uaa1570hl NXP Semiconductors, uaa1570hl Datasheet - Page 24

• Manufacturer Part Number: uaa1570hl
• Description: Global Positioning System Gps Front-end Receiver Circuit
• Manufacturer: NXP Semiconductors

Philips Semiconductors
7.4
The VCO consists of a single transistor in common
collector configuration with internal positive feedback
realized by capacitors connected from emitter to ground
and emitter to base. Together with the external
resonator/inductor, this is a typical Colpitts circuit. With the
transistor’s base connected to pin 10 (TANK), the internal
circuit arrangement produces a large negative input
impedance at this pin.
At V
initially biased with approximately 1.5 mA of start-up
current. This value is relatively constant varying from
approximately 1.56 mA at T
T
1999 May 10
higher due to the large amplitudes involved leading to
rectification and bias point shifting.
The VCO operates as a negative impedance oscillator with
a suitable external inductive resonator.
handbook, full pagewidth
amb
Global Positioning System (GPS) front-end
receiver circuit
(1) T
(2) T
(3) T
Fig.15 Simulated VCO small signal negative resistance (top diagram) and capacitance (below), without strays.
CCA
= 55 C. During operation, the average currents are
(pF)
( )
100
100
R
C
VCO
amb
amb
amb
0
4
2
0
= 3 V and T
0
= 55 C.
= +27 C.
= +120 C.
amb
(2)
(3)
= 27 C the oscillator transistor is
(1)
0.5
amb
= +120 C to 1.4 mA at
1
24
1.5
Series or parallel resonators can be used, while tuning is
achieved by an external varactor diode.
VCO gain as well as the out-of-loop bandwidth phase
noise are dependent on the choice of external elements
used here. Consequently, they should be selected with
great care; their quality factor is especially important and
should be as high as possible.
Finally, the VCO is followed by a differential buffer stage
with emitter follower inputs splitting the signal to the divider
and LO driver stage path to increase isolation between
mixer and synthesizer and their wanted or unwanted
signals.
This first buffer stage is followed by two other specialized
buffer amplifier stages in the individual signal paths
bringing the signal to the required levels for driving a mixer
or a divider.
2
2.5
UAA1570HL
Product specification
f (GHz)
(1)
(2)
(3)
MHB278
3