TX CN CN
IN TRL1 TRL0
TX MOD 8 17 18
& Bias Control
ASH Transceiver Block Diagram
VCC1: Pin 2
VCC2: Pin 16
GND1: Pin 1
GND2: Pin 10
GND3: Pin 19
RREF: Pin 11
CMPIN: Pin 6
& RF Amp Bias
ASH Transceiver Block Diagram
Figure 2 is the general block diagram of the ASH transceiver. Please refer
to Figure 2 for the following discussions.
The only external RF components needed for the transceiver are the an-
tenna and its matching components. Antennas presenting an impedance in
the range of 35 to 72 ohms resistive can be satisfactorily matched to the
RFIO pin with a series matching coil and a shunt matching/ESD protection
coil. Other antenna impedances can be matched using two or three com-
ponents. For some impedances, two inductors and a capacitor will be re-
quired. A DC path from RFIO to ground is required for ESD protection.
The output of the SAW filter drives amplifier RFA1. This amplifier includes
provisions for detecting the onset of saturation (AGC Set), and for switching
between 35 dB of gain and 5 dB of gain (Gain Select). AGC Set is an input
to the AGC Control function, and Gain Select is the AGC Control function
output. ON/OFF control to RFA1 (and RFA2) is generated by the Pulse
Generator & RF Amp Bias function. The output of RFA1 drives the SAW
delay line, which has a nominal delay of 0.5 µs.
The second amplifier, RFA2, provides 51 dB of gain below saturation. The
output of RFA2 drives a full-wave detector with 19 dB of threshold gain. The
onset of saturation in each section of RFA2 is detected and summed to pro-
vide a logarithmic response. This is added to the output of the full-wave de-
tector to produce an overall detector response that is square law for low
signal levels, and transitions into a log response for high signal levels. This
combination provides excellent threshold sensitivity and more than 70 dB
of detector dynamic range. In combination with the 30 dB of AGC range in
RFA1, more than 100 dB of receiver dynamic range is achieved.
The detector output drives a gyrator filter. The filter provides a three-pole,
0.05 degree equiripple low-pass response with excellent group delay flat-
ness and minimal pulse ringing. The 3 dB bandwidth of the filter can be set
from 4.5 kHz to 1.8 MHz with an external resistor.
The filter is followed by a base-band amplifier which boosts the detected
signal to the BBOUT pin. When the receiver RF amplifiers are operating at
a 50%-50% duty cycle, the BBOUT signal changes about 10 mV/dB, with
a peak-to-peak signal level of up to 685 mV. For lower duty cycles, the mV/
dB slope and peak-to-peak signal level are proportionately less. The de-
tected signal is riding on a 1.1 Vdc level that varies somewhat with supply
voltage, temperature, etc. BBOUT is coupled to the CMPIN pin or to an ex-
ternal data recovery process (DSP, etc.) by a series capacitor. The correct
value of the series capacitor depends on data rate, data run length, and
other factors as discussed in the ASH Transceiver Designer’s Guide.
When an external data recovery process is used with AGC, BBOUT must
be coupled to the external data recovery process and CMPIN by separate
series coupling capacitors. The AGC reset function is driven by the signal
applied to CMPIN.
When the transceiver is placed in the power-down (sleep) or in a transmit
mode, the output impedance of BBOUT becomes very high. This feature
helps preserve the charge on the coupling capacitor to minimize data slicer
stabilization time when the transceiver switches back to the receive mode.
The CMPIN pin drives two data slicers, which convert the analog signal
from BBOUT back into a digital stream. The best data slicer choice de-
pends on the system operating parameters. Data slicer DS1 is a capacitive-
ly-coupled comparator with provisions for an adjustable threshold. DS1
provides the best performance at low signal-to-noise conditions. The
RF Monolithics, Inc. Phone: (972) 233-2903
Fax: (972) 387-8148
Phone: 44 1963 251383
Fax: 44 1963 251510
©1999 by RF Monolithics, Inc. The stylized RFM logo are registered trademarks of RF Monolithics, Inc.
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