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2-Chip Stereo Radio This month, we complete construction of the AM Stereo Radio by giving the alignment details. Provided you follow the instructions carefully, you will finish up with a receiver capable of true hifi performance. By STEVE PAYOR Precise alignment of this receiver is a challenging task, requiring care and patience. The circuit may appear to be working well even before alignment has been attempted, but this is more a testimony to the sensitivity of the MC13024 than anything else. When it is fully aligned, it will work much better. 32 SILICON CHIP To achieve the maximum stereo separation ()25dB) and minimum distortion (<1 % ), the RF and IF response curves must be perfectly symmetrical in both amplitude and phase, either side of the carrier frequency. Not only does the alignment have to be perfect but the tuning must also be spot on - within a kHz or so. Normally, such precision can only be obtained with a digitally synthesised front end. However, this is where the phase locked tuning of the MC13024 comes into its own. Provided the IF circuits are all centred on 450kHz and the reference oscillator is tuned to exactly 8 times this frequency, the PLL will automatically centre the signal carrier on 450kHz whenever it is within the ± 3kHz capture range. Block diagram Before we start randomly twiddling the coil slugs, it would be a good idea to review the way all the tuned circuits relate to one another, so we VOLTAGE CONTROLLED LOCAL OSCILLATOR ··rn ~ Jf, r----1 I I c4 osc I PHASE COMPARATOR TRIMMER 450kHz 981-2052kHz (:t 3kHz-VCO SHIFT) % llffi C3 ,fo 6MHz REFERENCE OSCILLATOR LS 3688 OSCILLATOR TUNED CIRCUIT AERIAL TUNED CIRCUIT f1 PHASE LOCKED LOOP ' CONTROL VOLTAGE L2 T1342 GANGED TUNING (CONSTANT 450kHz DIFFERENCE) 11 450kHz REF 531-1602kHz AERIAL TRIMMER ,fo 2ND IF TUNED CIRCUIT 1ST IF TUNED CIRCUIT MIXER II Ll FERRITE ROD AERIAL COIL 450kHz CERAMIC FILTER SFP-450D L3 T1341 IF AMPLIFIER ro 450kHz IF OUTPUT TO DETECTOR ANO STEREO DECODER L4 T1340 I --- - ----- ~----WIDE -12kHz t +12kHz -12kHz SIGNAL CARRIER FREQUENCY t +12kHz 450kHz -12kHz t +12kHz 450kHz Fig.12: this block diagram shows the tuned circuits that have to be adjusted during the alignment procedure. This involves setting the IF stages to 450kHz, adjusting the aerial and oscillator circuits, and setting the 3.6MHz reference oscillator. can formulate a suitable plan of attack. Fig.12 is a block diagram showing just those circuit elements of importance to the alignment procedure. The response curves of the RF & IF tuned circuits are shown approximately to scale, to help you visualise what is happening with each adjustment. The alignment process has four main aims: (1). Setting the centre frequency of both IF tuned circuits to the centre of the ceramic filter passband (4g0kHz ± 2kHz). (2). Setting the 3.6MHz reference oscillator to exactly 8 times the IF centre frequency. (3). Adjusting the oscillator coil and trimmer capacitor so that the dial calibrations are correct over the whole tuning range. (4). Adjusting the aerial coil and trimmer so that the aerial tuned circuit "tracks" the oscillator over the whole tuning range. Unfortunately, these four objectives cannot be achieved simultaneously by any instant "one shot" alignment process - at least not without a lot of test equipment. Instead, you need to repeat a series of adjustments, with each round bringing you closer to the desired result. Tuning meter The minimum requirement in the way of test equipment is an analog moving coil multimeter (remember those'?). If you don't have one, buy or borrow one now! Digital multimeters are almost useless here as they make finding the optimum adjustment unbearably tedious. If you have a 10M0 FET input type [ie, the equivalent of the good old VTVM), simply connect it between the AGC test point and ground. Alternatively, for a more expanded scale (useful if you are trying to make do with a bargraph display on a digital multimeter), try connecting it between the AGC test point and the + 1.0V reference. The ACG voltage dips slightly below + 1.0V on strong signals and will rise to around + 1.3V when the tuning is way off. On the other hand, connecting an ordinary moving coil multimeter directly between the AGC line and ground will kill the reception. This is because the low input impedance of the meter will pull the AGC voltage down and turn off the receiver gain completely. To overcome this problem, we have provided a "Buffered AGC" test point. So if you are using an ordinary multimeter as a tuning indicator, connect it between the Buffered AGC test point and ground. But regardless of what sort of tuning indicator you are using, remember this: optimum tuning means minimum AGC.voltage. Getting started The following settings are ''ball park" adjustments for all the coils and trimmers, based on our experience with two prototypes: • Slide the aerial coil (L1) along the ferrite rod until it is flush with the end of the rod. • Set the oscillator coil slug (L2) about one turn out of the top of the can. NOVEMBER 1989 33 Fig.13: the 'Sharp', 'Medium' or 'Wide' bandwidth option is selected by bridging solder pads on the PCB. We recommend that you select the 'Wide' option as shown at right. (Note: repeated from last month's issue - see footnote). • Set the aerial trimmer (associated with C3) and the oscillator trimmer (across C4) to half mesh. The aerial trimmer adjustment screw is adjacent to the C3 marking on the back of the tuning gang. The oscillator trimmer adjustment is below the C4 marking. • Set the slugs of both IF coils (13 & 14) level with the top of the can. Don't worry too much about the 3.6MHz reference oscillator at this stage. In fact, it is better left way off frequency to start with. This will disable the phase locked loop and make the initial alignment procedure the same as for any ordinary superhet receiver. (As long as the tuning indicator is NOT lit, you are in control of the tuning). IF alignment This is not a bad place to start but remember that you will have to come back and re-align this part later on, when the other tuned circuits are a bit closer to their final adjustment. Tune in a clear, un-crowded station near the top end of the band and peak the IF coils (13 & 14) for minimum AGC voltage. Temporarily peak the aerial trimmer as well and re-check the IF coils. Now swing the tuning knob a little to either side of the tuned station and, using your ears, locate each edge of the ± 12kHz passband of the ceramic filter. The distortion caused by the steep cutoff at each edge is unmistakable. Temporarily affix a slip of paper under the perspex cursor and carefully mark each edge of the passband, then put the cursor line exactly in the middle. Before starting alignment, the tuning shaft must be rotated fully anti-clockwise and the cursor aligned with the horizontal line on the dial. Don't forget to preload the Teflon washer and tuning gang bearings before tightening the grubscrew. 34 SILICON CHIP Don't worry at this stage that the station frequency doesn't match its location on the dial - this adjustment comes later. Also, if the tuning indicator LED comes on, get rid of it by de-tuning the reference oscillator. Having found the middle of the ceramic filter passband, go back and re-peak the IF coils (13 & 14) and the aerial trimmer again. The reason for peaking the aerial tuned circuit here is to prevent its response peak from masking the true centre frequency of the IF coils. As you can see from Fig.12, the aerial tuned circuit is quite sharp in its own right. This gives the receiver excellent image rejection and cross modulation performance but is a bit of a nuisance during the IF alignment. Getting the dial markings right Contrary to what you may first think, it is the oscillator frequency which determines the receiver tuning and not the aerial tuned circuit. Of course, the aerial circuit is tuned to the signal frequency but it has nothing to do with positioning the signal on the dial. It simply maximises the signal at a given position. To get the cursor to line up with the correct dial marking for a given station, you have to adjust the oscillator coil (12) and the oscillator trimmer capacitor (across C4). The trick is to always adjust the coil at the low frequency end of the band and the trimmer at the high frequency end. First, set the dial to the exact frequency of a station at the low end of the band and peak the oscillator coil for maximum signal. This done, move the cursor to a station frequency at the top end of the band, then move the station to this position using the oscillator trimmer. If you now go back to the low end of the band you will find that the first station has moved slightly (but not very much), so re-adjust the oscillator coil. Now you will find that the station at the top end of the band has moved a bit too, so readjust the oscillator trimmer. After about the third pass, both stations should be spot on. This photo will allow you to quickly locate the various coils and trimmers during alignment. The 3.6MHz reference oscillator (L5) should initially be left way off frequency to disable the phase locked loop (see text). · Getting the tracking right This step can be done simultaneously with the oscillator adjustments but we are describing it here separately for clarity. Basically the procedure is the same: you adjust the aerial coil at the bottom end of the band and the aerial trimmer at the too end. The trimmer adjustment (at the top end of the band) is straightforward but the aerial coil adjustment is a little tricky, since you will have to slide the coil along the rod with your fingers. Now putting your fingers on the coil will de-tune it quite drastically, so it's best to know which way to move it before you touch it! Try this simple test: tune to the exact centre of a station at the low frequency end of the band and note the effect of de-tuning very slightly to either side (by very slightly we mean staying within the flat region of the IF passband). If the indicated signal strength increases to the low side, then the resonant frequency of the aerial circuit is a little low. To correct this, you will need to slide the coil towards the end of the ferrite rod, and vice-versa. So now you know which way to move the coil, although how much is still a matter of trial and error. You must remove your fingers from the coil to check each position. Of course, all this peaking of tun- Where to buy the kit A complete kit of parts for this project has recently been made available by Dick Smith Electronics. You can buy it at your nearest DSE store or by mail order from PO Box 321, North Ryde, NSW 2113 . Phone (02) 888 2105 . The kit comes with all parts & even includes a low-cost pair of headphones to get you going . The front panel is of aluminium & is supplied pre-punched together with an attractive adhesive label. The price is $79.95 (does not include batteries) plus postage charges. Quote Cat. K-5200 when ordering . ed circuits assumes that the IF coils are correctly aligned to start with. Repeat the adjustments for these once more before making the final touchups to the oscillator and aerial tuned circuits. Remember: the oscillator adjustments set the stations to their correct positions on the dial, while the aerial adjustments peak the reception at these positions. Using stations at the extreme ends of the band is helpful for the initial tracking adjustments, since there is minimal interaction between the coil and trimmer adjustments. The final tracking adjustments should be done using stations about one quarter to one third of the way in from each end of the band. This will give a "best fit" linear approximation to the dial positions. With careful adjustment, it should be possible to get the station frequencies within a pointer's width of the station position over most of the tuning range. Setting the reference oscillator Tune to a nice clear station and, as before, mark the approximate passband edges on a piece of paper NOVEMBER 1989 35 Fig.14: these tuning scales are for Melbourne, Brisbane, Adelaide, Perth, Hobart and Launceston, and the Northern Territory. They fit in the blank central area of the tuning dial. One approach is to make a transparency or adhesive label from the artwork (try your local printer), which can then be affixed to the front panel label. Alternatively, you can use a combined artwork to produce a single 3M Dynamark (formerly Scotchcal) label. slid under the cursor. Now set the cursor as close as possible to the centre of the passband as your ears and eyes can detect and adjust the reference oscillator slug (15) with a plastic alignment tool until the tuning indicator LED comes on. Now carefully tune across the passband and note whether the "Locked" tuning range is centred within the audible tuning range. It will be fairly narrow - about 1/4 of the IF bandwidth. If it is a little to the high side of centre, screw the reference oscillator slug a teeny weeny bit out, and vice-versa. Persevere until you have the locking range exactly centred. Note: With the reference oscillator coil, maximum inductance occurs with the slug near the top of the can. The inductance decreases as you screw the slug in. If you find the slug needs to be screwed all the way out to make the frequency correct, try screwing it in instead. Stereo? Once the "Locked" indicator is lit, the receiver should switch to stereo mode if you leave the tuning knob alone for more than 300ms (ie, the LED should jump from half brilliance to full brilliance). If not, then something is amiss with the 25Hz pilot filter circuitry or the station is not transmitting in stereo. 36 SILICON CHIP The stereo mode is much more sensitive to phase errors than mono mode. For example, if you have chosen the Medium or Sharp bandwidth options for the 2nd IF tuned circuit, the alignment symmetry will be quite critical. With the Sharp setting, try detuning 14 ·slightly while listening to a stereo signal with headphones. The sound will appear to go in one ear and out the other, or even rotate around inside your head! Of course, the stereo separation will be thoroughly shot. For this reason, the Wide setting is recommended (at least initially) since it is the least critical with regard to alignment. Other test equipment? If you have a digital frequency meter and are thinking of hooking it up to the reference oscillator, then forget it. Even the SILICON CHIP 1GHz DFM (which has a lower input capacitance than most commercial units) can't get a proper reading, even with a 3pF divider probe. In any case, the MC13024 has a habit of shutting down the reference oscillator at the first sign of any disturbance. The best approach is to set the reference frequency indirectly. Hook the DFM up to an RF signal generator and adjust the signal fre- quency to exactly 450kHz (unmodulated). Run a length of insulated wire from the signal generator and lay it near the MC13024 chip to inject a little of this signal into the IF circuitry. As you tune through a station, you will hear a faint whistle which decreases in frequency until the LED comes on. After that, the whistle frequency will remain constant. This constant audible frequency is the error in the PLL setting so, while the LED is on, carefully adjust 15 until the whistle becomes a low growl, or even a rapid flutter if you are very lucky. Check the adjustment by tuning through the station again. When the whistle frequency becomes less than a few kHz, it should suddenly drop to a low frequency growl (as the LED comes on) and stay there until the LED goes off again. Now you can peak all the IF circuits (13 & 14), knowing that when the LED is on, the IF is exactly 450kHz. However, while this is an accurate way of setting up the reference oscillator frequency for a 450kHz IF, it doesn't help if the centre frequency of the ceramic filter is 2kHz off (worst case). Once again, you should perform an "eyes and ears" verification that the locking range is centred within the filter passband. Final touches With the reference oscillator working, you can now make some last minute touch-ups to the alignment. In particular, the local oscillator can now be precisely adjusted so that the locking range is symmetrical (about one cursor line thickness) either side of the chosen station markings. Also, the IF coils can now be repeaked any time the LED is on, as this guarantees that the IF signal is centred correctly. You will appreciate that this is a classic "chicken before the egg" situation but that's life, as they say. Operating hints If you are listening to " fringe area" stations with headphones, the 9kHz inter-carrier whistle can be annoying, especially at night. In these cases, try rotating the radio to minimise reception of the interfering carrier. The same applies to other sources of man-made interference. On local stations, however, inter. carrier interference will not be a +Trim to outside of board outline + SILICON CHIP AM STEREO Fig.15: the Rev.2 version of the PC board will be available only with the Dick Smith kit (see panel). The Rev.1 version (published last month) is available from the usual suppliers. problem, even if you use the Wide setting. In any case, we plan to describe an add-on whistle filter to improve the audio quality from distant stations. Look for it in a future issue. Finally, if you are driving speakers (rather than headphones) and the stereo mode drops out when you cO turn up the volume, it's time to change the batteries. Footnote: unfortunately, the colour overlay for Fig.5 in last month's issue was not precisely positioned. 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