Application Hints

External Component Descriptions

Pin

Component

Typical Values

Pin Description

Component Function

1

L1 C1

500 mH – 50 mH

Second gain stage output

Set the operating frequency (fO) for

250 pF – 22 nF

transmitter oscillator

the transmit oscillator and receiver

2

C2

500 pF – 10 nF

Second gain stage input

Couples first and second gain stage

3

R3

51 kX

First gain stage output

Terminates emitter-follower output

4

C4

100 pF – 10 nF

First gain stage input

Input coupling for the first gain stage

6L6

50 mH – 10 mH

Transmitter output

Matches LM1812 to the transducer

7

Transmitter driver

8R8

1 kX –10 kX

Transmitter key

Current limiter for keying pulses up to 12V

9

C9

100 nF – 10 mF

Receiver second stage delay

Sets the receiver turn-on delay after

transmit (

Figure 10 )

11

C11

220 nF – 22 mF

Detector output duty cycle limit

Limits the duty cycle of the detector

output (short to ground to defeat)

13

C13

100 mF – 1000 mF

Transmitter supply decoupling

Decouples the transmitter power supply

14

T14

Lp t 50 mH

Detector output

Drives neon display lamp

NS NP j 10

16

Output driver

17

R17 C17

22k – Open

Pulse integrator

Controls integration time constant

10 nF – 10 mF(

Figure 13 )

18

C18

1 nF – 100 mF

Pulse integrator reset

Controls integrator reset time constant

(

Figure 14 )

TRANSDUCERS

The most common transducer used with the LM1812 is the

piezo-ceramic type which is electrically similar to a quartz

crystal Piezo-ceramic transducers are resistive at only two

frequencies termed the resonant and antiresonant (fr fa)

frequencies Elsewhere these transducers exhibit some

reactance as shown in

Figure 3

TLH7892 – 3

FIGURE 3 Phase and Magnitude

of Transducer Impedance

For transmitting (to maximize electrical to mechanical effi-

ciency) the transducer should be operated at its resonant

frequency For receiving (to maximize mechanical to electri-

cal efficiency) optimum operation is at antiresonance In

two-transducer systems the resonant frequency of the

transmit transducer is matched to the antiresonant frequen-

cy of the receiver

The LM1812 is primarily used with a single transducer per-

forming both transmit and receive functions In this mode

maximum echo sensitivity will occur at a frequency close to

resonance

Transducer ringing is a troublesome phenomenon of single

transducer systems After a transducer has been electrically

driven in the transmit mode some time is required for the

mechanical vibrations to stop Depending on the amount of

damping this ringing may last from 10 to 1000 cycles This

mechanical ring produces an electrical signal strong enough

(l200 mVp-p) to hold the detector ON thus masking any

echo signals occurring during this time

A solution to this ring problem is to vary the receiver gain

from a minimum just after transmit to a maximum when the

ring signal has dropped below the full-gain detection thresh-

old Since near-range echo signals are much stronger than

ring signals close echos will still be detected in spite of the

reduced gain

The gain is varied by attenuating the signal between pins 2

and 3 of the LM1812

Figure 4 shows such an arrangement

An externally generated 12V pulse (

Figure 17 ) keys the

transmitter and activates the attenuator This pulse charges

C to a voltage set by P8 turning the FET OFF C slowly

discharges through R decreasing the gate voltage which in

turn decreases the attenuation of the signal passing from

pin 3 to pin 2 R and C are selected so that the FET is not

3