uaa1570hl NXP Semiconductors, uaa1570hl Datasheet - Page 28

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

Philips Semiconductors
7.6
The second mixer is a standard Gilbert cell mixer operating
with a total tail current of 4.3 mA, which is Proportional To
Absolute Temperature (PTAT). The RF input, or lower tree
of the mixer, is not internally terminated other than by 5 k
biasing resistors to each input. With no significant emitter
degeneration, the real part of the RF port input impedance
is dominated by the two differential junction R s in parallel
with the bias elements. The differential output of the Gilbert
cell mixer is open-collector to allow the conversion gain
and matching to the second IF filter to be optimized over a
wide range of frequencies and to many types of IF filters.
The conversion voltage gain is determined by the tuned
real part of the effective output load. This load may consist
of the IF filter input impedance as well as fixed filter input
matching compensation terminations and losses.
The voltage conversion gain can be estimated by
multiplying the effective differential conversion
transconductance value (0.0294 A/V) by the total effective
differential load at the output of the mixer.
The conversion power gain is best described relative to
specified mixer input and output impedance environments.
The power conversion gain is calculated by subtracting
of the voltage conversion gain of the mixer. It should be
noted that differential second mixer input terminations may
be DC-coupled.
We can simplify and estimate the second mixer conversion
gain in the default application by noting that the input
impedance environment is approximately 663 , while the
effective output loading of 854
0.7
(0.7
power conversion gain is approximately 24.8 dB.
The factor of 0.7 in the calculation of the impedance level
is explained in Section 7.7.
It should be noted that a balanced coupled k filter can be
converted to a single-ended equivalent with a
single-ended input impedance exactly equal to that of the
full differential filter by keeping the same tank resonator
components, but placing the series coupling capacitors in
single-ended series (i.e. halving the differential value) and
AC grounding one side of the tanks.
1999 May 10
output environment is approximately 1394 . With
be expected to be approximately 25.1 V/V or 28 dBV.
The power correction from 663
10
Global Positioning System (GPS) front-end
receiver circuit
log
2
2
output resistance environment
------------------------------------------------------------------------------- -
Second IF mixer
input resistance environment
996 ) the voltage conversion gain can therefore
996 ) at the output is 3.2 dB, so the resulting
(2.2 k in parallel with
at the input to 1394
from the dBV value
28
The demonstration board was converted from a balanced
second IF filter in this manner.
To optimize the noise figure of the second mixer the input
termination admittance should be reduced. However, this
will be at the expense of the mixers input compression and
3rd-order intercept characteristics. Since the RF input of
the IF mixer is a simple differential stage, the input 1 dB
compression point and 3rd-order intercept points are
relatively fixed at approximately 67 and 215 mV
(peak value), respectively, in the second mixer. This
results from noting that an undegenerated differential input
can be expected to have an input 1 dB compression point
of approximately 36.6 mV (peak value) differential. With a
small additional extrinsic emitter degeneration the 1 dB
compression point is raised to approximately
67 mV (peak value) differential. This is approximately
input environment with the 3rd-order intercept point being
approximately 10 dB higher at 14.6 dBm. Another
important second mixer parameter is its 2nd-order input
intercept point, which will extrapolate to approximately
79.8 V (peak value) differential or 36.8 dBm in the 663
mixer input environment.
The peak output voltage swing of the IF mixer should be
limited to a peak differential swing of less than
approximately 1 V (peak value) or 2 V (p-p) differential to
prevent clipping by the internal output ESD protection
diodes and to prevent mixer output saturation. This implies
that an effective differential output load of approximately
3.2 k could result in clipping at the output of the mixer.
The IF mixer supports single-ended first and second
IF filter applications. A single-ended input is implemented
by AC bypassing one side of the IF mixer input to ground
and accepting an associated drop in mixer conversion
voltage gain. It should be noted that single-ended input
terminations can still be DC-coupled to the mixer input pins
by using the above mentioned bypass capacitor. The IF
mixer output can be made single-ended by connecting the
unused mixer output to the supply rail.
Extra care should be taken to characterize single-ended
first IF applications. Using a single-ended second IF filter
in combination with a balanced first IF filter may help reject
common mode signals not rejected by the first IF filter.
However, it should be noted that the differential tank
capacitors of the fully differential IF filters can be replaced
by common mode capacitors by doubling the differential
value and connecting two of these capacitors to ground.
Any distribution between these two extremes is also
acceptable.
24.7 dBm in the 663
second mixer GPS application
UAA1570HL
Product specification