MICRF004/RF044

Micrel

MICRF004

8

February 9, 2000

cludes all signals within the sweep range. This same range

reduction also occurs with superregenerative receivers as

their RF bandwidth is also generally 2% to 3% around the

nominal transmit frequency. Any superregenerative receiver

application can instead use a MICRF004 in sweep mode.

IF Bandpass Filter

Rolloff response of the IF Filter is 5th order, while the

demodulator data filter exhibits a 2nd order response. The

multiplication factor between the reference oscillator fre-

quency f

fixed mode, and 32.25

× for sweep mode (that is, for f

6.00MHz in fixed mode, f

Bandwidth

The inputs SEL0 and SEL1 control the demodulator filter

bandwidth in four binary steps (550Hz to 4400Hz in sweep,

1100Hz to 8800Hz in fixed mode). Bandwidth must be

selected according to the application. See “Applications

Information” for the bandwidth programming table.

Slicing Level

Extraction of the dc value of the demodulated signal for

purposes of logic-level data slicing is accomplished using the

external threshold capacitor C

capacitor “resistor” R

the effective resistance of R

can be considered a low-pass RC filter with source imped-

ance of 124k

Ω.

Slicing level time constant values vary somewhat with de-

coder type, data pattern, and data rate, but typical values

range from 5ms to 50ms. Optimization of the value of C

required to maximize range.

Automatic Gain Control

The signal path has AGC (automatic gain control) to increase

input dynamic range. An external capacitor, C

connected to the CAGC pin of the device. The ratio of decay-

to-attack time-constant is fixed at 10:1 (that is, the attack time

constant is 1/10th of the decay time constant), and this ratio

cannot be changed by the user. However, the attack time

constant is set externally by choosing a value for C

The AGC control voltage is carefully managed on-chip to

allow duty-cycle operation of the MICRF004 in excess of

100:1. When the device is placed into shutdown mode (SHUT

pin pulled high), the AGC capacitor floats, to retain the

voltage. When operation is resumed, only the voltage droop

on the capacitor due to leakage must be replenished, there-

fore a relatively low-leakage capacitor is recommended for

duty-cycled operation. The actual tolerable leakage will be

application dependent. Clearly, leakage performance is less

critical when the device off-time is low (milliseconds) and

more critical when the off-time is high (seconds).

To further enhance duty-cycled operation of the IC, the AGC

push and pull currents are increased for a fixed time immedi-

ately after the device is taken out of shutdown mode (turned-

on). This compensates for AGC capacitor voltage droop

while the IC is in shutdown mode, reduces the time to restore

the correct AGC voltage, and therefore extends maximum

achievable duty ratios. Push-pull currents are increased by

45 times their nominal values. The fixed time period is based

on the reference oscillator frequency f

6.00MHz, and varies inversely as f

Reference Oscillator

All timing and tuning operations on the MICRF004 are de-

rived from the internal Colpitts reference oscillator. Timing

and tuning is controlled through the REFOSC pin in one of

three ways:

1. Connect a ceramic resonator

2. Connect a crystal

3. Drive this pin with an external timing signal

The third approach is attractive for lowering system cost

further if an accurate reference signal exists elsewhere in the

system, for example, a reference clock from a crystal- or

ceramic-resonator-controlled microprocessor. An externally

applied signal should be ac-coupled and resistively-attenu-

ated, or otherwise limited, to approximately 0.5Vpp. The

specific reference frequency required is related to the system

transmit frequency and to the operating mode of the receiver

as set by the SWEN pin.

Wake-Up Function

The wake-up circuit is available for reducing power consump-

tion of the overall wireless system. WAKEB is an output logic

signal, which goes active low when the IC detects a constant

RF carrier “header” in the demodulated output signal. This

output may be used to enable external circuits, such as a data

decoder or microprocessor, when there is a detection of an

incoming RF signal. The wake-up function is unavailable

when the IC is in shutdown mode.

The wake-up function consists of a resettable counter, based

on an internal 23.4kHz clock (created from a 6.0MHz refer-

ence frequency). When this constant carrier is detected,

without interruption for 128 clock cycles of 25kHz or 5.12ms,

WAKEB will transition low and stay low until data begins. This

approach is utilized over others because constant tones in

excess of 5ms are rare, resulting in few false detections, and

this technique does not require the introduction of a signal

path offset which impacts achievable range.

Shutdown Function

The shutdown function is controlled by a logic state applied

to the SHUT pin. When V

low-power standby mode, consuming less than 1

µA. This pin

is pulled high internally. It must be externally pulled low to

enable the receiver.