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RF2917 Datasheet(PDF) 8 Page - RF Micro Devices

Part # RF2917
Description  433/868/915MHZ FM/FSK RECEIVER
Download  14 Pages
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Manufacturer  RFMD [RF Micro Devices]
Direct Link  http://www.rfmd.com
Logo RFMD - RF Micro Devices

RF2917 Datasheet(HTML) 8 Page - RF Micro Devices

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11-136
RF2917
Rev B2 010118
11
may be used. Phase noise is generally more critical in
narrowband
applications where adjacent channel
selectivity is a concern, but it can also contribute to
raising the noise floor of the receiver, thereby degrad-
ing sensitivity.
For the interface between the LNA and mixer, the cou-
pling capacitor should be as close to the RF2917 pins
as possible, with the bias inductor being further away.
Once again, the value of the inductor may be changed
to compensate for trace inductance. The output imped-
ance of the LNA is on the order of several k
Ω, which
makesmatchingto50
Ω difficult.Ifimage filteringis
desired, a high impedance filter is recommended. If no
filtering is used, the match to the mixer input need not
be a good conjugate match, because of the high gain
of the IF amplifier stages. In fact, a conjugate match
between the LNA and mixer will not significantly
improve sensitivity, but will have an adverse effect on
system IIP3 and increase the likelihood of IF instability.
Because of the high gain of the IF section, care should
be taken in laying out the IF filtering and discriminator
components to minimize the possibility of instability. In
particular, inductive feedback may occur between the
inductor of a discrete (LC) discriminator and any induc-
tor(s) in the IF interstages. Orthogonal placement of
inductors will generally minimize coupling. Indicators
that an instability may exist include poor sensitivity and
a high RSSI level when no input signal is present.
The quadrature tank of the discriminator may be imple-
mented with ceramic discriminators available from a
variety of sources. This design works well for wideband
applications, and where the temperature range is lim-
ited. The temperature coefficient of a ceramic discrimi-
nator may be on the order of +50ppm/°C. An automatic
frequency control loop may be implemented using the
DC level of the FM OUT for feedback to an external
varactor on the reference crystal. An alternative to the
ceramic discriminator is an LC tank. The DEMOD IN
pin has a DC bias and must be DC-blocked. This can
be done either at the pin or at the ground side of the LC
tank (this must also be done if a parallel resistor is
used with a ceramic discriminator). The decision
whether to use an LC or a ceramic discriminator
should be based on the frequency deviation in the sys-
tem, discriminator Q needed, and frequency and tem-
perature tolerances. Tuning of the LC tank is required
to overcome the component tolerances in the tank.
PREDICTING AND MINIMIZING PLL LOCK TIME
The RF2917 implements a conventional on-chip PLL.
The VCO is followed by a prescaler, which divides
down the output frequency for comparison with the ref-
erence oscillator frequency. The output of the phase
discriminator is a sequence of pulse width modulated
current pulses in the required direction to steer the
VCO’s control voltage to maintain phase lock, with a
loop filter integrating the current pulses. The lock time
of this PLL is a combination of the loop transient
response time and the slew rate set by the phase dis-
criminator output current, combined with the magni-
tude
of
the
loop
filter
capacitance.
A
good
approximation for total lock time of the RF2917 is:
where D is a factor to account for the loop damping, FC
is the loop cut frequency, C is the sum of all shunt
capacitors in the loop filter, and dV is the required step
voltage change to produce the desired frequency
change during the transient. For loops with low phase
margin (30° to 40°), use D= 2, whereas for loops with
better phase margin (50° to 60°), use D=1.
To lock faster, C needs to be minimized.
1. Design the loop filter for the minimum phase margin
possible without causing loop instability problems; this
allows Ctobe keptata minimum.
2. Design the loop filter for the highest loop cut fre-
quency possible without distorting low frequency mod-
ulation components; this also allows C to be kept at a
minimum.
LockTime
D
F
C
-------
35000 CdV
⋅⋅
+
=


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