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RF2917

433/868/915MHZ FM/FSK RECEIVER

RF Micro Devices 

RF2917

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 kΩ, which

makes matching to 50Ω difficult. If image filtering is

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:

LockTime

=

--D----

FC

+

35000

⋅

C

⋅

d

V

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 C to be kept at a 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.

11-136

Rev B2 010118

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