MICRF004/RF044

Micrel

MICRF004

12

February 9, 2000

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5

Table 2. Common Transmitter Frequencies

Selecting REFOSC Frequency f

T

(Sweep Mode)

Selection of the reference oscillator frequency f

mode is much simpler than in fixed mode due to the LO

sweeping process. Also, accuracy requirements of the fre-

quency reference component are significantly relaxed.

In sweep mode, f

(3)

f

f

32.25

T

LO

=

Connect a ceramic resonator of frequency f

pin on the MICRF004. Two-decimal-place accuracy is gener-

ally adequate. A crystal may be used. A crystal may be

mandatory in some cases to reduce receive frequency ambi-

guity if the transmit frequency ambiguity is excessive.

Use Equation 3a to compute sweep-mode frequency band

coverage (f

(3a)

f

0.5f

2f

f

BC

TIF

BW

=+

+

Example:

fMHz

f

5

MHz

.27

f

170

150

0.86MHz

f

170

150

0.43MHz

then:

f

5.07MHz

centered symmetrically about 170MHz.

Selecting Capacitor C

TH

The first step in the process is selection of a data-slicing-level

time constant. This selection is strongly dependent on sys-

tem issues including system decode response time and data

code structure (that is, existence of data preamble, etc.). This

issue is covered in more detail in Application Note 22.

Source impedance of the CTH pin is given by equation (4),

where f

(4)

R

124k

4.65

f

SC

T

=Ω

Assuming that a slicing level time constant

τ has been

established, capacitor C

(5)

C

R

TH

SC

= τ

A standard

±20% X7R ceramic capacitor is generally suffi-

cient.

Selecting C

Selection of C

AGC control voltage by using a sufficiently large capacitor.

Factory experience suggests that C

vicinity of 0.47

µF to 4.7µF. Large capacitor values should be

carefully considered as this determines the time required for

the AGC control voltage to settle from a completely dis-

charged condition. AGC settling time from a completely

discharged (zero-volt) state is given approximately by Equa-

tion 6:

(6)

∆t 1.333C

0.44

AGC

=−

where:

C

Selecting CAGC Capacitor in Duty-Cycle Mode

Use of 0.47

µF or greater is strongly recommended for best

range performance. Use low-leakage type capacitors (dipped

tantalum, ceramic, or polyester)for duty-cycled operation to

minimize AGC control voltage droop.

Generally, droop of the AGC control voltage during shutdown

should be replenished as quickly as possible after the IC is

“turned-on”. As described in the functional description, for

about 10ms after the IC is turned on, the AGC push-pull

currents are increased to 45 times their normal values.

Consideration should be given to selecting a value for C

AGC

and a shutdown time period such that the droop can be

replenished within this 10ms period.

Polarity of the droop is unknown, meaning the AGC voltage

could droop up or down. Worst-case from a recovery stand-

point is downward droop, since the AGC pullup current is

1/10th magnitude of the pulldown current. The downward

droop is replenished according to the Equation 7:

(7)

I

C

V

t

AGC

= ∆

∆

where:

I = AGC pullup current for the initial 10ms (67.5

µA)

C

∆t = droop recovery time

∆V = droop voltage

For example, if user desires

∆t = 10ms and chooses a 4.7µF

C

same equation with 200nA worst case pin leakage and

assuming 1

µA of capacitor leakage in the same direction, the

maximum allowable

∆t (shutdown time) is about 0.56s for

droop recovery in 10ms.