Micrel

MICRF009

January 18, 2005

8

M9999-011805

(408) 955-1690

315MHz

9.81MHz

390MHz

12.140MHz

418MHz

13.01MHz

433.92MHz

13.51MHz

Table 2. Sweep-Mode Recommended Reference Oscillator

Values For Typical Transmit Frequencies

Step 3: Selecting C

Extraction of the DC value of the demodulated signal for

purposes of logic-level data slicing is accomplished using

switched capacitor “resistor” RSC, shown in the block

diagram.

Slicing level time constant values vary somewhat with

decoder type, data pattern, and data rate, but typically

values range from 5ms to 50ms.This issue is covered in

more detail in “Application Note 22.” Optimization of the

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

level time constant. This selection is strongly dependent on

system issues including system decode response time and

data code structure (that is, existence of data preamble,

etc.) This issue is also covered in more detail in

“Application Note 22.”

The effective resistance of RSC is listed in the electrical

characteristics table as 145kΩ at 315MHz, this value

scales linearly with frequency. Source impedance of the

CTH pin at other frequencies is given by equation (4),

9.7940

(4)

τ of 5x the bit-rate is recommended. The effective

resistance of RSC is listed in the electrical characteristics

table as 145kΩ at 315MHz, this value scales inversely with

frequency. Source impedance of the CTH pin at other

τ

(5)

A standard ±20% X7R ceramic capacitor is generally

selection examples.

Step 4: Selecting C

The signal path has AGC (automatic gain control) to

increase input dynamic range. The attack time constant of

the AGC is set externally by the value of the CAGC

capacitor connected to the CAGC pin of the device. To

maximize system range, it is important to keep the AGC

the control voltage has attained its quiescent value. For

this reason, capacitor values of at least 0.47µF are

recommended.

The AGC control voltage is carefully managed on-chip to

allow duty-cycle operation of the MICRF009. When the

device is placed into shutdown mode (SHUT pin is pulled

high), the AGC capacitor floats to retain the voltage. When

operation is resumed, only the voltage droop due to

capacitor leakage must be replenished. A relatively low-

leakage capacitor is recommended when the devices are

used in duty-cycled operation.

To further enhance duty-cycled operation, the AGC push

and pull currents are boosted for approximately 10ms

immediately after the device is taken out of shutdown. This

compensates for AGC capacitor voltage droop and

reduces the time to restore the correct AGC voltage. The

current is boosted by a factor of 45.

Selecting CAGC Capacitor in Continuous Mode

A CAGC capacitor in the range of 0.47µF to 4.7µF is

typically recommended. Caution! If the capacitor is too

large, the AGC may react too slowly to incoming signals.

AGC settling time, from a completely discharged (zero-volt)

state is given approximately by Equation 6:

∆t = 1.333 × C

(6)

where:

Selecting CAGC Capacitor in Duty-Cycle Mode

Voltage droop across the CAGC capacitor during shutdown

should be replenished as quickly as possible after the IC is

enabled. As mentioned above, the MICRF009 boosts the

push-pull current by a factor of 45 immediately after start-

up. This fixed time period is based on the reference

capacitor and the duration of the shutdown time period

should be selected 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. The worst-case from a recovery

standpoint is downward droop, since the AGC pull-up

current is 1/10th magnitude of the pull-down current. The

downward droop is replenished according to the Equation

7:

I

=

∆V

∆t

(7)

where:

I = AGC pull-up current for the initial 10ms (67.5µA)