e991018.pdf

RADIO, TELEVISION & VIDEO

receiver

part 1: circuit descriptions

This two-

part article

describes an

AM/FM/SSB

receiver for the

frequency range

0.15 – 32 MHz,

which is generally

(but incorrectly)

referred to as ‘the

shortwave bands’.

The receiver is micro-

processor controlled

and avoids many of

the pitfalls traditionally

associated with RF

construction.

Main Specifications

➧ Double conversion superheterodyne receiver, 1st IF 45 MHz,

2nd IF 455 kHz

➧ Microprocessor control of synthesizer tuning and other receiver

functions

150 kHz to 32 MHz tuning range in 1-kHz steps.

➧ Selectable selectivity: 3 kHz (narrow) or 12 kHz (wide)

➧

Internal 6-band preselector with automatic band switchover

➧ 12-key keyboard for frequency entry,

mode and bandwidth selection

➧ 16-character LCD shows receive mode, bandwidth, frequency

and preselector band

➧ Memory for 21 frequencies, incl. bandwidth and mode

➧

Spurious product rejection >50 dB

➧ Power supply 15 V, max. 400 mA (approx. 90 mA without audio

and LCD backlight)

Audio output power approx. 1 W into 8

Design by G. Baars, PE1GIC

Elektor Electronics

1/99

general-coverage

➧

The receiver we’re about to describe is

the product of many hours of design-

ing, testing and programming by the

author, a licensed radio amateur from

the Netherlands. Throughout the

design process, the emphasis has been

on repeatability, ease of construction

and avoidance of many of the pitfalls

commonly associated with building

radio equipment. As many of you will

avow, the two best known pitfalls are

winding your own coils and non-avail-

ability of specialized test equipment to

align the receiver, or, indeed, any other

RF project you may want to build.

So how are these problems solved?

Well, the present receiver has only one

inductor you have to wind yourself,

and the use of ready-made filters and

main purpose is to reduce the risk of

interference and cross-modulation

products caused by very strong signals.

The preselector is manually tuned for

best performance. The second function

of the preselector is to make the

receiver input virtually independent of

the antenna used: in fact, anything

ranging from a simple telescope

antenna to a full-blown ‘beam’ (with a

cable impedance of 50

suppression of the reference frequency

(here, 1 kHz). Like a number of other

sub-circuits in the receiver, the synthe-

sizer is digitally controlled by a central

microprocessor.

The output signal of the first mixer

is taken through a 45 MHz filter with a

bandwidth of about 15 kHz. The main

function of the filter is to suppress the

image frequency of the second mixer,

which occurs at 44.090 MHz

(44.545–0.455).

The first IF signal (45 MHz) is het-

erodyned down to 455 kHz by means

of the second mixer and the second LO

signal, which is supplied by a crystal

oscillator operating at 44.545 MHz. The

mixer is followed by two bandpass fil-

ters, one with a width of 3 kHz for ‘nar-

44.545 MHz

DEM.

455 kHz

PRESELECTOR

IF 1

MIXER 1

45 MHz

IF 2

455 kHz

12 kHz

DEM.

455 kHz

MIXER 2

SSB DET.

3 kHz

Band

Select

RF GAIN

VCO

AGC

÷ 64

÷ 65

TUNING

S meter

WIDE

BFO

VOLUME

rotary

encoder

Synthesizer

NARROW

Keyboard

SSB

LCD

Figure 1. Block diagram of the general coverage receiver.

The design is a double conversion superheterodyne with

high-side injection for the first LO. The use of a ‘high’ first

IF (45 MHz) guarantees a minimum of in-band generated

spurious products while also reducing the risk of IF break-

through. Note that many functions are controlled by a cen-

tral microprocessor.

980084 - 11

transformers in the IF sections obviates

the need for complex constructions

and adjustments. If you are a careful

builder with some experience in RF

technology, then the receiver should

work spot-on, and a minimum of

adjustments is needed to tweak it for

optimum performance. The good

news is that these adjustments only

require the built-in S meter, your hear-

ing ability, and possibly a voltmeter.

most important design considerations

is to keep strong signals away from the

input of the next stage, the mixer. If

you are new to shortwave reception,

then remember that your main con-

cern is not dredging in the noise to get

the weakest possible signal into the

receiver, but to keep multi-megawatt

signals out .

The local oscillator (LO) signal for

the first mixer is supplied by a synthe-

siser circuit which can be tuned in

steps of 1 kHz across the range

45.150 MHz to 77.000 MHz. The syn-

thesizer consists of the usual ingredi-

ents: a VCO (voltage-controlled oscil-

lator) a prescaler, and a loop filter for

row-band’ mode (SSB), and one with a

width of 12 kHz for FM and AM recep-

tion. The gain of all IF amplifier stages

(45 MHz and 455 kHz) is controlled by

an AGC circuit (automatic gain con-

trol). Because the AGC voltage is a

measure of the received signal

strength, it can also be used to drive

the S-meter.

The last 455-kHz amplifier drives

two demodulators (for AM/FM recep-

tion), and a product detector (for SSB

reception.) The oscillator in the prod-

uct detector can be pulled a little to

allow USB/LSB selection. The relevant

control is labelled BFO (beat frequency

oscillator). Analogue switches are used

T HE CONCEPT

The block diagram of the general-cov-

erage receiver is shown in Figure 1 .

The design is that of a double-conver-

sion superheterodyne receiver with a

‘high IF’, which means that the first

intermediate frequency (IF) is well

above the highest receive frequency.

The antenna signal is first taken

through a preselector stage whose

Elektor Electronics

1/99

) or a long-

wire may be connected. Alternatively,

for indoor use, consider a small mag-

netic-loop antenna such as the superb

DJ8IL design described in the Septem-

ber 1998 issue of Elektor Electronics .

The preselector is followed by a pre-

amplifier stage with manually

adjustable gain. Here, again, one of the

C83

1 st

MIXER

6p8

10n

C10

10n

C12

10n

220p

R12

R67

R15

L12

C21

C20

100n

C1 04

100n

1mH5

D14

0 µ H56

22p

1mH5

0mH82

120

BB112

C23

15p

L13

0...9V

R18

C19

L10

H68

100k

C15

100n

0...2V8

D10

D12

R10

330k

220p

2V1

2V9

L11

BF961

45M15AU

D11

D13

1V8

C14

BF961

H33

H22

R11

R13

R14

R16

R17

C11

C13

C16

C17

10p

C18

10p

C22

BB112

100n

50k

100n

PRESELECTOR

RF-GAIN

C24

zie tekst

* see text

* siehe Text

* voir texte

D1...D12 = BA479S

56p

L23

100

R23

R24

R44

R47

C100

L15

C70

100n

C71

100n

C76

100n

C79

100n

C82

100n

SFR455J

63V

R20

2K2

C26

R21

10k

WIDE

VFO

R46

15k

C25

100n

5V5

4V6

2V3

10n

C27

1V7

R22

10k

C67

100n

C69

NARROW

R45

C75

R53

0V/5V

3kHz

0V2

IC4

CLK

SCLK

SDATA

SENABLE

100n

D15

10n

D16

L14

2V5

MB501-L

C78

DATA

BF245C

IC5

C68

2V4

OUT

FIN

L21

BA182

L16

C77

4V8/0V2

A55GGP

5p6

R55

C28

C72

BFR91

SW1

R19

2K2

R25

10k

C64

150p

1V2

NARROW

10n

MC145156-2

R48

47k

WIDE

10n

C29

0V2

0V5

SW2

LMC4101

R26

10k

12kHz

0V7

PDOUT

0V2/4V8

D17

10n

D18

R54

82k

RA1

0...8V

OSC

R43

47k

R49

12k

OUT

SYNTHESIZER

R52

IC3

B A182

2V4

C30

10n

C31

100n

C32

100n

D24

C66

100n

D25

R50

C65

R51

C74

R68

C33

100n

C81

R27

C73

1V5

C80

100p

C38

100n

MC33171

1MHz

BAT82

220n

BB509

220n

1 µ 16V

40p

MIXO

IFIN

USSB

1V5

RFIN

IFDEC

IFOUT

L18

D20

D22

C34

100n

C47

47p

IC1

BAT85

C53

10p

C54

100n

1V5

TCA440

BAT85

R35

82k

IFIN

3V3

R29

39k

R40

15k

R32

L19

OSC

AGC

C48

100n

1V4

MULIN MULIN

RFIN

AGC

MOUT

C61

22n

C62

3n3

LMC4101

INA

INB

OSC

OUTA

OUTB

C40

C49

1V4

IC2

D19

UAM

UFM

5V8

4V3

4V9

C39

C 55

100n

NE612

4p7

100p

BAT

OSC

C41 1n

YMCS17105R2

CSB455A

C60

10n

L17

R30

BFO

C42

C44

C45

BF245C

4V3

C59

R41

330k

C 35

C37

100n

C36

100n

C46

3n3

R31

R33

R34

R36

D21

C50

10n

C51

C52

2n2

C 56

22p

4 µ 7

16V

2 µ 2

16V

470p

50k

100n

R37

22k

L20

1mH

L22

1mH

44.545MHz

R42

H56

100p

C63

D23

BS170

12V

S +

R66

0...0V3

BAT85

R38

22k

10k

BB509

100n

C96

R39

R28

C57

100n

C58

100n

12V

C43

SIGNAL

T7 BS170

C84

220

16V

IC7

UAM

0mA1...1mA5

78L05

1V4

100n

+12V

R62

3k9

C97

T8 BS170

C85

1V4

USSB

C101

R61

16V

100n

3V5

63V

T9 BS170

C86

R64

12k

R65

47k

C95

UFM

C99

LS1

100n

C90

C91

10n

IC6

IC8

BS170

BF245

0V/5V

5V/0V

0V/5V

16V

BF961

LS1

78L09

22n

4n7

C98

100n

R56

R57

R58

R59

BC549C

C87

C88

C89

R60

R63

C103

63V

C102

C92

10n

C93

3n9

C94

LM386-3

50k

10 0n

63V

SSB

980084 - 12

SSB

to feed one of the

demodulator/detector

outputs to the input of

the audio amplifier, by

way of a ‘speech’ filter

with roll-off points at 450 Hz and

3.3 kHz.

The microprocessor circuit controls

the preselector, the synthesizer, the IF

bandwidth (wide/narrow), the mode

selection (AM/FM/SSB, and the LCD

(liquid crystal display). Its ‘input

devices’ are a rotary encoder for the

receiver tuning, and a small keyboard

for direct frequency entry and several

other functions like channel memory

control, manual bandwidth selection

(3 kHz/12 kHz), etc.

Figure 2. Practical circuit of the RF sec-

tions of the general coverage receiver.

Most of the functions defined in the

block diagram will be easy to find back

in this schematic.

to the small capacitive load presented

by the DG MOSFET. The preselector

has six ranges:

ure 2 may look large and complex at

first, its operation is relatively easy to

understand thanks to the previous

description of the block diagram. Let’s

take the sub-circuits one by one.

1: 150 – 370 kHz

2: 370 – 900 kHz

3: 900 – 2200 kHz

4: 2200 – 5400 kHz

5 : 5400 – 13200 kHz

6: 13200 – 32000 kHz

Preselector

The active element is a type BF961

dual-gate MOSFET, T1, which guaran-

tees minimum loading of the inductors

in the preselector. PIN diodes are used

to allow the outputs of a decimal

counter to switch the requisite induc-

tors on and off. The counter, in turn, is

controlled by the microprocessor. For

the sake of repeatability, ready-made

miniature chokes from the E12 series

are used in the preselector. Their Q fac-

tors remain as high as possible thanks

The inductive part of the preselector is

brought to resonance by the capaci-

tance formed by a pair of varicap

diodes, D14-D13. The varicap control

voltage has a range from 0 to 9 V, and

is supplied by the wiper of the prese-

lector tuning control, P1.

The gain of the DG MOSFET is con-

trolled in traditional fashion by means

of a direct voltage on gate 2. Although

the preselector already affords consid-

erable suppression of unwanted fre-

quencies, the MOSFET is followed by

P RACTICAL CIRCUIT

Drawing a block diagram is one thing,

actually implementing the functions

with real components is quite another.

Although the circuit diagram in Fig-

Elektor Electronics

1/99

220

an additional low-pass filter with two

‘notch’ sections, L9-C17 and L11-C18,

for virtually complete suppression

(–50 dB) of image frequencies and out-

of band products.

double-balanced type (DBM) is

employed as the first mixer to guaran-

tee excellent large-signal behaviour.

The main dis-

advantages of

a passive DBM

are the high

level of the LO

Figure 3. The microprocessor control

circuit is based on a PIC16F84. To

keep receiver-internal interference to

a minimum, the PIC is in ‘sleep’

mode most of the time.

signal (typically 7 dBm), and the inher-

ent conversion loss of about –7 dB. The

present receiver employs a DG MOS-

FET in the first

mixer. As

opposed to a

DBM, the

MOSFET

1st mixer and synthesizer

In many up-market SW receivers, a

" "

S6'

S1'

S0'

S5'

S4'

S8'

S0'

SRG4

"7"

C1/

IC4b

S1'

"4"

S2'

"1"

S3'

S10

S11

S10'

S9'

S11'

S7'

S3'

S2'

KEYB'

SRG4

"0"

S4'

C1/

IC4a

"8"

IC3, IC4 = 4015

S5'

"5"

S6'

KEYB

"2"

S7'

SRG4

100p

C1/

100n

S10

IC3b

"#"

D10

S8'

S10

S11

10k

D11

S11

"9"

S9'

DEN

D12

S12

"6"

S10'

D13

S13

"3"

S11'

C14

IC2

100n

KEYB'

100n

ENCODER

IC2

RA0

RB0

RB1

RB2

RB3

RB4

RB5

RB6

RB7

PRESET

PCLK

KEYB

74HCT4017

CTRDIV10/

RA1

IC1

DEC

RA2

RA3

PIC16

RA4

F84-

04/P

1N4148

SCLK

SDATA

SENABLE

MCLR

SCLK

SDATA

SENABLE

OSC1

OSC2

12V

CT=0

100n

27p

100n

10k

SRG4

12V

C1/

IC3a

500mW

IC5

IC6

+ M

+ B

+ 12V

+15V

D14

7 812

7 8L05

SSB

M1 BACKLIGHT

– M

– B

400mA

1N4001

C10

C11

C12

C13

IC3

IC4

100n

25V

100n

BS170

980084 - 13

Elektor Electronics

1/99

470

offers a conversion gain of about 10 dB,

and it works fine at a relatively small

LO signal.

The combination of a synthesizer IC

type MC14156-2 (from Motorola) and

a dual-modulus (÷128/÷129 or

÷64/÷65) divider type MB501L (from

Fujitsu) forms a phase-locked loop

(PLL) whose step size equals the refer-

ence frequency of 1 kHz, which is

derived from quartz crystal X3 by an

on-chip divider. The MC14156-2 is con-

trolled by means of serial information

supplied by the microprocessor. The

error signal supplied by the synthe-

sizer IC is filtered by a loop filter built

around an opamp type MC33171 (IC3).

Because the 1-kHz reference-frequency

component has to be minimized in the

filter, the PLL should allow for a rela-

tively long lock time. Here, the largest

frequency change of the local oscillator

(45.150 MHz to 77.000 MHz) takes

about 100 ms. Using the single-ended

‘PDOUT’ terminal of the MC14156-2

allows the loop filter to be kept simple.

The MC33171 is used here because it is

capable of supplying a rail-to-rail

swing of the output voltage. This is a

must if the VCO based on FET T5 is to

cover the required frequency range

(theoretically, 45.15 MHz to 77 MHz)

without ‘dying’ as a result of a low var-

icap control voltage. In practice, the

VCO is slightly overdimensioned, cov-

ering a frequency range of 37-85 MHz

with a control voltage of 0-9 V. The

VCO output signal is capacitively cou-

pled to the first mixer (T2) as well as to

a buffer stage around T6, which is

designed to drive the ECL inputs of

the MB501L divider chip.

2 kHz is suffi-

cient for USB and LSB reception

(upper/lower sideband) if you turn the

BFO control pot.

The FM demodulator is a classic

ratio detector with a FET amplifier in

front of it. This detector has been

designed to supply enough output

even if an NBFM (narrow-band fre-

quency modulation) signal is received.

NBFM is commonly used in the 27-

MHz (11-m) CB band.

The AM demodulator consists of a

single diode, D20, which also supplies

the AGC drive signal.

The three tuneable inductors in this

part of the circuit are all 455-kHz,

ready-made types from Toko. These

units contain internal tuning capaci-

tors. Other 455-kHz transformers than

the ones shown here may be used, as

long as the primary-to-secondary turns

ratio is 20:1 (in case of L14 and L18),

and the tap is exactly at the centre of

the primary (in case of L19).

cheapest clock option, an R-C network

(R1-C1), is used. The processor runs at

about 4 MHz, however, it is only

‘active’ when its action is required, for

example, when a key is pressed, or the

synthesizer has to be reloaded. To keep

spurious signals to a minimum in the

receiver, the PIC will be ‘asleep’ most

of the time!

Three of the four shift registers type

4015 expand the I/O functionality of

the PIC into a 12-bit shift register

which is used to drive the keyboard

and the LCD. The keyboard is not a

matrix type. As indicated by the circuit

diagram, each switch has a separate

connection, while the other goes to a

‘common’ rail. Pressing a key causes an

interrupt which serves both as a wake-

up call and a service request for the

‘sleeping’ processor. Turning the rotary

encoder also generates a hardware

interrupt and causes the processor to

wake up. The encoder used here is a

Bourns type with 24 turns per full rota-

tion. It enables the complete tuning

range of the receiver to be covered —

just keep turning until the LCD shows

the desired frequency, and then care-

fully adjust the preselector for best

reception. Alternatively, type the

desired start frequency into the key-

pad, and tune from there. The rotary

encoder is connected directly to two

PIC I/O pins. Debouncing is effected

by hardware and software.

The remaining I/O pins of the PIC

are used to control the serial synthe-

siser (RB5, RB6, RB7), and the preselec-

tor, by way of decimal counter IC2

(RB2, RB3).

The power supply is conventionally

based on 3-pin fixed voltage regulators

from the 78 and 78L series. Three volt-

ages are supplied: 12 V, two times 5 V,

and 9 V. The latter and one of the 5-V

supplies are part of the main receiver

circuit discussed above (refer back to

Figure 2). They obtain their input volt-

age from the 12-V regulator on the

microprocessor board. The heaviest

loads on the 12-V rail are obviously the

audio power amplifier IC, the S-meter

lighting and the LCD backlight (if

used). The unstabilized input voltage

should be at least 15 V. An inexpensive

power mains adaptor may be used, but

do note that the receiver may draw up

to 450 mA, so go for a relatively pow-

erful adaptor.

Audio signal sections

Three BS170 FETs are used as analogue

switches, feeding either the FM, AM or

SSB signal to filter/amplifier T10. The

control signals at the gates of the FETs

are, again, supplied by the micro-

processor circuit. The audio bandfilter

is designed for speech at radio com-

munications quality, i.e., roll-off points

are defined at 450 Hz and 3.3 kHz to

keep out most unwanted noise, and in

the case of SSB, neighbouring stations.

The LM386 audio amplifier, finally,

supplies about 1 watt into 8 ohms,

which is good for a small external loud-

speaker in your shack, or a pair of low-

impedance headphones (preferred by

veteran DXers).

IF amplifiers, AM/FM demodulators

and SSB detector

Referring back to the block diagram,

the good news is that all sub-circuits

between the first IF filter and the out-

put of the last IF amplifier are con-

tained in a single IC, the TCA440. This

old faithful from Siemens contains a

preamplifier, an oscillator, an IF ampli-

fier, and an AGC with a dynamic range

of no less than 100 dB (which is no

mean requirement for SW listening).

The two 455-kHz IF filters for narrow

(3 kHz BW) and wide (12 kHz BW)

reception are connected into and out of

the TCA440 external circuitry by means

of PIN diodes and control signals sup-

plied by the microprocessor. Other fil-

ters than the Toko types indicated here

may be used as long as their input

impedance is 2.2 k

M ICROCONTROLLER

SECTION

The schematic of the microcontroller

section in the receiver is given sepa-

rately in Figure 3 . This circuit also con-

tains most of the power supply com-

ponents.

The microcontroller used is the

familiar PC16F84 from Microchip.

Here, it executes a user program of

about 1 kBytes from its on-chip ROM.

The PIC controller is supplied ready-

programmed by the Publishers.

The on-chip EEPROM is used to

store and retain frequencies. Because

the processor clock does not have to be

particularly stable or accurate, the

(980084-1)

, and the respec-

tive 3-dB bandwidths are about 3 kHz

(narrow) and 12 kHz (wide). The

TCA440 drives the S-meter directly via

its AGC output. Meters with different

sensitivities are accommodated with

preset P3.

The injection signal for the second

mixer is supplied by the oscillator

Ω

The construction, adjustment and operation of

the receiver will be discussed in next month’s

concluding instalment.

Elektor Electronics

1/99

inside the TCA440. This oscillator only

needs an external quartz crystal and a

couple of passive parts to supply a

rock-steady 44.545 MHz signal.

The SSB detector is built around the

familiar NE612 (or NE602), which con-

tains a balanced mixer and an oscilla-

tor. The latter is connected to an inex-

pensive 455-kHz ceramic filter which is

‘pulled’ by a varicap, D23. The result-

ing deviation of about

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