MICRF003 データシートの表示（PDF） - Micrel

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MICRF003

QwikRadio™ 900 MHz UHF Receiver

Micrel 

MICRF003

QwikRadio tm

Micrel

The signal path has automatic gain control (AGC) to

increase input dynamic range. An external capacitor,

CAGC, must be connected to the CAGC pin of the device.

The ratio of decay-to-attack time-constant is fixed at 10:1

(i.e., the attack time constant is 1/10th the decay time

constant), and this ratio cannot be changed by the user.

However, the attack time constant is selectable by the user

through the value of capacitor CAGC.

[By adding resistance from the CAGC pin to VDDBB or

VSSBB in parallel with the CAGC capacitor, the ratio of

decay-to-attack time-constant may be varied, although the

value of such adjustments must be studied on a per-

application basis. Generally the design value of 10:1 is

adequate for the vast majority of applications.] See

“Application Note TBD”.

To maximize system range, it is important to keep the AGC

control voltage ripple low, preferably under 10mVpp once

the control voltage has attained its quiescent value. For this

reason capacitor values ≥ 0.47uF are recommended.

The AGC control voltage is carefully managed on-chip to

allow duty-cycle operation of the MICRF003 in excess of

100:1. When the device is placed into SHUT mode (i.e.,

SHUT pin pulled high), the AGC capacitor is “floated”, to

retain the voltage. When operation is resumed, only the

voltage droop on the capacitor associated with leakage

must be replenished. Thus a relatively low-leakage

capacitor is recommended for duty cycle 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-cycle operation of the IC, the AGC

push and pull currents are increased for a fixed time

immediately after the device is taken out of Shutdown mode

(i.e., turned-on). This compensates for AGC capacitor

voltage “droop” while the IC is in Shutdown, reduces the

time to reacquire the correct AGC voltage, and thus extends

maximum achievable duty ratios. Push/Pull currents are

increased by 45X their nominal values. The fixed time

period is based on the REFOSC frequency Ft, 9.7msec for

Ft = 6.75MHz, and varies inversely as Ft varies, by the

scale factor Ft / 6.75, where Ft is in MHz.

system (e.g., a reference clock from a crystal or ceramic

resonator-based microprocessor). An externally applied

signal should be AC-coupled, and resistively-divided down

(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 device

as set by the SWEN control pin. See “Application Note

TBD” for a discussion of frequency selection and accuracy

requirements.

Wakeup Function

The Wakeup function is made available for the purposes of

further reducing power consumption of the overall wireless

system. WAKEB is an output logic signal, which goes

active when the IC detects a constant RF Carrier “header” in

the demodulated output signal. Sense of the signal is

active-low. This output may be used to “wakeup” other

external circuits, like a data decoder or microprocessor,

only at times when there is a reasonable expectation of an

incoming RF signal. The Wakeup function is unavailable

when the IC is in SHUT mode.

The Wakeup function is composed of a resettable counter,

based on an internal 27.5kHz clock (based on a 7.04Mhz

reference frequency). To utilize this function, a burst of RF

Carrier in excess of 5msec must be placed at the start of

each data code word, or a single 5msec RF carrier tone at

the start of the data pattern (clearly the former is preferred

to improve communication reliability). When this constant

carrier tone is detected, without interruption, for 128 clock

cycles of 27.5kHz, WAKEB will transition low, and stay low

until data begins. This particular approach is utilized over

others (1) since constant tones in excess of 5msec are very

rare, leading to few false-positive indications, and (2) this

technique does not require the introduction of a signal path

offset, which impacts achievable range.

[Note: For designers who wish to use the Wakeup function

while “squelching” the output, a positive squelching offset

voltage must be used. This simply requires the squelch

resistor be taken to a voltage more positive than the

quiescent voltage on pin CTH, so that the data output is low

in absence of a transmission.]

Shutdown Function

Reference Oscillator (REFOSC) and External Timing

Element

All timing and tuning operations on the MICRF003 are

derived from the REFOSC function. This function is a

single-pin Colpitts-type oscillator. The user may handle this

pin in one of three possible ways:

(1) connect a ceramic resonator, or

(2) connect a crystal, or

(3) drive this pin with an external timing signal.

The third approach is attractive for further lowering system

cost if an accurate reference signal exists elsewhere in the

The Shutdown function is controlled by the logic state of

SHUT. When SHUT is high, the device goes into low-power

standby mode, consuming less than 1µA. This pin is pulled

high internally, and so must be pulled low to engage the

device.

MICRF003 Frequency and Capacitor Selection

Selection of the REFOSC frequency Ft, Slicing Level (CTH)

capacitor, and AGC capacitor are briefly summarized in this

section. Please see Application Note TBD for complete

details.

October 1999

9

MICRF003

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