MICRF005

Micrel

MICRF005

8

October 2001

Application Information

Bypass and Output Capacitors

The bypass and output capacitors connected to V

have the shortest possible lead lengths. For best perfor-

mance, connect V

(that is, keep V

return path). V

together at the IC pins. A 10

Ω resistor in series with the supply

line plus three decoupling capacitors is recommended. The

suggested capacitor values are 1nF, 10nF and 100nF.

VDD

C1

100nF

C2

10nF

C3

1nF

5V

VSS

R1

10R

To MICRF005

Figure 8. Supply Bypassing

External Timing Signals

Externally applied signals should be ac-coupled and the

amplitude must be limited to approximately 0.5Vpp.

Optional BandPass Filter

For applications located in high ambient noise environments,

a fixed value band-pass network may be connected between

the ANT pin and V

ity and input overload protection.

Frequency and Capacitor Selection

Selection of the reference oscillator frequency f

capacitor (C

marized in this section.

Selecting Reference Oscillator Frequency f

T

As with any superheterodyne receiver, the difference be-

tween the internal LO (local oscillator) frequency f

incoming transmit frequency f

center frequency. Equation 1 may be used to compute the

appropriate f

(1)

ff

f

915

LO

TX

TX

=± 







2 496

.

Frequencies f

f

and “low-side mixing,” and there is generally no preference of

one over the other.

After choosing one of the two acceptable values of f

Equation 2 to compute the reference oscillator frequency f

(2)

f

f

64

T

LO

=

Equations (1) and (2) can be simplified to:

f

Frequency f

frequency f

place accuracy on the frequency is generally adequate. The

following table identifies f

quencies when the MICRF005 is operated.

Transmit

Reference Oscillator

Frequency (f

Frequency (f

868.35MHz

13.6050MHz

915MHz

14.3359MHz

916.5MHz

14.3594MHz

Table 2. Common Transmitter Frequencies

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 C

where f

(3)

R3

14.3359

f

SC

T

= 0Ω

Assuming that a slicing level time constant

τ has been

established, capacitor C

(4).

(4)

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 (5):

(5)

∆t 1.333C

0.44

AGC

=−

where:

C

Selecting C

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 [tbd]ms 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 [tbd]ms period.