參數(shù)資料
型號(hào)： MC13158
廠商： Motorola, Inc.
英文描述： WIDEBAND FM IF SUBSYSTEM FOR DECT AND DIGITAL APPLICATIONS
中文描述： 寬頻帶調(diào)頻中頻DECT和數(shù)字應(yīng)用子系統(tǒng)
文件頁(yè)數(shù)： 15/24頁(yè)
文件大?。?/td> 394K
代理商： MC13158
MC13158
15
MOTOROLA ANALOG IC DEVICE DATA
Input Matching/Components
It is desirable to use a SAW filter before the mixer to
provide additional selectivity and adjacent channel rejection.
In a wideband system 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 k
in parallel 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 k
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
; thus, interface matching 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.
Figure 15. Mixer Input Impedance
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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 transistor (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 sets 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:
[(
F2–1
)
G1
]
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á
f
ááá
á
Rs
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á
Note:
the proceeding terms are defined as linear
relationships and are related to the log form for gain and
noise figure by the following:
log–1[(NF in dB) 10] and similarly
log–1[(Gain in dB) 10]
Xs
ááááá
–330
Rp
ááááá
á
ááááááá
ááááááá
Xp
ááááá
á
Cp
áááááá
ááááááááááááááááááááááááááááááá
150
270
–400
860
–580
1.8
200
170
–320
770
–410
1.9
ááááááááááááááááááááááááááááááá
300
400
500
63
250
110
71
á
–140
130
–250
ááá
á
370
–270
680
580
ááááá
690
–300
–220
–170
á
1.9
1.8
1.8
á
á
1.85
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ááááááááááááááááááááá
ááááááááááááááááááááááááááááááá
600
–190
ááááá
áááááááááá
ááááá
ááááá
Fsystem
F1
[(
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
F
G
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 the system noise figure
but it increases the reverse isolation 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
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