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A synthesised stereo AM tuner If you have the right equipment, stereo AM radio can sound fantastic. This new stereo AM tuner will really deliver the goods. It's based on a stereo AM receiver chip and features pushbutton tuning and digital frequency readout. By JOHN CLARKE & GREG SWAIN In September 1989, we published a 2-Chip Portable AM Stereo Radio based on Motorola's new high-performanc e MC13024 IC. That design is very popular but, since publication, there has been a steady stream of requests for a mains powered version with digital readout. This new tuner is our response to those many pleas. It's very easy to Despite its "high-tech" design, the new tuner is easy to build. Most of the parts are mounted on two PC boards which are soldered together at right angles. This view shows the completed assembly, before installation in the case. 22 SILICON CHIP build and align, yet boasts many impressive features such as 6-station memory, synthesised tuning with microproc essor control, digital readout and, of course, AM stereo decoding. It delivers low-distortion wideband stereo sound which, as far as most people are concerned, is every bit as good as FM stereo sound. One of our main concerns in producing a "high-tech" design such as this was that the unit had to be easy to build. This has been achieved by mounting most of the parts on two PC boards which are then soldered together to give a neat assembly. This assembly then fits into a slimline rack mounting case to give the tuner a modern appearance. All the control switches, the indicator LEDs and the digital displays are mounted on the display board. The switches and indicator LEDs protrude through holes in the front panel while the four digital displays are located behind a red perspex window. You don't have to get involved in too much metal bashing though - at least one kit supplier (Dick Smith Electronics) will be supplying a kit with a fully pre-punched front panel that will feature screen-printed gold lettering. A bevelled pers pex window that slots into place in the front panel cutout will be included as part of the deal. Front panel controls Let's take a look at the front panel layout. At the extreme left is a pushbutton on/off switch, whi le immediately to its right are a 5-LED signal strength indicator, the frequency display window and a stereo indicator LED. Next in the lineup are pushbutton switches for station seek and memory enable, and then the six station memory buttons. Finally, at the extreme right, there are two pushbutton switches for down/up tuning. 522·1629tllz FREQUENCY DISPLAY FERRITE ROD TUNED CIRCUIT 3.6MHz OSCILLATOR AUDIO AMPLIFIERS AND 9kHz _....__ NOTCH FILTERSLEFT r----'-----.VARICAP ERROR ~Sm'fE OlJTF-==---t MICROPROCESSOR CONTROLLER MUTE STOP LOCAL OSCILLATOR 972-207!111Hz 450kHz 1ST IF MIXER BUFFER 450kHz CERAMIC FILTER CQUAM STEREO DECODER AGC R 450kHz 2ND IF AGC STATION MEMORY SWITCHES AND LEDS SEEK, UP, SIGNAL LEVEL LED METER DOWN, TUNING MUTE CONTROL Fig.I: block diagram of the Stereo AM Tuner. It operates on the superheterodyne principle, which means that the local oscillator always runs 450kHz higher than the tuned RF (radio frequency) signal. The tuning is fully synthesised & is controlled by a microprocessor chip which also drives the LED displays. Most of the tuner functions shown are contained in a single IC - the MC13024. These controls are all easy to use. For example, when the Seek control is pressed, the tuner automatically scans up the frequency band and locks onto the next available station. The Memory Enable control allows you to store pre-selected stations in any one of the six memories. You simply tune to the desired station, then press the Memory Enable and the desired Memory switch to store the station setting. If the station switch is not pressed within five seconds, the Memory Enable LED extinguishes. Of course, you can tune manually if you wish and that function is provided by the Down/Up pushbuttons. When these buttons are pressed, the tuner steps up or down in 9kHz steps. If either button is held down, the tuner scans at a fast rate until the button is released. The stereo indicator LED lights whenever a station is received in stereo. Ferrite rod antenna For ease of antenna adjustment, we opted for a ferrite rod assembly instead of a long wire loop antenna. This is installed on an adjustable mount on the rear panel and provides excellent signal pickup compared to a balanced loop configuration. The antenna coil is a commercial unit by the way, so you don't have to go to the trouble of winding it. Nor do you need any special equipment to align this tuner - just a couple of plas- tic alignment tools and a multimeter. The alignment procedure is carried out using off-air stations and by measuring the AGC voltage. Block diagram Take a look now at Fig.1 which is the block diagram of our new tuner. It operates on the superheterodyne principle which means that the local oscillator frequency is always 450kHz above the tuned radio frequency (ie, the station frequency) . Both the RF and local oscillator stages are tuned using varicap diodes. These diodes are connected in parallel with inductors to form tuned circuits and vary their capacitance according to a control voltage. The microprocessor controller plays a very important role in the operation of this circuit. In addition to driving the front panel display, it also provides synthesised tuning for the AM tuner front end plus audio output switching and the station seek function. There are several inputs to the _microprocessor which control its operation. These include an input from the local oscillator, a Stop input from the station detector, and various inputs from the front panel switches (Up/Down tuning, Memory switches, Memory Enable and Seek). Basically, the microprocessor functions as a phase lock loop consisting of three sections: a reference frequency oscillator, a programmable divider and a phase comparator. In operation, the value of the programmable divider is set by inputs from the external switches. Depending on these inputs, it divides the external 4.5MHz crystal frequency to provide a reference frequency in the range 5221629kHz. This reference frequency is always some multiple of 9kHz, which corresponds to the station spacing. Inside the microprocessor there is also a counter which subtracts the 450kHz offset from the local oscillator. The resulting frequency is then compared with the reference frequency from the programmable divider. This produces an error output voltage which is then fed to the varicap diodes to lock the tuner to the desired station. In addition to frequency synthesis, the microprocessor also has outputs which drive the 4-digit frequency display and the memory enable and memory selection LEDs. These displays are all multiplexed. There is also a Mute output and this is used to switch out the audio amplifiers during tuning to eliminate noise. Tuner section Although shown as separate blocks, most of the AM tuner section of the circuit is contained in a single IC - the Motorola .MC13024. Those parts inside the chip include the local oscillator, the mixer, the two IF stages, AGC circuitry, a stereo pilot tone detector and a C-QUAM stereo decoder. To these, we have added the necessary tuned circuits for the RF and IF stages, plus signal strength indication, a station detect function and audio output stages. FEBRUARY1991 23 elude 9kHz notch filters. The notch filters remove any 9kHz whistles which can be generated by adjacent stations beating with the received station. Main circuit J The ferrite rod is installed on a small PC board along with its varicap tuning diode & a few other parts. This assembly in turn mounts on an adjustable bracket on the rear of the chassis, so that the ferrite rod can be oriented for best signal pickup. Following the mixer, the signal is fed to the 1st IF stage and thence to a wideband 450kHz ceramic filter. This filter has a response which is only 6dB down at ±12kHz but is then sharply rolled off to be 35dB down at ±20kHz to reduce noise. The output from the filter is then fed to the 2nd IF stage and thence to the stereo decoder. The output from the 2nd IF stage is also fed to a narrow band station detector block. This block consists of a narrow band ceramic filter, a gain stage and a comparator. Because the ceramic filter has a very narrow bandwidth (about 2.5kHz), the output of the comparator goes high only when the tuner is locked to the exact station frequency. This high is applied to the Stop input of the microprocessor and ensures that the tuner locks to the next available station when the Seek function is used. Stereo decoding An AM stereo transmission consists of an L+R (left plus right) mono signal plus a phase encoded L-R signal. To decode the L-R signal, the CQUAM decoder compares the phase changes in the 450kHz IF signal with a reference signal. This signal is derived from a 3.6MHz oscillator, which is divided by eight internally to give the required 450kHz reference. In operation, the decoder compares th e phase changes in the IF signal 24 SILICON CHIP against the reference and derives an error voltage using a PLL circuit. This error is then used to adjust the 3.6MHz oscillator, to bring it into lock with the IF signal. The phase fluctuations in the IF signal are interpreted by the IC as L-R stereo information. Once this signal is extracted, all that remains is to add and subtract it to the L+R mono signal to get the required left and right stereo outputs . The MC13024 will not switch into stereo mode on just any signal, however. The signal has to be confirmed as a genuine CQUAM stereo signal by detecting a 25Hz pilot tone in the L-R signal. If this tone is not present, the decoder will remain in mono. AGC voltage In addition to stereo decoding, the CQUAM decoder also derives an AGC (automatic gain control) voltage from the recovered audio. This voltage is used to control the gain of the preceding mixer and IF stages, so that the audio output level remains constant regardless of signal strength. The AGC voltage is also used to drive a signal level indicator. In thi_s circuit, the signal level indicator is a bargraph made up of five rectangular LEDs. Finally, the left and right outputs from the MC13024 IC are fed to audio amplifier stages. These stages boost the output from the IC and also in- Now take a look Fig.2. This shows the main tuner circuit diagram. While the circuit looks rather daunting at first glance, it's really not that bad. If you look at the centre of Fig.2, you will find the MC13024 (IC2) and this does most of the work. As we 've already pointed out, it is virtually a complete stereo AM tuner on a single chip. The remainder of the circuitry provides the narrow band station detect function (Q2, CF2 & IC4a-b), the signal level indicator (IC6, IC7 & LEDs 2-6) and the audio output stages (IC3ad). The incoming RF signals are picked up by antenna coil 11 which is tuned to resonance at the signal frequency by varicap diode VC1. A 4.7pF capacitor is wired in parallel with VC1 to provide the extra capacitance required to cover the high frequency end of the band. Another varicap diode, VC2, is used to tune local oscillator coil 12. This varicap is connected in parallel with trimmer Cl which is used to adjust the local oscillator so that it is always exactly 450kHz higher than the signal frequency. This 450kHz difference is equal to the intermediate frequency (IF) of the tuner. The series 470pF capacitor reduces the capacitive effect ofVC2 by up to a factor of 2 at the low frequency end. This is done because the local oscillator tuning range from 972-2097kHz is a 2.16 ratio , while the tuning range of the L1/VC1 antenna circuit from 522-1629kHz represents a 3.12 fre quency ratio . The tuning voltage for the varicap diodes is derived from the microprocessor-based control circuit. This voltage is applied to VC1 via a 100kQ isolating resistor and to VC2 via a 1MQ isolating resistor. In practice, VC1 , its two associated capacitors (.0lµF and 4.7pF) and the ferrite rod antenna are installed on a small PC board which is mounted on the back of the chassis. The output is taken from the secondary winding on the ferrite rod and fed to the RF input (pin 10) via a length.of shielded cable. DISK DRIVES AT RIDICULOUS PRICES (-------- - ------------ -- - - - -- - - -------, · Hard disk drive package deals with savings of up to $500 NEC 45 Mb AT BUS (IDE) Special Package deal for both the Drive and an IDE/FDD controller card NEC 42 Mb MFM Hard Disk Drive and MFM Hard Disk Drive Controller Card Only $495.00 Normally $695.00 You Save $200.00 This month only $549.00 Normally $795.00 You Save $250.00 NEC 115 Mb AT Bus IDE Drive and IDE/FDD Controller Card Reduced to only $995.00 Normally $1495.00 You Save $500.00 . ~-- - - ---- ------- -- - -- --------- ---------J SAVE $50.00 ON EACH OF THESE FLOPPY DRIVES Now is the time to add a second floppy drive to your system 1.2 Mb Floppy Disk Drive 1.44 Mb Floppy Disk Drive only $125.00 only $135.00 Normally $175.00 Normally $185.00 ~ ------------------- - --------- ---------, Notebook Computer - smaller than a ream of A4 paper 1 80C286 6/12Mhz 286 Weight only 3.2 kgs 1 1 I In such a small package you will find many of the features of desktop computers. 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As well there are 2 x RS232 serial ports, parallel printer port, external 16 bit expansion port, I external VGA socket and external 1.2Mb FDD port. \, sx 16Mhz, 40 Mb HOD Available April 1991 Only $3995.00 1 •; I I I I · -···--··~··--·········--·····--·· i . /J ;\,"::,.~, ·1:.E:l .. ___ ______ ______________ _______________ IAN'S PERSONAL GUARANTEE * All products carry a 14 day money back guarantee ( except software and hard disks). * All prices include sales tax. * All cards come with full documentation All motherboards carry a full 12 month warranty. * All other products carry a full 3 month warranty. * Due to Technical advances, products we supply may in some cases vary from those pictured. In all cases the produ,cts supplied are guaranteed to perform to an equal or higher standard than those pictured - WHOLESALE ENQUIRIES WELCOME I VlSA I r------------------- 1 Send us this coupon to receive your FREE 1991 Catalogue: Electronic Solutions 5 Waltham St Artarmon 2064 PO Box 426 Gladesville 2111 Telephone: (02) 906 6666 Fax: (02) 906 5222 I Mr/Mrs/Ms:_ _ _ _ _ __ __ _ _ _ _ __ I I Address:_ _ _ _ _ _ _ _ _ _ _ _ _ _ __ I Suburb:_ _ _ _ _ _ _ _ _ State: Postcode:_ __ I I. Note: Please do not send back this coupon if you purchased from us in 1 990 or returned the previously issued card to be included on the mailing list (you will receive the catalogue automatically) . 02 /91 I I ·--------------------------· PARTS LIST 1 black rack-mounting case, 44mm high 1 bevelled red Perspex sheet for front panel 1 PC board, code SC01101911, 352 x 120mm 1 PC board, code SC01101912, 341 x36mm 1 PC board, code SC01101913, 101x11mm 1 SPOT plastic.rocker switch 10 black pushbutton switches 4 20-way Molex pin strips 1 dual RCA panel socket 1 mains cord & plug 1 cord grip grommet 1 Ferguson PC-mounting transformer, PL 12/5VA 1 4 rubber feet 2 solder lugs 1 150mA 3AG fuse 1 piece of Elephantide insulation (352 x 120mm) 1 panel mount fuse holder 5 5mm spacers 1 TO-220 heatsink 13 PC stakes 3 small cable ties 1 4.5MHz parallel resonant crystal 2 5kQ miniature horizontal trimpots 2 10kQ miniature horizontal trimpots Semiconductors 1 017106-227 microprocessor (IC1) 1 MC13024 AM stereo receiver (IC2) 1 LF347 quad op amp (IC3) 1 LM339 quad comparator (IC4) 1 4066 CMOS quad bilateral switch (IC5) 1 LM324 quad op amp (IC6) Note that the shielded side of the cable connects to the 1V reference of IC2 so that the RF input is biase d correctly. The output from the local oscillator tuned circuit is fed to pin 9 of IC2 via a l0Q isolating resistor. It also feeds buffer transistor stage Ql via a 47pF capacitor. The buffe red oscillator output appears at the collector of Ql and is fed to the local oscillator input (pin 18) ofICl. It is this frequency that is compared with the reference frequency derived 26 SILICON CHIP 1 LM358 dual op amp (IC?) 1 UA9667 Darlington driver (IC8) 1 4049 hex inverter (IC9) 2 BC548 NPN transistors (01,02) 5 BC328 PNP transistors (03-07) 2 BC549 NPN transistors (08,09) 2 B8212 double varicap diodes (VC1,VC2) 1 B8809 varicap diode (VC3) 4 common anode ?-segment LED displays (red, 13mm high) 13 red rectangular LEDs (LED1-13) 1 LM317 3-terminal regulator (REG1) 2 7805 5V regulators (REG2,REG4) 1 78L05 5V regulator (REG3) 61N4002 diodes (01-04,014,015) 10 1N4148 signal diodes (05-013, 016) Inductors & Filters 1 3/8-inch dia. x 100mm ferrite rod 1 prewound AM broadcast band antenna coil (L 1) 1 A7BRS-T1080UH Toko 7P coil (L2) 1 A7NRES-T1341AYN Toko 7P mixer coil (L3) 1 A7NRES-T1340AYN Toko 7P IF coil (L4) 1 A 119ANS-18287RS Toko 7P coil (L5) 1 SFP450D Murata ceramic filter (CF1) 1 SFZ450C Murata ceramic filter (CF2) Capacitors 1 2200µF 25VW axial electrolytic 2 4 70µF 16VW PC electrolytic 1 100µF 16VW PC electrolytic 2 47µF 16VW PC electrolytic from ICl (the microprocessor), as described previously. The error output from the microprocessor controls the tuning voltage and this in turn adjusts the capacitance of VCl & VC2 so that the tuner locks to the station . Normally though , a PLL circu it inside IC2 w ould control the local oscillator. However, as w e h ave just stated, the local oscillator in this circuit (insi de IC2) is controlled by a tuning voltage derive d from the microprocessor (ICl ). This has been 1 33µF 16VW RBLL electrolytic 1 22µF 16VW PC electrolytic 3 10µF 25VW PC electrolytic 10 10µF 16VW PC electrolytic 1 3.3µF 16VW RBLL electrolytic 1 2.2µF 16VW PC electrolytic 5 1µF 16VW PC electrolytic 1 0.4 7µF 16VW PC electrolytic 1 0.1 µF metallised polyester 1 .068µF metallised polyester 2 .047µF 2% metallised polyester 16 .01 µF ceramic 2 .0012µF metallised polyester 1 .001 µF ceramic 1 470pF polystyrene 8 270pF 1% polystyrene 1 120pF polystyrene 2 100pF ceramic 2 47pF ceramic 2 22pF ceramic 1 4.7pF NP0 ceramic 2 1.8-22pF trimmers Resistors (0.25W, 4 1.8MQ 9 1MQ 21MQ1% 1 330kQ 1% 1 180kQ 1% 1 180kQ 2 150kQ 7 100kQ 4 68kQ 1% 2 68kQ 2 47kQ 4 33kQ 218kQ 1 15kQ 5%) 6 10kQ 1 8.2kQ 1% 54.?kQ 7 3.3kQ 1 2.?kQ 2 2.2kQ 1 1.8kQ 9 1kQ 6 680Q 2 220Q 1 120Q 2 1000 7 47Q 4 10Q Miscellaneous Shielded cable (1 metre), machine screws & nuts, heatshrink tubing, hookup wire. achieved by using a high-Q tuned circuit for the local oscillator. IF stages The signal from the RF input is amplified and mixed w ith the local oscillator signal to produce an intermediate frequenc y (IF) signal of 450kHz. This appears at pin 11 and is resonated in the tuned circuit based on coil L3. A low-impedance tapping on L3 is then used to drive the SFP450D ceramic filter. TMl102C Sweep/ Function Generator Has Built-In Frequency Counter Metrix DMMs First To Offer New Level of Safety · lftll Metrix have announced that their popular MX50 Series digital multimeters have been tested and approved to safety standard VDE411, Class 2. <at> [g] They are the first manufacturer in the world to gain this approval. The MX50 Series now carries the VDE logo ind icating their compliance to the standard which requires protection against hazards such as electric shock or burn, excessive temperature, liberated gases and explosion or implosion. Among the safety features of the MX50 Series are fuse protected current ranges - even on the 10A range, totally sealed case (to IP66 standard) and a unique battery and fuse compartment accessed via the front face of the instrument. To gain access it is necessary to disconnect the safetydesigned test leads and Secur'XTM adaptor, thereby protecting the operator from accidental contact with hazardous voltages. il Goldstar Release 100MHz, 3 Channel Scope Under $2000 The new OS-81 oo 100MHz scope joins the popular Goldstar low cost oscilloscope range. It offers 3 channel, eight trace capability using a new 150mm rectangular CRT with internal graticule. Key features of the OS-8100 include high sensitivity (1 mV max) and a bandwidth limiting circuit. For X-Y phase measurements the bandwidth can be expanded. Trigger view is also incorporated together with single sweep.'B' sweep sampling enables improved observation of critical parts of waveforms. A variable 'Hold-Off' circuit and TV sync separation circuits are also provided. The Goldstar OS-8100 is very competitively priced at $1950 ex tax .. .Joins Popular Scope Range Goldstar also produce 20MHz and 40MHz dual trace and a 20MHz readout oscilloscope with measuring cursors. • 0.02Hz to 7MHz in 7 ranges • Multifunction Output with Sweep • Built-In 4 Digit Counter • External Sweep Control • CMOS/TTL Output • <0.5% Distortion from 10Hz to 20kHz • Variable DC Offset Order TMIT102C $340 ex tax Low Cost Topward Bench Supplies Have Extensive Metering • Tough aluminium cases for good heat dissipation • Constant Voltage & Constant Current Modes • Short Circuit Protection • Single & Triple Outputs • Two or Four Meters • Compact Design Model 2303 4303 Outputs 30V 3A Dual 30V, 3A plus fixed 5V, 3A Australia's Leading Test & Measuring Instrument Company NEW SOUTH WALES 18 Hilly Street, MORTLAKE · P.0.Box 30, CONCORD NSW2137 Tel. (02) 736 2888 Fax: (02) 736 3005 VICTORIA 12 Maroondah Higl1way, RINGWOOD P.O.Box 623, RINGWOOD VIC3134 Tel· (03) 87~ 2322 Telex: AA30418 Fax: (03) 870 8972 QUEENSLAND 192 Evans Hoad, SALISBURY P.O.Box 274 SALISBURY OLD4107 Tel. (07) 875 1444 Fax . (07) 277 3753 SOUTH AUSTRALIA 241 Churchill Road, PROSPECT P.O.Box 154 PROSPECT SA5082 Tel. (08) 344 9000 Telex. AA87519 Fax. (08) 269 641 'I W. AUSTRALIA 32 Teddington Road, VICTORIA PARK, WA 6100 Tel: (09) 4701855 Fax: (09) 470 31 73 Metrix 40 Series Ideal For Harsh Industrial Applications ~~ !di] • Exceptional All-Weather Performance • Four Year Warranty • 4000 Count Resolution . • Autoranging with Manual Override • Hold & Peak Functions • True RMS AC or AC+DC • Safe to IEC348 Class 11 from $220 ex tax 4.7k 100k 47pF 220n SEEK SET C2 2-22pF +5V +12V + + 101 16VWJ 10 16VWJ .01! 10ll .01! -:- 1°1T 01---1. 100k 120pF STYRO L5 18287 10!2 OUT 11 MIXER 16 IF IN 91VCLOt :::.>":---.--'wll.--"1 A1 ,. 01J .,. 22 REF 19 IF OUT L1 FERRITE ROD ANTENNA PLAIN PLL 3 TUNINGF- - 0UT osc IC2 MC13024 7 AGC>-- - - ;.- 10 RF IN .01 15 +1V REF PILOT 25Hz OUT 24 25Hz IN 1M 1% +5V 180k 1M 1% R OUT 20 L OUT LOCK 13 23 10 + 16VWI: 330k 1% 1% .01 -I +1V 01r 01+ .047 2% 8.2k 1% 10 16VW+ MUTE----+---..--- 100k +12V -:- TUNING VOLTAGE LOCAL OSCILLATOR OUT F1 150mA A D1-4 4x1N4002 A 100k IC5b 1 16VW - 10 + 16VWJ 1M No----~ EO>--~-, CASE 01 CASE 470 16VWJ .,. 6□ •12 •• 4• • 3 L2-L5 STEREO AM TUNER 28 SILICON CHIP VIEWED FROM BELOW + The ceramic filter contains four coupled resonant elements and these provi de an almost rectangular bandpass characteristic of 450kHz ±12kHz at the -6dB points. Its output is amplified by the 2nd IF stage (inside IC2) , the output of which is tuned to 450kHz by coil L4 and its associated internal capacitor (pin 19). A lO0kQ resistor across the coil damps it sufficiently to give the required ±12kHz bandwidth . 1000 .01 4.7k STOP 10k .sv 10k' 100k . 1M 6800 10n • 12v 01+ 15k 2.2k Stereo decoding A STEREO LED1 1M 'A 6800 4.7k .,. 1M 6800 .12v TP1 •1.3V-0.9V 1M 680 \l 3.3 .12v • 01l 16VW.J .12v ~ .,. 1M 68k 1% SIGNAL LEVEL DISPLAY 68 k 16VW 21orJ 100pFl 2% 10k s? ·rs 1 LEFT OUTPUT lk 270pF 10 • 16VWJ .12v--- 2% 270pF 2% The output of the IF amplifier is internally connected to an envelope detector, much the same as in a conventional (mono) AM radio. Wh en the MC1 3024 is in monaural mode, the output from the envelope detector is the L+R signal and this is fed to both audio outputs . The stereo decoder components comprise the 3.6MHz oscillator, a 25Hz pilot filter, and a lock filter at pin 13. The 3.6MHz reference oscillator is at pin 22 and is tuned by coil L5 and its parallel 120pF capacitor. Further tuning of th is stage is provided by varicap di ode VC3 which provides the small frequen cy shifts necessary to lock the 3.6MHz oscillator to the 450kHz IF signal when a stereo signal is present. The tuning voltage for VC3 is provided by the PLL output at pin 7 of IC2 an d, as stated previously, is derived by comparing the phase of the divided 3.6MHz oscillator with the 45 0kHz IF signal. Wh e n phase lock has been achieved, the deco der will switch to stereo provided th e 25Hz p ilot tone is present in th e demodulated L-R signal. In order to detect this pilot ton e, the L-R signal is first fed internally to. pin 8 an d filtered using a l00kQ resistor and .068µF capacitor. This re- - -+ 10 16VW: r 68k 68k 1% 1% 1 14 270pF .,. 10k 10 + 16VWJ 270pF 2% 270pF 100pF! '~ RIGHT Fig.2 (left): the main tuner diagram. Most of the work is done by IC2 which is virtually a complete stereo AM tuner on a single chip. It is tuned by varicap diodes VCl & VC2 which vary their capacitance according to a control voltage from the microprocessor circuit. Q2, CF2, IC4a & IC4b form the narrow band station detector, while IC6a-d & IC7a,b form the signal level meter. FEBRUARY1991 29 STEREO AM TUNER - CTD moves most of the audio but the pilot tone is still too small to be recognised at this stage. To overcome this problem, the filtered signal is buffered by an amplifier with a gain of two, between pins 5 & 4, and then fed to a high-Q 25Hz bandpass active filter between pins 1 & 24. This effectively extracts the 25Hz pilot tone from the low frequency noise. The output of the 25Hz bandpass filter at pin 24 goes to the MC13024's pilot tone detector. If the 25Hz tone is present for more than seven consecutive cycles, the decoder switches into stereo mode. When a new station is selected, the decoder immediately drops out of stereo and then returns to stereo again when the new pilot tone is detected. Because it takes 300ms to cciunt seven cycles of a 25Hz signal , there is a slight time delay before the stereo mode switches in. IC2 's pin 3 output is used to drive the stereo indicator circuit. When no station is present, pin 3 is at +5V. However, if a station is detected, the voltage on pin 3 drops to about 3.5V and, finally, to 0V if a stereo signal is decoded. Thus, when a stereo signal is detected , pin 9 of comparator IC4c is pulled low and so its pin 14 output goes low and drives stereo indicator LED 1 via a 3.3kQ resistor. The associated 15kQ and 4. 7kQ resistors set the voltage on IC4c 's inverting input to 1.2V. This ensures that pin 14 can only switch low when pin 3 of IC2,is at 0V; ie, when stereo is detected. Station detect circuit Q2, CF2, IC4a & IC4b form the narrow band station detect circuit. Its job is to stop the microprocessor from scanning further up the band as soon as a station is detected, when the tuner is in the seek mode. A tapping on IF coil L4 provides the signal drive for the station detect circuit. This signal is coupled to the base of common emitter amplifier stage Q2 via trimmer capacitor C2 which sets the sensitivity of the seek function. The amplified signal is taken from the collector of Q2 and is applied to 30 SILICON CHIP narrow band ceramic filter CF2. Its output drives the inverting input of comparator stage IC4a. When the input signal exceeds 20mV p-p, IC4a's output swings between the +5V and 0V supply rails. This signal then charges a .0lµF capacitor on pin 4 of comparator IC4b via diode D16. Two 10kQ resistors set the bias on pin 5 of IC4b to +2.5V, while the lO0kQ feedback resistor sets the hysteresis to about ±240mV. When the voltage on pin 4 drops below +2.26V (due to the .0lµF capacitor charging up), pin 2 switches high and delivers the stop signal to IC1. If there is no signal out of CF2 (ie, when tuning between stations), IC4a's output remains high and the .0lµF capacitor discharges via a 68kQ resistor. Signal level meter In addition to controlling the gain of the IF and mixer stages, the AGC voltage developed by the MC13024 is made availabl e at pin 17 and is used to drive the signal level indicator. This AGC voltage is first buffered by op amp IC7a which then drives a comparator chain consisting of op amps IC6a-d & IC7b. These drive the five signal level LEDs via 3.3kQ current limiting resistors. A resistive divider chain is used to set the voltages on the non-inverting comparator inputs, ranging from 0.8V on pin 6 of IC7b to 1.2V on pin 3 of IC6a. Thus, all the LEDs will be on for AGC voltages of less than 0.8V, indicating a strong signal, while all the LEDs will be off for AGC voltages of greater than 1.2V. For voltages in between these values, one of more of the LEDs will be lit to indicate the relative signal strength. The 1MQ resistors connected between the outputs and non-inverting inputs provide a small amount of hysteresis for each comparator. This is done to prevent the LEDs from flickering at the threshold points. Audio output stages The audio outputs appear at pins 20 & 23 of IC2 and are shunted by .0lµF capacitors to filter out high frequency noise. The left and right channels are then fed to op amps IC3a & IC3b via CMOS gates IC5b & IC5a re- spectively. These CMOS gates switch the signal through when the mute signal from the microprocessor is high (ie, when a station is detected) but go open circuit to mute the output when the control input is low (ie, during tuning and at power up or power down). IC3a & IC3b both operate with a gain of 11, as set by their 10kQ & lkQ feedback resistors. A .0012µF capacitor across each 10kQ feedback resistor sets the upper frequency rolloff to about 13kHz. Immediately following the amplifiers are the twin-T filter networks, based on IC3c, IC3d and their associated close-tolerance RC networks. These twin-T filters provide a sharp notch at 9kHz, to filter out signal beats from adjacent stations. VR1 & VR3 allow the total resistance in the bottom leg of each twin-T filter to be adjusted to the correct value, while VR2 & VR4 allow precise setting of the notch frequency. The 100pF capacitors on the noninverting inputs of IC3c & IC3d are there to prevent instability in the twin-T filters, and to provide a small amount of high frequency cut above 12kHz. Control circuit Now let's take a look at the control circuit for the new tuner, shown in Fig.3. At the heart of the control circuit is IC1 which is a standard NEC tuner control chip. Among other things, it provides the tuner control voltage and drives the 7-segment LED displays and memory LEDs in multiplex fashion. The Sa-Sg outputs drive the display segments and memory LEDs via Darlington buffer IC8 (UA96677), while the D1 -D5 outputs drive inverter stages IC9b-IC9f. These in- Fig.3 (right): the control circuit is based on microprocessor ICl. It's basically a phase lock loop that compares the local oscillator signal with a divided reference frequency & produces an error voltage at pin 8. This error voltage is then buffered by QB to give the tuning voltage for the varicap diodes in the tuner front end. ICl also drives the LED displays & decodes the switch inputs. ~ ... ..... co co ..... >-<: > :i:l c:: :i:l ~ t%J !:Cl I + 10 25VW! B SEEK ~o D13~ 1N4148 + IN 7 10 25VW+ A I I I 4 t 44 lk2 43L ~ + "T 29IFM STOP -:- I D1 4 1N4DD2 MUTE +5V DISPLAY p INT 5 voo 17 12 CE 2 MUTE 18 AM osc 13 STOP X1 ..,. GN0 9 5=+R" 0 XTA 4.5M LOCAL OSCILLATOR ":' 100 16VWI vv• 22pF I ?? t r-M"' I I I 33kl I"'~ "-~ I """% I - '-~- UP ~ ~ r-.~ 421Ko -:- GN0 10 IC1 D1710G-227 7 31 6 1 .01 '""' ':' J: 470 16VWI 1.8k 1 +12V 08 BC549 -f .,. D.47 + 16VW I 220 .,. i'.'N D TP2 TUNING VOLTAGE I t g 05 1N4148 I t D6 1N41 48 05 BC328 COM 3,8 DISPLAY 3 I '-Y 04 BC328 t /c /b COM 3,8 - d- 1/ ., a OISPLAY 2 IN I 4xZ-4145 COMMON ANODE DISPLAYS 07 1N4148 CDM 3,8 DISPLAY1 03 BC328 10 g 9I +5V~ 1_~ _ _.,u, ... _... 15 1e 14 4C 6b 7a 2d 7!~?P· 13 4 7kf 1 16VW -:- IC8 UA9667 TUNER CONTROL ME 20 D1 02 52 03 51 D4 50 D5 49 VDD 1 ~ I t1 '""~ 34 35 36 37 38 4D 41 a b C d e I g 12 ~o OUT B EOc I! l6 GN0 '~"' ~ .P. VIE WED FROM BELOW R ~ +5V DISPLAY OISPLAY 4 This view shows the completed tuner, minus its silk-screened front panel. It's easy to build & delivers superb stereo AM performance. verters then drive transistors Q3-Q7 which switch the display digits and the memory LEDs. ·The ME or 'memory enable' output drives the ME LED via inverter IC9a and a 1kQ resistor. In addition to driving the display segments, the Sa, Sb, Sc , Se, Sf & Sg outputs drive the switch matrix via isolating diodes D8-D13. The other sides of the switches are connected to the K0-K3 inputs of IC1, which are normally pulled low via 33kQ pulldown resistors. However, if a switch is pressed, one of the segment outputs will pull the corresponding K0K3 input high and the switch function is then decoded and executed by the microprocessor. Also connected to the Kl & K2 inputs are feature selection diodes D5D7. These are driven by the D1 & D2 outputs of 1C1 and are used to select various options; eg, 9kHz or 10kHz station spacing, discrete LEDs or a 7segment display for memory indication, etc. Because IC1 is designed to control both AM and FM tuners, it always comes on in FM mode when power is initially applied. Since we want to control an AM tuner only, Q9 has been included to automatically switch IC1 back into AM mode. It works quite simply. When power is first applied, pin 29 of IC1 goes high and turns on Q9 after a brief delay, as set by the 2.2µF capacitor. This in turn allows Sf to pull the K3 input high, which is the command 32 SILICON CHIP for IC1 to switch to AM control. A 4.5MHz crystal (X1), connected to pins 3 & 4, provides the timebase for the IC1 and this is loaded by two 22pF capacitors to ensure optimum operating conditions. Pin 8 (EO1) is the error output and this drives buffer stage Q8 via a 10kQ resistor. The output appears at the collector of Q8 and, after filtering, provides the tuning voltage for the varicap diodes in the tuner front end. The .01µF capacitor reduces high-frequency noise on the collector of Q8, while the series 1.8kQ resistor and 1µF capacitor form a low-pass filter. Other inputs and outputs are the stop input (pin 13), the local oscillator input (pin 18) and the mute output (pin 2). The oscillator input is derived from buffer stage Q1 on the main tuner circuit (Fig.2) as discussed previously, while the mute output controls CMOS switches IC5a & IC5b. Power supply Power for the AM Stereo Tuner is derived from a mains transformer which provides 12VAC at 5VA (see Fig.2). This drives bridge rectifier D1D4 and the resulting DC output filtered by a 2200µF capacitor to give an unregulated supply rail of about 17V. This 17V rail is then used to drive a number of 3-terminal regulators. For the main tuner circuit, the 17V rail is switched by S1 and fed to adjustable 3-terminal regulator REG1 to derive a regulated +12V supply rail. This +12V supply is used to power most of the op amp and comparator circuitry and also supplies the collector of Q8 via a 4. 7kQ resistor. It also drives REG4 which provides a +5V rail to power IC2, IC4 & IC5. Note the 470µF capacitor across the 1kQ voltage adjust resistor on REG1. This ensures a slow turn on to prevent large thuds in the audio output when power is first applied. (You can't get slow turn-on from a 7812 but with a capacitor on the ADJ terminal, you can with an LM317). The 0.1µF capacitor connected between chassis and circuit earth is necessary to keep the circuit earth at the same AC potential as the chassis. Without this capacitor, the sensitive 25Hz pilot detector in IC2 would be overloaded by 50Hz mains signal and there would be no stereo reception. Two separate supplies are used to power the microprocessor chip (IC1) - see Fig.3 again. First, a +5V supply is derived via REG3 from the unswitched side of S1 and this is applied to pins 5 & 17 via D14. This standby supply is on permanently (unless the mains is switched off at the wall socket) and keeps the microprocessor powered up to maintain the station memories when the tuner is switched off. If the mains supply is interrupted (or the tuner is switched off at the wall), the 33µF low leakage capacitor at pins 5 & 17 maintains the standby supply for several hours, to ensure that station settings are not lost. Diode D14 prevents the 33µF capacitor from rapidly discharging back into the regulator if the mains supply does fail. To compensate for the voltage drop across this diode, diode D15 jacks up the output of REG3 to 5.6V. The second +5V supply rail for the microprocessor is derived from the switched side of S 1 and is fed to the chip enable (CE) input via REG2. This supply rail is also used to power the LED displays. At first glance, the use of four separate regulators may seem unnecessary. Why not just use two: one to provide the +12V rail and another to power all the +5V circuitry? The answer is that separate +5V regulators are necessary to keep the multiplex noise generated by the control circuitry out of the sensitive tuner circuits. That's all we have space for this month. Next month, we will continue with the full construction details. SC