MC13142D View Datasheet(PDF) - Motorola => Freescale
Part Name
Description
Manufacturer
MC13142D Datasheet PDF : 16 Pages
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MC13142
current sources to the Cell and can be programmed (via
Pin 15) for more current. The current is often adjusted for
minimum third order response. In this Fixture it is fixed biased
for most conversion gain.
The Mixer circuit is balanced where both oscillator and RF
are suppressed. This provides IF signals at Pins 9 and 10
which are equal in amplitude and 180 degrees out of phase.
To realize a positive gain one needs to reflect a higher
impedance from the load impedance (50 Ω for this fixture) to
the Mixer output or outputs. Maximum signal transfer would
require a balance to unbalance network. Center tapped
tuned transformers can perform this function but are quite
expensive. If one can afford 3.0 dB less signal, a simple LC
circuit at one of the outputs will work well. The other output is
unused and bypassed to ground.
The most gain is realized when no shunt capacity is added
and L4 is selected to resonate with the terminal capacity.
Adding shunt capacity will lower the gain and increase the
circuit’s bandwidth. A small value series capacitor C4 to the
50 Ω output will control the reflected impedance and
complete the circuit. L4 and C4 will vary in value depending
on the IF frequency.
VCO
The base of the device is the source for driving both the
Gilbert cell and prescaler buffer stages. Because of this, the
oscillator device will operate and drive the Mixer only in the
grounded collector configuration. Additional dc bias is added
through a 1.3 kΩ resistor (tapped for minimum VCO loading)
to reduce the off–set between base and supply.
The external circuit is a modified Colpitts where the
capacitance between base and emitter (Pins 4 and 5), along
with a capacitor from emitter to ac ground, forms the circuit
capacity and the feedback that sustains oscillations. The
effective circuit inductance (looking from the top of the circuit,
the transistor base) consist of L3 in series with varactor diode
D1 and a blocking capacitor. This circuit must appear
inductive for the VCO to operate properly. If the capacity is
too small, the feedback ratio is reduced and the VCO can
cease oscillating. When it becomes to large, it will not vary
the frequency due to the limiting effect of the series loop
capacitance.
In this application, the VCO is not required to cover a large
tuning range. Limiting the tuning range to no more than is
required to cover the band (making allowance for
temperature and aging effects) will result in a VCO less
susceptible to on board noise sources. To assure oscillation
while controlling the tuning range the varactor (plus series
capacitor) minimum capacity is chosen to be about equal to
the capacity from Pin 5 (transistor base) to RF ground. The
maximum tuning ratio could be no greater than 1.41 because
the circuit capacity could only double whatever the upper
value capacity the varactor attained. An upper limit on the
varactor capacity along with the effects of the series
capacitor reduces the VCO tuning range to about 1.2 times.
The varactors chosen for the test fixtures were Loral KV2111.
The VCO buffer, as most emitter follower circuits, has the
potential of generating a parasitic oscillation. When a
collector is RF bypassed, a tuned LC circuit is formed
consisting of the bypass capacitor, bond wire plus package
pin inductance and the device effective output capacity. If the
base is low impedance, there is normally enough distributive
collector to emitter capacity for the device to oscillate in the
common base mode. A simple fix without affecting the buffer
otherwise, is to place a small value series resistor in the
collector lead. This will lower the Q of the circuit where it
cannot sustain oscillations. Without the series resistor at Pin 8
or some other damping element, the buffer will oscillate.
PLL
A phase lock loop is added to the test board to evaluate
the VCO. The MC12179 multiplies the crystal reference
frequency by 256 to obtain lock. In a frequency agile system,
the MC12210 would control the VCO and its reference
derived from a crystal. The crystal frequency would be
selected to coincide with the required VCO frequencies and
channels spacing requirements.
Expected Performance
As stated earlier, the MC13142 performance in any of the
systems should mirror the performance obtained in the test
fixture. Fixture power gains of 15 dBm and noise figures of
5.5 dB are typical. The Mixer current can be varied to
enhances battery life as well as alter its output characteristic
for peak performance of a desired or undesired response.
MOTOROLA RF/IF DEVICE DATA
11
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