MICRF218 datasheet(18/23 Pages) MICREL

18 / 23 page

Micrel

MICRF218

November 2011

M9999-111111

(408) 944-0800

Selection of CTH and CAGC Capacitors

Capacitors C6 and C4, Cth and Cagc respectively

provide time-based reference for the data pattern

received. These capacitors are selected according to

data profile, pulse duty cycle, dead time between two

received data packets, and if the data pattern has or

does not have a preamble. See Figure 11 for an

example of a data profile.

Preamble

Header

PW1 PW2

PW2 = Narrowest pulse width

t1 & t2 = data period

Figure 11. Example of a Data Profile

For best results, the capacitors should always be

optimized for the data pattern used. As the baud rate

increases, the capacitor values decrease.

Table

shows suggested values for Manchester Encoded

data, 50% duty cycle.

SEL0

JP1

SEL1

JP2

Demod.

(hertz)

Cth

(C6)

Cagc

(Cagc)

Short

1625

100nF

4.7µF

Open

Short

3250

47nF

2.2µF

Short

Open

6500

22nF

1µF

Open

13000

10nF

0.47µF

Table 10. Suggested Cth and Cagc Values.

Other components used include C5, which is a

decoupling capacitor for the Vdd line; R4 reserved for

future use and not needed for the evaluation board;

R3 for the shutdown pin (SHDN = 0, device is

operation), which can be removed if that pin is

connected to a microcontroller or an external switch,

and R1 and R2 which form a voltage divider for the

AGC pin. One can force a voltage in this AGC pin to

purposely decrease the device sensitivity. Special

care is needed when doing this operation, as an

external control of the AGC voltage may vary from lot

to lot and may not work the same for several devices.

DO, RSSI and Shutdown Functions

Three other pins are worthy of comment. They are the

DO, RSSI, and shut down pins. The DO pin has a

driving capability of 0.6mA. This drive current is good

enough for most of the logic family ICs in the market

today. The RSSI pin provides a transfer function of the

RF signal intensity vs. voltage. It is very useful to

determine the signal to noise ratio of the RF link,

crude range estimate from the transmitter source and

AM demodulation, which requires a low Cagc

capacitor value.

Shut Down Control

The shut down pin (SHDN) is useful to save energy.

When its level close to Vdd (SHDN = 1), the device is

not in operation. Its DC current consumption is less

than 1µA (do not forget to remove R3). When toggling

from high to low, there will be a time required for the

device to come to steady state mode, and a time for

data to show up in the DO pin. This time will be

dependent upon many things such as temperature,

choice of crystal used, and if the there is an external

oscillator with faster startup time. Normally, with the

crystal vendors suggested, the data will show up in

the DO pin around 1msec time, and 2msec over the

temperature range of the device. See Figures 12.

Figure 12. Time-to-Good Data After Shut Down Cycle,

Room Temperature

MICRF218 Datasheet(PDF) 18 Page - Micrel Semiconductor

MICRF218 Datasheet(HTML) 18 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL