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RF2905

433/868/915MHZ FM/FSK/ASK/OOK TRANSCEIVER

RF Micro Devices 

The quad tank of the discriminator can be implemented

with ceramic discriminators available from a couple of

sources. This design works well for wideband applica-

tions and where the temperature range is limited. The

temperature coefficient of a ceramic discriminator can

be in the order of +/- 50ppm per degree C. An auto-

matic frequency control loop can be implemented

using the DC level of the FM OUT for feedback to an

external varactor on the reference crystal. An alterna-

tive to the ceramic discriminator is a LC tank. Figure 2

shows a schematic implementation of a LC tank.

28

C17 7 pF

27

C16 10 nF

39 pF

3.3 µH

4-22

pF

R

opt.

Figure 2. LC Type Discriminator Circuit

The DEMOD IN pin has a DC bias on it 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 used a LC or a ceramic dis-

criminator should be based upon the frequency devia-

tion in the system, discriminator Q needed, and

frequency and temperature tolerances. Tuning of the

LC tank is required to overcome the component toler-

ances in the tank.

PREDICTING AND MINIMIZING PLL LOCK TIME

The RF2905 implements a conventional PLL on chip,

with a VCO followed by a prescaler dividing the output

frequency down to be compared with a signal from the

reference oscillator. The output of the phase discrimi-

nator is a sequence of pulse width modulated current

pulses in the required direction to steer the VCO’s con-

trol 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 discriminator output

current combined with the magnitude of the loop filter

capacitance. A good approximation for total lock time

of the RF29.5 is:

Lock time=D/fc+35000*C*dV

Where D is a factor to account for the loop damping.

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. fc is the loop cut frequency. C is the

sum of all shunt capacitors in the loop filter. dV is the

required step voltage change to produce the desired

frequency change during the transient.

Rev B11 010516

RF2905

To lock faster, we need to minimize C.

1. To this end, use the divide by 128 rather than the

64, and a correspondingly lower frequency refer-

ence crystal to achieve the desired output fre-

quency.

2. Design the loop filter for the minimum phase margin

possible without causing loop instability problems;

this allows C to be kept at a minimum.

3. Design the loop filter for the highest loop cut fre-

quency possible without distorting low frequency

modulation components; this also allows C to be

kept at a minimum.

CRYSTAL SELECTION

Several issues arise in the selection of the crystals.

Timing specifications such as start-up and switching

are related to the crystal specifications, as well as

external circuitry. The tolerance of the crystals are also

an issue in optimum radio performance. In general,

tighter tolerance crystals lead to better performance

and are more critical to higher data rates. Frequency

offsets between the TX crystal, RX crystal and discrim-

inator generate duty cycle variations in the receive

demodulator.

The crystals used on the RF2905 evaluation boards

are specified as a parallel resonant, 30pF crystal with

a maximum ESR of 80Ω. The initial tolerance is

+20ppm and temperature stability is +30ppm for -10°C

to 70°C. The transistor oscillator will work with a variety

11 of different crystals and the final crystal specifications

should be evaluated for each application.

Faster start-up or switching times are achievable by

specifying crystals with low motion inductance and low

motional resistance. Additionally, the feedback caps of

the oscillator can be changed to increase the voltage

on the crystal. Generally, crystals in the leaded

HC-49U packages will provide better start-up times

than the smaller surface-mount types used on the eval-

uation board.

11-63

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