參數(shù)資料
型號(hào)： UAA1570HL
廠商： NXP SEMICONDUCTORS
元件分類： 通信及網(wǎng)絡(luò)
英文描述： Global Positioning System (GPS)baseband processor(通用定位系統(tǒng)基帶處理器)
中文描述： SPECIALTY TELECOM CIRCUIT, PQFP48
封裝： PLASTIC, SOT-313, LQFP-48
文件頁(yè)數(shù)： 26/76頁(yè)
文件大?。?/td> 890K
代理商： UAA1570HL
1999 May 10
26
Philips Semiconductors
Product specification
Global Positioning System (GPS) front-end
receiver circuit
UAA1570HL
7.5
First IF filter
The first IF filter provides four functions:
1.
It provides selectivity to protect the 2nd mixer (the IF
mixer) from high level spurious RF signals which pass
through the wide band-pass envelope of the RF filters,
typically 40 to 60 MHz.
2.
The filter attenuates thermal noise and spurious
signals in the 2nd mixer image band.
3.
It can provide impedance matching/transformation
from the RF mixer output to the IF mixer input.
4.
It can reject spurious common mode and/or differential
signals generated by high level local sources such as
harmonics of the reference clock or sample clock.
The first IF can be structured to support a wide range of
single-ended or balanced filters including LC or SAW
realizations. High RF gain provides first IF signal levels
high enough to accommodate first IF filter losses of 15 dB
with optimum RF matching and conversion gain in the first
mixer.
The Philips application board uses a 6th-order coupled
resonator filter based on the butterworth response.
The design method is described in the “Handbook of
FILTER SYNTHESIS”by Anatol Zverev. The handbook
tables formulate single-ended filter designs which we later
convert to a balanced form.
The initial centre frequency and bandwidth were 41.82 and
4.5 MHz, respectively. The following list illustrates the
tabular design 3 dB down k and q parameters from Zverev
that were developed for the initial single-ended structure.
R
s
= 331.4
R
L
= 689.6
q
0
= 5.0; insertion loss = 4.742; q
1
= 0.8226;
q
n
= 1.7115; k
12
= 0.6567; k
23
= 0.7060.
This tabular listing was chosen based on the desired
selectivity and minimal insertion loss, which could be
realized with available surface mount inductors operating
with quality factors (Q) in the range of 40 to 50.
The impedance level is determined by the choice of design
inductance (165 nH), with foresight given to eventual
balancing of the design. Maximizing the load presented to
the first mixer was also a consideration. With some
frequency plans stability in both the first and second IF will
also need to be considered when choosing the impedance
level of the design.
The handbook calculations result in a preliminary
single-ended three shunt tank structure with a coupling
capacitor between each tank as follows:
R
s
= 331
Tank 1 = 165 nH in parallel with 81.6 pF
Coupling capacitor 1 = 6.2 pF
Tank 2 = 165 nH in parallel with 74.9 pF
Coupling capacitor 2 = 6.7 pF
Tank 3 = 165 nH in parallel with 81.1 pF
R
L
= 690
.
To convert this filter to a balanced design it can be
mirrored in the ground plane which would result in the
following balanced structure. It should be noted that the
tank design inductances have doubled while the tank
capacitances have halved, which can be seen by
removing the virtual ground plane. The series elements
remain unchanged in the balanced design, while the
differential source and load have of course doubled.
R
s
= 663
Tank 1 = 330 nH in parallel with 40.8 pF
Coupling capacitor 1 = 6.2 pF
Tank 2 = 330 nH in parallel with 37.5 pF
Coupling capacitor 2 = 6.7 pF
Tank 3 = 330 nH in parallel with 40.6 pF
R
L
= 1380
.
To optimize the power developed by the first mixer its load
was maximized by driving the 1380
side of this filter.
It was also decided to bias the output of the first mixer
through Tank 3 components by breaking the former
differential 330 nH inductor back into two 165 nH inductors
connected to the supply which also acts as a virtual
ground.
The filter was then resimulated in ‘SPICE’ to optimize
against available discrete surface mount component
values with finite quality factors. All filters must be driven
by their design impedances to produce their prescribed
response. Since the finite quality factors of the filter
inductors emulate an equivalent shunting load of
approximately 2
× π ×
f
×
L
×
Q, the source and load
terminating impedances can be increased to compensate
for this parasitic element while maintaining ideal filter
response and minimizing losses. In the default application,
R322/306 in conjunction with the equivalent parallel
resistance at the respective filter input and output give the
desired terminating impedance.
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