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SUMMER CIRCUITS COLLECTION

SW Converter

for Digital AM Car Radio

070

P. Laughton, VK2XAN

This circuit is purposely pre-

sented with many loose ends (not

literally, of course) to stimulate

experimenting with RF circuitry

at a small outlay.

Looking at the circuit diagram

you may recognize a modified

version of the SW Converter for

AM Radios described elsewhere

in this issue. The modifications

were necessary to make the cir-

cuit compatible with a digital

rather than analogue AM car

radio. The main difference

between digital AM radios and

their all-analogue predecessors

is that tuning is in 9 kHz (some-

times 4.5 kHz steps) in compli-

ance with the international fre-

quency allocation for the band.

Obviously, that particular step

size, desirable as it may be on

MW, is a stumbling block if you

want to use a digital AM receiver

in combination with a frequency

step-up converter for SW, where

chaos reigns and there is no

fixed step size.

The first attempt was to make the crystal oscillator vari-

able by about 5 kHz each way. Unfortunately, despite seri-

ous efforts, the crystal could not be pulled more than 1 or

2 kHz so another solution had to be found.

After studying the NE/SA602/612 datasheet, it was found

that a variable LC based oscillator was the best alternative.

The circuit worked after winding a resonant LC circuit and

adding a 0.1

RADIO

S1.C

R2

1k

+12V

*

see text

SW

D1

C12

C13

K1

*

S2

BFO ON

voir texte

ANT

100n

470µ

16V

RADIO

5V6

400mW

IN

S1.B

SW

8

K2

Tr1

RADIO

S1.A

ANT

C1

1

5

OUT

C2

220p

IC1

SW

NE602N

2p2

SA612AN

*

2

4

*

Tr2

BFO TUNE

10.7MHz

IFT

3

7

6

C3

R1

C7

T1

33p

C4

500p

C6

68p

100n

455kHz

BC109

IFT

C9

BC549

2N2222

L1

*

C11

8p2

C8

C10

220p

365p

365p

BAND SET

BAND SPREAD

000164 - 11

input stray capacitance of the NE602 (612) does the trick.

A BFO (beat frequency oscillator) was added to enable

SSB (single sideband) signals to be received. The BFO

built around T1 is simple, has a heap of output and is sta-

ble enough to hold an SSB signal for a few minutes without

adjustment. The BFO frequency is tuned with C3. Tr2 is a

ready-made 455 kHz IF transformer whose internal capac-

itor was first crushed and then removed with pliers. When S2

is closed the BFO output signal is simply superimposed on

the NE602 (612) IF output to the MW radio.

The converter should be built into a metal box for shield-

ing. If you find that the BFO gives too much output, discon-

nect it as suggested in the circuit diagram and let stray

coupling do the work.

Sensitivity, even on a 1-metre length of car radio aerial,

is quite amazing. Bearing in mind that most of the major

international SW broadcasting stations like Radio NHK

Japan, Moscow, BBC etc.) generate enough power to make

sure that you will hear them, it is still quite exciting to hear

such signals for the first time on your car radio.

F series capacitor to block the DC compo-

nent on pin 6 of the NE602 (612). When the tuning was

found to be a bit sharp with the original capacitor, a simple

bandspread (or fine tuning) feature was added by shunting

the LC resonant circuit with a lightly loaded 365 pF tuning

capacitor (C10) which, like the main tuning counterpart,

C8, was ratted from an old transistor radio.

The tuning coil, L1, consists of 8 to 10 turns of 0.6-0.8mm

dia. enamelled copper wire (ECW) on a 6-8 mm dia. for-

mer without a core. With this coil, frequency coverage will

be from about 4 MHz to 12 MHz or so.

Details on Tr1 may be found in the referring article. Note

that no tuning capacitor is used on the secondary — the

(000164-1)

104

Elektor Electronics

7-8/2001

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