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An FM wireless microphone for musicians This new FM wireless microphone looks good & works well. It uses a well-proven circuit which has excellent frequency stability & good range. It operates from a 9V battery with a current drain of 3.5 milliamps. Design by BRANCO JUSTIC FM wireless microphones can be temperamental devices to use, particularly as far as frequency drift is concerned and there are several causes for this. The first of these is due to a drop in the supply voltage as the battery ages. The second is due to capacitance effects between the user’s body and the dangling antenna. Third, and not usually recognised, is drift due to change in temperature. When you set up an FM wireless microphone to operate at a particular frequency, say 95MHz, you don’t expect it to drift much. If it only drifts by a small amount, the AFC (automatic frequency control) circuits of your FM tuner should cope with the change in frequency so that the signal is always received clear­ly. But there is a limit to the AFC range of any FM tuner, per­ haps ±100kHz, and beyond that, the signal will start to dis­tort badly and ultimately, will not be received at all. That is why drift caused by body capacitance can be so annoying as ANTENNA S1 22k Q2 BF199 10k 560  B 0.1 0.1 220k 0.1 B 8.2k 100pF MIC 100k .047 Q3 BF199 B C 1k 33pF External features 1pF 12k L1 B FM WIRELESS MICROPHONE C E 15pF E 100pF 6.8k C E 22pF 9V Q1 BC549 270 E 680  Q2, Q3 C Q1 B E C VIEWED FROM BELOW Fig.1: Q1 functions as a preamplifier, while Q2 & Q3 form a modulated oscillator with good isolation between the antenna & the tank circuit. 66  Silicon Chip it varies all over the place. We make these comments about drift essentially because this design does not have these problems. We tested it in a number of ways, including heating up the PC board with a hot air gun and even then, drift was not a problem. After five minutes under a heat gun, the operating frequency shifted from 95.422MHz to 95.452MHz and by that time the circuit components were pretty hot. That order of change is only +0.03%. In fact, drift due to supply voltage variations of ±1V for a 9V supply is also quoted as less than ±0.03%. Operating range is quoted as better than 100 metres with a good quality tuner. Other relevant specs are: signalto-noise ratio >60dB; pre-emphasis 50µs; frequency response 40Hz to 15kHz. 15pF The unit is housed in a rugged black anodised aluminium tube measuring 210mm long and 40mm in diameter. At one end of the tube is a miniature slide switch and exit hole for the wire antenna. At the other end, which is open, is the PC board and electret microphone insert with is covered by a foam plastic windshield, mak- 560  10k 100pF 22k 270  6.8k 0.1 220k 0.1  MIC 100k 1k .047 9V Q1 8.2k Q2 15pF 680  22pF 100pF Fig.2: install the parts on the PC board exactly as shown in this wiring diagram. ANTENNA Q3 L1 15pF 1pF 12k ing the unit quite professional in its appearance. The PC board measures only 26 x 44mm and is held inside the aluminium tube by foam plastic. Also inside the tube is a stan­ dard alkaline 9V battery and battery snap connector. Fig.1 shows the circuit which uses three NPN transistors. Transistor Q1 is an audio preamplifier which steps up the signal from the electret microphone insert. The output of Q1 is coupled via a 0.1µF capacitor and 8.2kΩ resistor to the base of Q3 which is the lower half of a cascode oscillator circuit. The cascode con­ figuration is the secret of this circuit’s excellent rejection of body capacitance effects on the operating frequency. The operating frequency is set by the parallel network comprising the 1pF capacitor and adjustable coil L1 at the base of Q3. By virtue of the cascode configuration, the components which set the operating frequency are well and truly isolated from the antenna which is connected to the collector of Q2. Building it Assembling the board is simply a matter of inserting and soldering the components into the board and this is a pretty straightforward process. The 33pF most important point to remember is to keep all the component leads to an absolute minimum length because at the operating frequency of the FM band, even short lead lengths have significant inductance and this can prejudice the circuit operation. The second point to consider is that the PC board is actu­ally double sided, with the top of the board being a ground plane. Hence some component leads will need to be soldered to the copper on both sides of the board. This means that all the com­ponent leads which connect to the 0V line in the circuit must be soldered on both side of the board. This includes the negative lead from the battery, the negative lead of the electret and the can of the adjustable coil L1. The negative lead of the electret supplied in the kit is the one connected to the case. All the resistors are soldered “endon” to save space on the tiny PC board. The length of the antenna wire is up to you. You can have it short and unobtrusive or long and thereby obtain better range. We suggest a length of about 80-90cm as the best length for overall range. Any longer and the range will be re­duced. Once all the components are soldered to the board, you are ready to test continued on page 93 PARTS LIST 1 PC board, coded FMTX, 44mm x 27mm 1 electret microphone insert 1 9V alkaline battery 1 9V battery snap 1 subminiature former with core, can and base (L1) 1 SPST miniature slide switch (S1) 1 BC549 NPN transistor (Q1) 2 BF199 NPN RF transistors (Q2,Q3) Capacitors 3 0.1µF monolithic 1 0.047µF monolithic 2 100pF ceramic 1 33pF ceramic 1 22pF ceramic 2 15pF ceramic 1 1pF ceramic (see text) Resistors (0.25W, 5%) 1 220kΩ 1 8.2kΩ 1 100kΩ 1 6.8kΩ 2 22kΩ 1 1kΩ 1 12kΩ 1 680Ω 1 10kΩ 1 270Ω Kit availability This FM wireless microphone has been produced by Oatley Electronics who own the design copyright. They can supply the kit in several parts. First, a kit including the PC board, omnidirec­ tional elec­tret microphone insert and all the board parts is $11.00. A unidirectional insert is available for $6, while the black anodised tube & windshield is $9. Postage & packing is $4. The company’s address is PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985. Keep all leads as short as possible when mounting the parts on the PC board (above). The view at right shows how the completed assembly is wedged in position in the aluminium tube using pieces of foam rubber. October 1993  67 Several other computation functions are provided, such as mean value of phase or line voltage and mean value of phase current. An integration option is available, allowing meas­urement of amp-hours or watt-hours to an accuracy of ±0.2% + 1 digit up to a period of 999 hours. A further op­tion allows frequency measurement over the range of 20Hz to 200kHz with an accuracy of ±0.1 % + 1 digit. GPIB and RS232C options are provided, allowing the 2533E to be re­motely controlled and output data to be transferred to a PC. For further information, contact Tony Richardson, Yokogawa Australia Pty Ltd, Centrecourt D3, 25-27 Paul St North, North Ryde, NSW 2113. Phone (02) 805 0699. Economy soldering irons from Scope Two new low cost 25W and 40W utility irons for electronic work have been released by Scope Laboratories. These mains voltage irons feature long-life iron-plated tips that operate at around 380°C, a stainless steel barrel, a non-rolling impact-resistant handle and four tip shapes. For further information, contact Scope Laboratories by phone on (03) 338 1566. LS621 Loudspeakers – continued from page 28 are unable to verify this claim although the response is quite smooth overall. At the bass end there is usable response down to below 45Hz although if pushed hard, the woofer does tend to frequency double. At the high end, the tweet­er is a little prominent in the region of 7- 8kHz and then tapers off a little above that although it is smooth right to the limits of audibility. Efficiency is quoted as 87.5dB at one watt and one metre and the unit is claimed to be suitable for amplifiers rated from 15 watts to 150 watts. Our impressions were that you would need an amplifier of at least 40 to 50 watts and that anything much over 100 watts on program would be too much. That is backed up by the stated maximum SPL (sound pressure level) of 106dB. On music, the Magnet LS-621s give a good account of them­selves although the tweeter seems a little muted for our tastes. We found that they sound rather better with the grille cloth frames off and we think most people would listen to them in this way. On voice, they sound very natural without any tendency to chestiness or emphasis of sibilants. Our overall impression was that they were very satisfying on classical music, especially chamber works, and they give a good account of themselves on jazz material. If you are a heavy rock fan, you will want bigger guns and it would not be fair to expect them to do the job. Recommended retail price of the Magnet LS-621s is $1150 a pair and they are available from A-One Electronics, 432-434 Kent St, Sydney, NSW 2000. They have recently fitted out a sound lounge and to introduce the Magnets they have them on sale at $950 a pair, so get in quickly. Phone A-One Electronics on (02) 267 4819. (L.D.S.) SC FM Wireless Microphone – continued from page 67 it and set the operating frequency. For this you need an FM radio. Connect the 9V battery and turn on your FM radio. Now tune across the band until the speaker squeals. The frequency on your dial is now the operating frequency of the circuit. Now if you want to adjust the frequency of operation, you reverse the process. Tune your radio to a vacant part of the band. Let’s say this frequency is 99MHz. All you should be get­ting is hiss from the loudspeaker of the radio. Now adjust the slug of coil L1 until you get a continuous squeal from the radio. That’s it, the job is complete. In more detail, the tuning range of the wireless microphone can be adjusted upwards by removing the 1pF capacitor. With this capacitor in circuit, the tuning range of L1 will be in the lower region of the FM band: from This close-up view shows how the on/ off switch is fitted to the end-plate at one end of the tube. below 88MHz to about 102MHz. With the 1pF capacitor in circuit, the tuning range will be from about 95MHz. You have to decide which portion of the band you want your cir­cuit to operate in and then pull the capacitor out or leave it in. You then adjust the slug of L1 as described above. After you have adjusted coil L1 to your satisfaction, move the microphone well away from the radio so that the acoustic feedback squeal and distortion is no longer apparent. You should now be able to speak into the microphone and your voice should come from the radio with clean reproduction. You can now complete the construction of your microphone by wiring up the on-off switch and then installing the board and battery inside the anodised aluminium tube. They are held in place by pieces of foam plastic. The PC board is positioned so that the electret protrudes slightly from the end of the tube, after which the foam plastic windshield is fitted. The slide switch is attached to an endplate with epoxy adhesive and then the end plate itself is glued into the tube with the same epoxy. SC October 1993  93