signal-to-noise conditions. The threshold, or squelch, offsets the

comparator’s slicing level from 0 to 90 mV, and is set with a resistor

between the RREF and THLD1 pins. This threshold allows a trade-

off between receiver sensitivity and output noise density in the

no-signal condition. For best sensitivity, the threshold is set to 0. In

this case, noise is output continuously when no signal is present.

This, in turn, requires the circuit being driven by the RXDATA pin to

be able to process noise (and signals) continuously.

This can be a problem if RXDATA is driving a circuit that must

“sleep” when data is not present to conserve power, or when it its

necessary to minimize false interrupts to a multitasking processor.

In this case, noise can be greatly reduced by increasing the thresh-

old level, but at the expense of sensitivity. The best 3 dB bandwidth

for the low-pass filter is also affected by the threshold level setting of

DS1. The bandwidth must be increased as the threshold is in-

creased to minimize data pulse-width variations with signal ampli-

tude.

Data slicer DS2 can overcome this compromise once the signal

level is high enough to enable its operation. DS2 is a “dB-below-

peak” slicer. The peak detector charges rapidly to the peak value of

each data pulse, and decays slowly in between data pulses (1:1000

ratio). The slicer trip point can be set from 0 to 120 mV below this

peak value with a resistor between RREF and THLD2. A threshold

of 60 mV is the most common setting, which equates to “6 dB below

peak” when RFA1 and RFA2 are running a 50%-50% duty cycle.

Slicing at the “6 dB-below-peak” point reduces the signal amplitude

to data pulse-width variation, allowing a lower 3 dB filter bandwidth

to be used for improved sensitivity.

DS2 is best for ASK modulation where the transmitted waveform

has been shaped to minimize signal bandwidth (TR1100). However,

DS2 is subject to being temporarily “blinded” by strong noise pulses,

which can cause burst data errors. Note that DS1 is active when

DS2 is used, as RXDATA is the logical AND of the DS1 and DS2

outputs. DS2 can be disabled by leaving THLD2 disconnected. A

non-zero DS1 threshold is required for proper AGC operation.

AGC Control

The output of the Peak Detector also provides an AGC Reset signal

to the AGC Control function through the AGC comparator. The pur-

pose of the AGC function is to extend the dynamic range of the re-

ceiver, so that two transceivers can operate close together when

running ASK and/or high data rate modulation. The onset of satura-

tion in the output stage of RFA1 is detected and generates the AGC

Set signal to the AGC Control function. The AGC Control function

then selects the 5 dB gain mode for RFA1. The AGC Comparator

will send a reset signal when the Peak Detector output (multiplied by

0.8) falls below the threshold voltage for DS1.

A capacitor at the AGCCAP pin avoids AGC “chattering” during the

time it takes for the signal to propagate through the low-pass filter

and charge the peak detector. The AGC capacitor also allows the

hold-in time to be set longer than the peak detector decay time to

avoid AGC chattering during runs of “0” bits in the received data

stream. Note that AGC operation requires the peak detector to be

functioning, even if DS2 is not being used. AGC operation can be

defeated by connecting the AGCCAP pin to Vcc. The AGC can be

latched ON once engaged by connecting a 150 kilohm resistor be-

tween the AGCCAP pin and ground in lieu of a capacitor.

Receiver Pulse Generator and RF Amplifier Bias

The receiver amplifier-sequence operation is controlled by the Pulse

Generator & RF Amplifier Bias module, which in turn is controlled by

the PRATE and PWIDTH input pins, and the Power Down (sleep)

Control Signal from the Modulation & Bias Control function.

In the low data rate mode, the interval between the falling edge of

one RFA1 ON pulse to the rising edge of the next RFA1 ON pulse

terval can be adjusted between 0.1 and 5 µs. In the high data rate

mode (selected at the PWIDTH pin) the receiver RF amplifiers oper-

ate at a nominal 50%-50% duty cycle. In this case, the start-to-start

sistor over a range of 0.1 to 1.1 µs.

In the low data rate mode, the PWIDTH pin sets the width of the ON

0.55 and 1 µs. However, when the PWIDTH pin is connected to Vcc

througha1M resistor, the RF amplifiers operate at a nominal

50%-50% duty cycle, facilitating high data rate operation (TR1100).

In this case, the RF amplifiers are controlled by the PRATE resistor

as described above.

Both receiver RF amplifiers are turned off by the Power Down Con-

trol Signal, which is invoked in the sleep and transmit modes.

Transmitter Chain

The transmitter chain consists of a SAW delay line oscillator fol-

lowed by a modulated buffer amplifier. The SAW filter suppresses

transmitter harmonics to the antenna. Note that the same SAW de-

vices used in the amplifier-sequenced receiver are reused in the

transmit modes.

Transmitter operation supports two modulation formats, on-off

keyed (OOK) modulation, and amplitude-shift keyed (ASK) modula-

tion which is normally used by the TR1100. When OOK modulation

is chosen, the transmitter output turns completely off between “1”

data pulses. When ASK modulation is chosen, a “1” pulse is repre-

sented by a higher transmitted power level, and a “0” is represented

by a lower transmitted power level. OOK modulation provides com-

patibility with first-generation ASH technology, and provides for

power conservation. ASK modulation must be used for high data

rates (data pulses less than 30 µs). ASK modulation also reduces

the effects of some types of interference and allows the transmitted

pulses to be shaped to control modulation bandwidth.

The modulation format is chosen by the state of the CNTRL0 and

the CNTRL1 mode control pins, as discussed below. When either

modulation format is chosen, the receiver RF amplifiers are turned

off. In the OOK mode, the delay line oscillator amplifier TXA1 and

buffer amplifier TXA2 are turned off when the voltage to the TXMOD

input falls below 220 mV. In the OOK mode, the data rate is limited

by the turn-on and turn-off times of the delay line oscillator, which

are 12 and 6 µs respectively. In the ASK mode TXA1 is biased ON

continuously, and the output of TXA2 is modulated by the TXMOD

input current. Minimum output power occurs in the ASK mode when

the modulation driver sinks about 10 µA of current from the TXMOD

pin.

The transmitter RF output power is proportional to the input current

to the TXMOD pin. A series resistor is used to adjust the peak trans-

mitter output power. 0 dBm of output power requires about 450 µA

of input current.

Transceiver Mode Control

The four transceiver operating modes – receive, transmit ASK,

transmit OOK, and power-down (sleep), are controlled by the Modu-

lation & Bias Control function, and are selected with the CNTRL1

6