Diversity Antenna

JOHN

NEVES & BILL LEWIS

W H E N L I S T E N l N G T O A N FM STATION i n your car, have you ever noticed the sudden onset of noise -pops, clicks and hum- that lasts for iust a short time?

Maybe you siopped at a traffic light, heard the interference, but found that it disappeared when you drove away-less than a car length.

This audio annovance could be caused bv locaf sources of noise, or it could be caused by mdtipath-the convergence of

FM signals at your cars antenna that arrived by taking different paths from the FM transmitting antenna. The interference is commonly called "picket fenc- ing" because it comes and goes

EIiminate muIfipath noise from

FM

drive

with

this diversity

~ i r ~ ~ i t

antennasm

FM TRANSMITTER

A

9

9

Q iii

32

FIG. 1-MULTIPATH INTERFERENCE IN FM RECEPTION is caused by signals arriving out-of-phase at the antenna after traveling over paths of different length.

frequency which arrive at your antenna from the station tuned in. Radio waves of the same fre- quency that are out-of-phase, a s shown in Fig. 2, can cancel each other in certain locations and blank out the received signal, regardless of the FM station's transmitter power.

However, the effects of multi- path are more likely to show u p as partial cancellation of the re- ceived signal accompanied by extraneous noise. If you keep driving, you will soon pass out of this "noisy" region. Fluctua- tions in signal strength might occur just a quarter wavelength apart.

T h e FM b r o a d c a s t b a n d covers t h e r a d i o - f r e q u e n c y spectrum from 8 8 to 108 MHz.

Thus at the approximate mid-

20

b a n d frequency of 100 kHz,

8-

z

wavelength is 3 meters or about

3% yards. That's why moving

.,-

o

your car only a few feet can take it out of the noise region.

By contrast, the amplitude- m o d u l a t e d ( A M ) b r o a d c a s t b a n d , covers the much lower frequency spectrum of 540 to

1600 kHz. Thus at 1000 kHz, a n

AM signal has a wavelength of

3 0 0 meters-100 times t h e length of the FM signal. That's why AM reception is unaffected by multipath.

Diversity systems

In the FM reception situation described, if instead of moving the car out of the noisy region, a s e c o n d a n t e n n a were p o s i - tioned at least 30 inches away from the first, reception could be restored. Unfortunately, con- necting two antennas simulta- neously to a single car radio will not solve the problem.

The signals from the antenna in the noisy region would com- bine with the signals from the antenna "in-the-clear," and re- ception would not improve. The answer to this dilemma is find a means of switching automat- ically to the one of two antennas situated in the most favorable receiving position.

There is nothing new about the concept of switching anten- nas to improve reception. One method called

diversity recep-

tion was developed in the early days of radio to counter the effects of "fading" in shortwave reception. Shortwave or high- frequency (HF) signals, are ca- pable of traveling thousands of miles by "bouncing" off ionized layers 100 kilometers or higher in ionosphere. They were once the best method for long-range c o m m u n i c a t i o n , a n d fading could break that communica- tions link.

In those early high-frequency diversity systems, two separate a n t e n n a s positioned several miles apart fed two separate re- ceiver sections. Electronic cir- c u i t s compared t h e relative strengths of the two received signals, a n d automatically se- lected the strongest for further amplification a n d reception.

The selection was performed by automatic gain control (AGC).

The output

DC level was propor- tional to the strength of the sig- nal being received.

The same circuitry could im- prove mobile FM reception, but two complete receivers would be required-obviously imprac- tical and expensive. Moreover, opening a standard automotive receiver case to add circuitry could pose a problem due to space and power limitations.

The circuit described in this ar- ticle solves that problem.

A

novel

F M

In stereo FM broadcasting, the transmitter encodes left and right channels as sum and dif- ference signals. The difference signal, L

- kilohertz sub-carrier necessary for decoding the stereo chan- nels a t the receiver. It is not transmitted because of band- width restrictions. Instead, the

FM s t a t i o n transmits a

pilot

subcarrier at 19 kilohertz, half the subcarrier frequency. This pilot phase locks a 38-kilohertz oscillator in the receiver to de- code the stereo signal.

The 19-kHz pilot subcarrier is within the audio bandwidth, but its amplitude is so low that it doesn't disturb the listener.

However, the presence of this pi- lot subcarrier makes possible the FM diversity reception sys- tem discussed in this article.

T h e FM d i v e r s i t y c i r c u i t monitors the 19-kHz pilot sub- carrier signal.

Steadiness of the reception of this 19-kHz pilot signal in the audio portion of the FM trans- mission is a n indication of the quality of the received signal.

Whenever this signal falls below a specified threshold value, it will be lost in background noise.

The threshold establishes the criterion for switching anten- nas. In effect, the pilot thresh- old level f u n c t i o n s i n FM diversity a s the AGC level func- tions in HF diversity.

How

FM

diversity works

A second antenna, installed on your vehicle as far away from the original equipment antenna as practical, provides the sec- ond FM signal. Figure 3 is a sim- plified block diagram of the di- versity system.

The cables from both anten- nas are connected to the elec- tronic antenna switch. The 19- kHz pilot signal from the re- ceiver's audio output is passed through a high-gain bandpass active filter which attenua.tes a u d i o p r o g r a m m i n g t h a t i s much stronger t h a n the pilot signal. After amplification, the pilot s u b c a r r i e r becomes t h e reference frequency for a phase- locked loop (PLL) circuit. The output of the PLL locks to the

19-kHz pilot signal a n d func- tions a s a subcarrier detector.

When the reference frequency becomes noisy, the PLL will lose

"lock" and trigger the flip-flop whose output switches the state of t h e e l e c t r o n i c a n t e n n a switch. This action switches the alternate antenna into the system while disabling the orig- inal antenna.

If that second antenna is posi- tioned for better reception, the received signal will clear, a n d the PLL will again lock to the subcarrier and hold the switch in that state until the pilot sig- nal drops out again. If the sec- ond antenna does not restore the pilot signal reception after a

0.1 second delay, the primary antenna is switched back on.

When the radio is receiving

AM, the absence of a 19-kHz subcarrier will also reactivate t h e primary a n t e n n a t h a t is tuned to the receiver for the best

AM reception.

FM

diversity circuit

Refer to schematic diagram

Fig. 4. The audio signal from the FM receiver appears at con- nector 54. The two capacitors

Cl1 and C16 in the audio input section bypass any DC compo- nents in the radio output or overvoltages t h a t c o u l d b e caused by miswiring. Trimmer potentiometer R22 controls the input level. The LF347 quad op- erational amplifier IC3 is a n ac- tive filter with a gain of 50 at 19

KHz. It has four sections: a , b, c,

WAVELENGTH

(=

3

METERS FOR FM)

FIG. 2-OUT-OF-PHASE SINE WAVES represent out-of-phase FM signals arriving at a receiving antenna. They can combine constructively or destructively. An FM signal is about 3 meters long.

FIG. 3-BLOCK DIAGRAM OF THE FM

DIVERSITY SYSTEM to overcome multi- path interference or cancellation.

and d.

The active filter attenuates the audio so that the LM1800N phase-locked loop (PLL) IC1 can lock onto the 19-kHz pilot sub- carrier. With 2 millivolts input, the output level at pin 14 of IC3- d is about 100 millivolts. Light- emitting diode LED3 is the level indicator for IC3-d. Each of the four 47,000-ohm feedback re- sistors, R1, R6, R7, and R8, around the op-amp sections in

IC3 has a 1% tolerance. The feedback capacitors (reading from left to right) C6, C4, C19 and C23, and the input capaci- tors C7, C5, C20, and C21 have closer 10% tolerances to assure that the filter will tune in the 19- kHz region.

Trimmer potentiometer R21

(in series with resistor R10 at frequency pin 15 of IC1) sets the

PLL's operating frequency to 19

KHz. Resistor R9 a n d capaci- tors C2 and C12 form the loop filter between pins 13 and 14 of

ICl to set the PLL's locking characteristics including

cap- ture time

a n d

capture range.

The values shown result in a 1- millisecond capture time and 2- kHz bandwidth. Bandwidth is not critical in this circuit be- cause the center frequency is al- ways 19 kHz. However, the wider the bandwidth, the faster the capture.

;

2

q

2

c,

Every time the PLL locks to the incoming signal, it pro-

8

2, duces a low-level logic output at

LAMP pin 7 of IC1. When the in- put signal is lost, pin

7

of IC1 goes h i g h , toggling 7 4 C 7 4

?

CMOS dual flip-flop IC2-a. Com-

33

plementary output pins 5 and

6

of IC2-a control t h e

ONIOFF s t a t e s of the two MR901 RF transistors Q1 and Q2. They are switched through 33-kilohm re- sistors R13 a n d R12, respec- tively. When Pin 5 of IC2-a is high, Q1 turns on; when pin 6 id high Q2 turns on.

Transistors Q1 and Q2 are the active components in two iden- tical broadband, untuned, com- mon-emitter amplifiers. Each has a gain of about unity in the

AM broadcast band and about

6 dB in the FM band. That 6-dB gain overcomes cable and con- nector losses. The low AM gain prevents AM signal overload.

Transistor Ql's base is con- nected to antenna input jack 53 for Antenna 1 and transistor

Q2's base is connected to input jack 5 3 from antenna 2. The transistor collectors are con- nected, but only one transistor can be turned on at a time. As a result, the circuit works a s a fast electronic single-pole, dou- ble-throw antenna switch.

The load inductance for Q1 and Q2 presents a high imped- ance at

FM frequencies for good amplification. However, at AM frequencies this impedance is low, resulting in low amplifica- tion. The output signals a t Ql and Q2 feed back to the working antenna input jack.

The second half of the CMOS dual flip-flop IC2, section b, is a

0.1-second timer. If the FM pilot subcarrier is absent for more than 0.1 second, 0.1pF capaci- tor C17, charging through 1- megohm resistor R15, toggles flip-flop IC2-b, forcing pin 8 low.

That low output presets the IC2- a flip-flop, biasing Q1

ON and ac- tivating antenna 1. This feature is necessary for AM operation because antenna 1 is the car's original equipment or primary

AM antenna.

Flip-flop IC2-a also forces Q1 o n for non-stereo FM signals.

The illumination of

PILOT

LED 3 indicates that the PLL is locked to the pilot signal. The illumina- tion of

ANT

1

LED2 indicates that antenna 1 is active, and the il- lumination of

ANT 2

LED l indi- cates that antenna 2 is active.

Building

the

circuit

Readers are cautioned that this project is a relatively com- plex RF circuit that is not rec- ommended for beginners. Suc- cessful completion of this proj- ect will depend on the builder's skill and the care taken in cir- cuit assembly and soldering. An u n d e r s t a n d i n g of how im- properly placed and soldered

RF c o m p o n e n t s c a n c a u s e u n - desirable feedback is necessary.

Also, the installation of the cir- cuit calls for current knowledge of modern automotive radio and electrical systems.

E v e n s e a s o n e d b u i l d e r s should pay particular attention to details and work cautiously, especially in the

RF adjustment,

FIG. 4--SCHEMATIC DIAGRAM for the FM diversity circuit.

test and installation phases of the project. Experience in work- ing on automotive electrical and entertainment systems will be helpful.

Because the circuit must re- ceive clear high-frequency FM signals,

PC board construction is recommended. A partial kit, including a n etched and drilled double-sided PC board with a ground plane, is available from the source given i n the Parts

List. Foil patterns are provided for those who want to make their own boards.

The ground plane is copper foil laminated over most of the component side of the board to ensure stable RF reception. It shields the active components to prevent inadvertent signal ra- diation, t h u s preventing un-. wanted oscillations. All of the electronic components are stan- dard parts, stock items from mail-order d i s t r i b u t o r s a n d most electronics retail stores.

All wiring must be a s short as practical. For example, the leads of collector inductor L1 must be short to prevent unwanted os- cillations. However, parts place- ment and wiring in the audio section is not critical.

Refer to both the schematic

Fig. 4 and the parts placement diagram Fig. 5. Follow accepted parts placement practices, and do all soldering with a fine-tip- ped soldering pencil rated 30 watts or less, preferably with a t e m p e r a t u r e c o n t r o l s e t t o

700°F.

The cleanliness of the PC board and component leads is important for quality soldering.

Be sure that all solder joints are

2 smooth and shiny; cold solder

$ joints are usually dull gray and

G

irregular.

-8

Use sockets for the all ICs,

m,

and observe the location of pin 1 when installing all sockets. rr

$.

Mount the eight axial-leaded

G:

0.001 pF polyester capacitors

5

C4 to C7, C19 to 21, and C23,

-Z the 390 pF C3 and the 0.047 FF

C8 vertically. With needle-nose

35

FIG. !+PARTS PLACEMENT DIAGRAM for the FM diversity circuit.

g

2 pliers, grasp a lead at one end of the capacitor close to the body and bend the lead back 180" to form closely spaced radial leads.

Find the cathode band on di- ode Dl and insert the lead on that end in the cathode location shown in placement diagram

Fig. 5. The three light-emitting diodes LEDs 1, 2, and

3 in the prototype were in T-1-style radi- al-leaded packages. The longer lead on these LEDs is the anode lead (arrowhead side of the sym- bol), and the short lead identi- fies the cathode lead (bar in the symbol).

Insert a n d solder the three

LEDs w i t h o u t c u t t i n g t h e i r leads. M o u n t t h e m s o they stand off the board about 1%- inches so the leads can be bent to insert the reflectors into the pre-drilled holes in the case after the completed board i s mounted to the lower case half.

Transistors Q1 and Q2 have flat leads to minimize induc- tance. Position the transistors carefully o n the board before soldering them. Caution: The t r a n s i s t o r s ' o r i e n t a t i o n a n d spacing with respect to each

g

other and the other RF compo-

z

nents is critical.

E

36

Install the two polarized ca- pacitors, tantalum dipped, ra- dial capacitor C24 and alumi- num electrolytic C25, according to the polarity marks shown on

Fig. 5 . Caution: Be sure the minus

( - side of both capaci- tors is connected to ground; a n improper connection can de- stroy the capacitor.

Integrated circuit IC2 must be a CMOS B 74C74 because the power source is

+

12 volts. Com- parable parts in the HC a n d

HCT CMOS logic families are rated for only

5 volts.

After inserting and soldering all of the components on the PC board, cut, insert and solder the color-coded wires for the case- mounted connector jacks. The wires should all be cut about 3 inches long from No. 22 or 24

AWG insulated stranded hook- u p wire. Use red wire to indicate positive

(

+

) and black to indi- cate negative

( - 1 ground for power jack 55. Use any other color for the audio input signal to jack 54, but use black for the ground connection there also.

Insert and solder lengths of twisted black and red wire for the R F connector jacks J1, 5 2 and

53 with the red wires for the antenna and radio input sig- nals and black wires for ground.

Packaging

the circuit

The plastic enclosure spec- ified in the Parts List is recom- mended because t h e circuit board described in this article is sized to fit snugly in it. Refer to

Fig. 6 for the proper orientation of jacks with respect to the cir- cuit board, and mark the cen- ters of holes to be drilled for the five jacks J 1 to 5 5 on the ends of the lower half case using the hole-forming t e m p l a t e s i n - cluded in this article. Exact hole diameter and shape should be determined by measuring the actual jacks.

Mark the positions of t h e holes for the three LEDs in the side wall of the case with the aid of the template and a s shown in

Fig. 6. Drill the LED holes with a drill size that will permit the

LEDs to be press-fit in the side of the case so that no adhesive will be needed.

Clean the completed circuit board with cotton swabs dipped in cleaning fluids intended for that purpose. Insert the board in the lower half of the case a s shown in Fig.

6, and fasten it to the two internal sidebars of the case with small sheet-metal screws.

Next, install and fasten the three RF connector jacks, J 1 ,

52, and 53, power jack 55, and audio input jack J6. Solder the wires from the circuit board to the lugs on the jacks. Wire jacks

J l , J 2 , and 53 with the twisted pairs. They should be trimmed as short as possible before they are soldered to the connectors.

Solder the black wires for the ground connections to the jack shells and the red wires for the signal paths from the antennas and radio. (It might be neces- sary t o remove s o m e metal plating from the jack shells with emery cloth or a file to obtain a secure solder joint.)

Carefully b e n d b o t h LED leads together so that the reflec- t o r b o d y c a n be p r e s s - f i t through the previously drilled holes on the side of the case.

Double check the completed assembly carefully A magnify- ing glass will be helpful. Check for a n y incorrectly i n s e r t e d components, solder bridges, or cold solder joints.

Identify all the jacks a n d

LEDs on the outside of the case with a waterproof pen or dry transfer lettering. Label LED3 a s

PILOT,

LED1 a s

ANT 2 a n d

LED2 a s

ANT

1 .

Label J 1 as

RADIO

INPUT,

5 2 a s

ANT 1 a n d

53

a s

ANT 2.

Label

54 as

AUDIO

INPUT and 5 5 as

" +

12 V." Cover the

labels with transparent tape or coat them with nail lacquer. Fig- ure 8 is a photo of the author's assembled prototype.

Testing

and

tuning

Before applying power to the board, measure the resistance between the positive power sup- ply connection and ground. It should be 3000 ohms or higher, a f t e r t h e f i l t e r c a p a c i t o r charges. If it is lower, recheck the circuit for shorts or incor- rectly installed ICs.

The power source required to perform these tests can be a 12- volt nickel-cadmium battery, a

12-volt lead-acid batten3 or a 12- volt DC wall-outlet adapter. If you use a n adapter, be sure it has a standard 2.1 millimeter diameter axial pin in the plug.

Read the label on the adapter to be sure that the positive

( + ) conductor of the plug is the ax- ial lead a n d the negative

(

- conductor is the shell.

Put a %-ampere fuse in series with the power supply to pre- vent damage to the circuit if there are undetected shorts. An

FM radio or tuner with a n ear- phone or speaker plug con- nector is also required. The test setup shown in Fig. 7 emulates the wiring circuitry of a n auto- mobile installation.

Tune the FM receiver to a ster- eo FM station. (The stereo in- dicator should be illuminated).

Set the volume control from one quarter to one third of its max- imum angle. Then connect the audio input of the diversity cir- cuit to t h e FM radio's head- phone or speaker.

Apply power to the system.

With a plastic alignment tool or small screwdriver, adjust audio input-level potentiometer R22 to mid-position. Then adjust

PLL frequency-set potentiome- ter R21 slowly until LED3 is il- liminated. After t h i s adjust- m e n t , LED3 will t r a c k t h e radio's stereo indicator to verify the presence of the stereo pilot subcarrier signal with different thresholds.

The radio indicator works with all stereo stations, and the diversity circuit depends on the audio volume and the setting of input-level potentiometer R22.

Either LEDl or LED2 should be on. Turn down the volume con- trol on the FM radio until LED 3 e x t i n g u i s h e s . Observe t h e

LEDl a n d LED2 p a i r : They should alternate between o n and off each time LED3 goes out.

Turn the volume-control knob u p a n d down t o check t h e switching action. ' h r n the vol- ume down once or twice s o

LED2 stays on, then keep the volume low. After 0.1 second,

LEDl will light and LED2 will go off. That indicates the one-sec- ond timer is working.

Connect the

RADIO INPUT cable from J 1 to the FM radio's anten- n a input jack. If the existing FM radio receiver does not have ex- ternal antenna connections, re- tract its a n t e n n a completely, and convert it to the input con- nection with wires connected by

.

2718

INCHES

*

.

171a INCHES

-.

FOIL PATTERN FOR THE COMPONENT SlDE OF THE PC BOARD. FOIL PATTERN FOR THE SOLDER SIDE OF THE PC BOARD. 37

two alligator clips. 'l'he second connection is for ground, the outer part of the cable or jack.

Unless t h i s connection is made with a coaxial cable, the antenna will not function cor- rectly, and the test will be inval- id. Light-emitting diode LED2 will be illuminated after the one- second timeout. Antenna 1 will be the active antenna. Con- versely, LED1 will indicate that antenna 2 is active.

Connect two test antenna ca- bles to their jacks in the diver- sity circuit. One of these could be the extra antenna you pur- chased for installation in your car. A three-foot length of insu- lated hookup wire stretched ver- tically will serve as the second antenna, Regardless of what you use as an antenna, it must make contact with the axial conductor of the connector. A banana plug can serve a s a makeshift connector.

7hne in a n FM station, check to see which LED is illumi- nated, and then disconnect the related antenna. This step will permit the circuit to switch to the alternative a n t e n n a , a s shown by LED illumination.

If the signal from the FM station selected is strong, switching ac- tion can be prevented if the an- tenna connections are not well shielded.

Choose a different station and repeat the test. Then repeat the test for the other antenna.

Adjust the volume control to command antenna switching.

When the desired LED turns on, remove the corresponding an- tenna, and the ability of the

LEDs to turn on and off should be restored.

This test does not reproduce actual circuit operation be- cause it is not possible to simu- late, at a fixed location, the FM radio reception conditions to which your vehicle is exposed.

In a car installation, the proper

OF

CASE-MOUNTED components.

connectors and coaxial cables form well shielded connections without RF leakage. Neverthe- less, this bench test can demon- strate that the

RF switching is functional.

If you can perform this test with a n actual auto radio re- ceiver and the coaxial cables recommended in the Parts List, the test will still be a realistic approximation of a n a u t o - motive installation.

Installation

in a car

Refer again to Fig. 7, and in- stall the second antenna on your car. The greater the separa- tion between the primary and secondary antennas, the more effective will be the diversity cir- cuit's operation. If, for any rea- son, you do not want to install a second full-size antenna, you can purchase a flat antenna that adheres to the windshield glass.

The cable from the originally installed antenna probably will not reach the diversity circuit unless the a n t e n n a was in- stalled at the rear of the car. In that case, the secondary in- stalled antenna must be located at the front of the car. The an- tenna extension cables listed in the Parts List might be re- quired.

Alternatively, you can make your own extension cable from low-loss coaxial cable termi- nated by male a n d female

Motorola-type connectors.

Another possibility is a n au- tomotive M F M antenna whose design is based on that for a sur- face-mount cellular telephone a n t e n n a . A s o u r c e for t h e

03CH7516N antenna with 17 feet of cable is given in the Parts

List. However, a n extension ca- ble will not be needed for most cars.

A horizontal antenna such as the windshield type mentioned will provide polarization diver- sity because FM broadcast sta- tions transmit both vertically a n d horizontally polarized waves. This will be a n advan- tage even if the two antennas are not very far apart.

Access to the radio's electrical connections in most cars can be gained by removing the front

panel of the radio, and pulling it out to expose the antenna and speaker connections. Consult the maintenance m a n u a l for your car radio for details on how to remove t h e radio without damaging it.

The latest model car radios have RCA audio o u t p u t con- nectors. Those will will make it easy to provide the audio for the diversity circuit.

Alternatively, the audio signal source will be the radio's speak- er output terminals. Any of the stereo outputs will provide the signal depending, of course, on the stereo

BALANCE control set- ting. Identify all of the speakers' terminals and wiring. Consult your user's manual or read the labels on the wires.

Radio manufacturers do not all follow a uniform wiring color code, so the functions of the wires cannot be determined reliably from their colors. How- ever, several leading manufac- turers have agreed on green and gray for the left and right speak- ers, respectively, and black for common or ground. The ground

(or low side) goes to the ground side of the audio input jack of the diversity circuit. Only one audio source is required for the operation of this circuit.

Caution: Do not connect the unit to the output of a n external power booster.

Unfortunately, you have no g u a r a n t e e t h a t y o u r radio's power amplifier will have suffi- cient bandwidth to allow the 19- kHz subcarrier to pass. Tap the audio from the input cable to the power amplifier. If resistive

"faders" have been installed, tap the audio signal upstream of them.

Any connections spliced to the speaker wires must be insu- lated with quality electric tape so they will not be shorted to ground. Connect the original equipment car antenna to the antenna input jack

ANT 1 .

Then connect the second antenna to input jack

ANT 2.

Connect the diversity circuit to the

FM re- ceiver with a n extension cable terminated by two male plugs, one end plugged into the

RADIO

INPUT jack.

The required 12-volt power

FIG. 7-THE

FM DIVERSITY SYSTEM installed in an automobile with one antenna, a radio and speakers.

can be obtained from a fused cigarette lighter adapter cable.

Install a

Y4-

or ?h the fuse holder to protect the diversity circuit. However, if you want a more permanent con- nection, you can make one with a n in-line fused cable for the power connection to the spare lugs usually available in the car's fuse box. See the Parts

'

List.

Turn on the receiver and plug in the adapter. Tune to a stereo station, and set the receiver for normal listening while driving.

Readjust input-level trimmer potentiometer R22 for this nor- mal audio volume setting in the c a r u n t i l LED3 r e m a i n s o n without blinking. Set the tone or treble control to at least one- quarter of a turn towards max- imum. If this is not done, the 19 kHz pilot subcarrier signal will be too low. If LED3 doesn't light up, readjust the PLL frequency- set trimmer potentiometer R21 until it does.

Verify that LEDl a n d LED2 change state each time you lower the volume. Then change stations. That also will control the LEDl to LED2 switching.

Input-level trimmer potentiom- eter R22 controls switching sensitivity If it is set too low,

LED3 will not light. Set it so

LED3 r e m a i n s o n w i t h o u t blinking when a clear stereo sta- tion is being received. Trimmer

R21 must be set in the middle of its "lock" range, the span be- tween the two points in wiper rotation where LED3 turns on and off.

Check the 19-kHz pilot sub- carrier level at the audio output of a receiver with a high-Q band- continued on p a g e

85

consumer is happy with his Hitachi rice cooker, vacuum cleaner, or washing machine, he is likely to se- riously consider a Hitachi audio component. In other words, the brand names of audio-equipment manufacturers are a familiar part of

Japanese life. Large Japanese in- dustrial companies also encourage their employees with various sav- ings plans and discount arrange- ments to invest in their "house brand" audio equipment. The result of all this has been the development of a large number of hard-core au- diophiles plus a higher level of inter- est on the part of the general public.

CD costs

Unlike the situation with conven- tional record players where, by and large, the more you paid the better they got, even the cheapest CD players provide excellent quality.

Player costs have probably come down about as low as they are going to get. I've seen units advertised for slightly over $100.

I still don't understand the eco- nomics of player pricing. They are not like memory ICs whose cost per unit falls radically with time and sales because the inherent "parts" cost is insignificant to start with. CD players are crammed with precision- made mechanical parts that, it seems to me, are going to continue to keep the prices from going any lower. I'm grateful-and so should the rest of you be-that the prices of CD players have fallen as far down as they have.

Disc prices, on the other hand, have not fallen significantly from the

1983 initial introductions at $15 to

$20. Interestingly, during a 1981 re- port on a visit to Denon's Tokyo re- cording studio,

I quoted an engineer who said that, unlike the situation with CD players, the cost of the disc itself would not come down sub- stantially with time. However, con- sidering the effects of inflation and the significant rise in the yen with respect to the dollar, CD's are now something of a bargain. At least, if sales figures are an accurate indica- tion, the music listening public seems to think so.

In any case, Happy Tenth Birthday to the Digital Compact Disc-and long may it rotate.

R

I mobile dashboards and the lim- ited space available for mount- ing a n electronics case, univer- sal mounting directions cannot pass filter and a n oscilloscope. be given. After selecting a n ap-

Five millivolts of the pilot sub- propriate location where the in- carrier will ensure proper sys- put and output wires and cables tem operation. are free of stress, you can clamp

Temporarily fasten the diver- the package in position with a n sity circuit to the automobile adhesive-backed plastic clamp, dashboard with duct tape, but a strap that you fashion from put a cardboard hood over the plastic or light metal, or even

LED lamps so that they can be duct tape. seen more easily i n daylight.

Tmke the system for a test drive.

Application precautions

Ask a passenger to check the

It is possible that the diversity switching action by observing circuit will not work on auto- the three LEDs a s you drive. motive radio receivers that are

When you are satisfied that more than about 20 years old t h e diversity c i r c u i t works, because of their restricted audio m o u n t i t i n u n d e r t h e bandwidth. Nevertheless, t h e dashboard where it cannot be prototype was tested on several seen by passengers in the front different car radios with differ- seat, but where you can see the ing ages, and it worked satisfac- illuminated LEDs by ducking torily on all of them. The circuit your head down close to the will work with most late-model seat. Because of the wide varia- stereo receivers because they tions in the size and shape of typically have a s much a s

20-

the space available under auto- kHz audio bandwidth.

s2

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