should be selected to easily interface with the
mixer differential input impedance of approximately 2.0 kΩ in par-
allel with 1.0 pF. The PC board is dedicated to the Siemens SAW
filter (part number Y6970M); the part is designed for DECT at 112
MHz 1st IF frequency. It is designed for a load impedance of 2.0
kΩ in parallel with 2.0 pF; thus, no or little input matching is
required between the SAW filter and the mixer.
The Siemens SAW filter has an insertion loss of typically 10 dB
and a 3.0 dB bandwidth of 1.0 MHz. The relatively high insertion
loss significantly contributes to the system noise and a filter having
lower insertion loss would be desirable. In existing low loss SAW
filters, the required load impedance is 50 Ω; thus, interface match-
ing between the filter and the mixer will be required. Figure 15 is a
table of the single–ended mixer input impedance. A careful noise
analysis is necessary to determine the secondary contribution to
system noise.
SYSTEM NOISE CONSIDERATIONS
The system block diagram in Figure 16 shows the cascaded noise
stages contributing to the system noise; it represents the application
circuit in Figure 12 and a low noise preamp using a MRF941 trans-
lator (see Figure 17). The preamp is designed for a conjugately
matched input and output at 2.0 Vdc. VCE and 3.0 mAdc Ic. S-
parameters at 2.0 V, 3.0 mA and 100 MHz are:
S11 = 0.86, –20
S21 = 9.0, 164
S12 = 0.02, 79
S22 = 0.96, –12
The bias network and VCE at 2.0 V and Ic at 3.0 mA for VCC = 3.0
to 3.5 Vdc. The preamp operates with 18 dB gain and 2.7 dB noise
figure.
In the cascaded noise analysis the system noise equation is:
Fsystem = F1 + [(F2–1)/G1] + [(F3–1)]/[(G1)(G2)]
where:
F1 = the Noise Factor of the Preamp
G1= the Gain of the Preamp
F2 = the Noise factor of the SAW filter
G2 = the Gain of the SAW filter
F3 = the Noise factor of the Mixer
Note: the proceeding terms are defined as linear relationships and
are related to the log form for gain and noise figure by the follow-
ing:
F = log –1[(NF in dB)/10] and similarly
G = log–1[(NF in dB)/10]
The noise figure and gain measured in dB are shown in the system
block diagram. The mixer noise figure is typically 14 dB and the
SAW filter adds typically 10 dB insertion loss. Addition of a low
noise preamp having a 18 dB gain and 2.7 dB noise figure not only
improves th system noise figure but it increases the reverse isola-
tion from the local oscillator to the antenna input at the receiver.
Calculating in terms of gain and noise factor yields the following:
F1 = 1.86; G1 = 63.1
F2 = 10; G2 = 0.1
F3 = 25.12
Thus, substituting in the equation for system noise factor:
Fsystem = 5.82; NFsystem = 7.7 dB
Page 14 of 23
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