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©2008 by RF Monolithics, Inc.

TR1004 - 4/4/08

Figure 2

Since an amplifier-sequenced receiver is inherently a sampling

receiver, the overall cycle time between the start of one RFA1 ON

sequence and the start of the next RFA1 ON sequence should be

set to sample the narrowest RF data pulse at least 10 times.

Otherwise, significant edge jitter will be added to the detected data

pulse.

ASH Transceiver Block Diagram

Figure 2 is the general block diagram of the ASH transceiver.

Please refer to Figure 2 for the following discussions.

Antenna Port

The only external RF components needed for the transceiver are

the antenna 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 components. For

some impedances, two inductors and a capacitor will be required.

A DC path from RFIO to ground is required for ESD protection.

Receiver Chain

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 provide a logarithmic response.

This is added to the output of the full-wave detector 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 flatness 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 detected 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

external 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.

ASH Transceiver Block Diagram

RFA1

RFA2

TXA1

TXA2

SAW

Delay Line

SAW

CR Filter

Log

Antenna

RFIO

Tuning/ESD

Detector

Low-Pass

Filter

BB

AGC

Control

Peak

Detector

Pulse Generator

& RF Amp Bias

LPFADJ

PRATE

PWIDTH

RXDATA

TXMOD

CN

TRL1

CN

TRL0

AGCCAP

R

REF

THLD2

THLD1

Modulation

& Bias Control

Power Down

Control

Gain Select

AGC Set

AGC Reset

Threshold

Control

BBOUT

DS2

DS1

AND

dB Below

Peak Thld

Ref

Thld

PKDET

Ref

AGC

C

BBO

C

PKD

R

LPF

C

AGC

R

PR

R

PW

R

TH2

R

TH1

R

TXM

20

817

18

14

15

3

9

56

4

7

13

11

12

VCC1: Pin 2

VCC2: Pin 16

GND1: Pin 1

GND2: Pin 10

GND3: Pin 19

RREF: Pin 11

CMPIN: Pin 6

TX

IN

Tuning