Fullscreen HTML5Med Res (??MB)HTML4

Explore the mysteries of “slope detection” with this: Simple VHF FM/AM Regenerative Receiver If you want to build an AM/FM radio it takes at least one special IC or quite a raft of discrete components, doesn’t it? Wrong. As shown in this article, you can build a simple VHF receiver quite simply, particularly if it uses “slope detection” for the FM stations. By ANDREW WOODFIELD M ANY READERS will remember building their very first radio. The projects that you built after this are probably a forgotten memory now but that first receiver, well, is often quite special. Perhaps, like many, it was a simple crystal set. In my case, I built a one-transistor 86  Silicon Chip reflex receiver, the parts bought with money earned from helping to paint a holiday beach house. Tuning in those first sounds after hours of careful solder­ing, with the help of a long-suffering ham-radio-operator uncle, was nothing short of amazing for me. Then came an endless round of careful testing and adjusting of wire antennas and the earth connections to get the best performance out of that simple re­ceiver. (I don’t recall anything that made a real difference!) It all made for a memorable summer, listening to music, news and cricket broadcasts on the AM band. These days, most youngsters prefer listening to music and DJ chatter on the VHF FM band. Of course, you can buy an IC to build a radio but that hardly falls into the ‘simple’ category despite the modest number of parts required. Also, special ICs can be hard to find and expensive. Making a simple FM radio receiver, at least at first glance, therefore appears far more appealing for someone www.siliconchip.com.au Parts List 1 PC board, code 06212021, 37 x 31mm 1 battery holder for two AAA cells 2 x 1.5V AAA cells 1 crystal high impedance earphone 1 miniature slide switch 1 BF199 or equivalent RF small signal transistor (Q1) 1 BC549 or equivalent high gain small signal transistor (Q2) 1 10µH RF choke (L2) L1: see text Fig.1: the circuit uses just two transistors. Q1 and its surrounding parts form a regenerative detector stage, with the receiver’s frequency set by tuned circuit L1 and VC1. The output from this stage is fed to audio amplifier stage Q2. just starting out in the hobby. However, designing a simple radio for FM which is truly repeatable is more of a challenge. After all, FM cannot be received on a simple crystal detec­tor, can it? (Actually, it can, but it’s a complex and challeng­ing construction project.) And there’s no point in a design that won’t go first time. This article describes a simple FM radio that’s inexpensive to build. It uses only a few more parts than a basic one-transistor AM radio or a crystal set, yet it can receive speech and music with reasonable quality from FM stations. Based around a proven super-regenerative receiver design, it’s also easy to build, and all of the parts are readily available. Finally, with a little adjustment, local VHF AM airport radio services can be received equally well. Regenerative receivers Regenerative receivers, of which this design is an example, are tuned amplifiers which are held right on the edge of oscilla­tion. Any amplifier with too much feedback will oscillate - the loud squeal and howl of an audio amplifier with too much feedback is a www.siliconchip.com.au memory we don’t enjoy! In this case, however, the tuned amplifier’s gain is allowed to rise until it just begins to start to oscillate. The difference with a regenerative receiver is that as soon as it begins to oscillate, the circuit instantly reduces the amplifier’s gain so that it drops out of oscillation again. With careful design, this type of tuned amplifier/ oscillator can be made to fluctuate continuously in and out of oscillation, rapid­ly, right at this very high gain operating point. There is considerable debate about the exact manner in which a regenerative receiver operates. Perhaps because this time-shared amplifier-oscillator action is inherently non-linear, such high-gain amplifiers readily detect amplitude modulation speech and music on received radio signals. The typical amplifi­ er/oscillator quench frequency (as this on-off switching effect is called) varies between 10kHz and 100kHz. In simple regenerative receivers, like this design, the quench frequency is not fixed precisely. It changes with compon­ent characteristics, temperature, supply voltage, as well as with external effects such as the presence of metal objects, or even as your hands Capacitors 1 47µF 16V PC electrolytic 1 4.7µF 16V PC electrolytic 1 33nF (.033µF) MKT polyester 2 22nF (.022µF) MKT polyester 1 4.7nF (.0047µF) MKT polyester or ceramic 1 6-60pF plastic trimmer capacitor 1 6.8pF 50V disc ceramic 2 1 nF 50V disc ceramic Resistors (0.25W, 1%) 1 330kΩ 1 3.3kΩ 1 22kΩ 1 2.2kΩ 1 10kΩ 1 100Ω 1 4.7kΩ Miscellaneous Hookup wire, solder, case to hold PC board, etc. get closer to the circuit. Regardless, the tuned amplifier is still kept on the edge of oscillation. The quench rate is often controlled by a resistor and ca­pacitor in simple regenerative receiver designs. Such components cannot reliably control all of the dynamics of a high-gain ampli­fier when large changes occur, of course. If the receiver is tuned over large ranges, for example, the amplifier may stop oscillating at some point, or it may begin to oscillate and never properly quench. This is one reason why many simple regenerative receivers have a ‘reaction’ or ‘feedback’ control. This allows precise adjustment of quench to keep the re­ceiver as close as possible to the optimum setting. This design avoids this problem by selecting a compromise value for the resistor-capacitor pair and by limiting December 2002  87 2.2k 1nF 10k 22nF 6.8pF 10H + 4.7F 22nF Q1 4.7k 22k 12021260 L1 VC1 33nF 330k 100 4.7nF frequency is offset from the centre frequency of the receiver’s tuned circuits. Since the receiver is rapidly turning on and off at the quench frequency, this gives rise to considerable hiss in the received audio, especially when not receiving a signal. This is a very characteristic sound in regenerative receivers. Since the quench frequency is so audible, R5 and C8 are used to reduce the level of this hiss. C6 isolates the bias voltage on the audio stage around Q2 from the signal and bias voltages around Q1. Q2’s bias is set using a very simple bias chain using two resistors; R6 and R7. This requires that Q2 be a high gain transistor but these are no more expensive than similar types and readily obtainable. A crystal earphone in used to listen to the final detected sound. This minimises loading on the circuit, increasing the sound level considerably. It also saves a further amplification stage with another transistor, as well as the cost of a speaker and matching transformer. The audio received with this arrangement is amazing. One of the prototypes produced music and sound that could be clearly heard more than a metre away from the receiver. For simplicity and to save considerable cost, there is no volume control on the receiver. We did say this receiver was simple, didn’t we? If the audio level is too loud, R6 can be reduced to 220kΩ. A further major advantage of this receiver design is its miserly battery drain. Prototypes averaged well under 1mA with a pair of AAA batteries, allowing for many hours of use. This is probably one of the most important considerations for younger builders (and parents!) keen to avoid the continual cost of battery replacement or expensive rechargeable cells. Because the receiver oscillates mo- 2 x AA CELL HOLDER 3.3k 3.3k + 47F Q2 TO CRYSTAL EARPIECE S1 OFF ON the frequency range to the FM broadcast and nearby VHF aviation bands. Circuit description The receiver has two basic sections: (1) The regenerative detector, which amplifies and detects the signal; and (2) A simple one-transistor audio amplifier. The full circuit is shown in Fig.1. Q1 and surrounding components form the regenerative detector stage. The receiver’s frequency is set by the tuned circuit L1 and VC1. Capacitor C4 provides a path for RF feedback to encourage oscillation. The quench frequency is primarily set by R4 and C5 and as oscillation begins to rise, the increasing current through R4 ensures that Q1 is eventually limited, in turn halting oscillation. As the regenerative receiver is tuned across a signal, the current through R3 varies with the modulation on the received signal. With amplitude modulated signals, such as those used by airports, the strength of the signal changes in sympathy with the audio signal. If the receiver is tuned to this Fig.2: most of the parts fit on a small PC board which can be assembled in about 10 minutes. The receiver is tuned by adjusting trimmer capacitor VC1 with a plastic alignment tool (eg, a discarded knitting needle sharpened to fit the slot). signal, this variation in received signal level is detected and converted to small variations in collector current in Q1. In turn, this small signal is amplified by Q2. The process by which this receiver detects FM is a little more complex. FM signals are generated when the audio signal changes the frequency of the transmitter rather than it’s ampli­ tude. When a very selective tuned circuit is adjusted closer and closer to the frequency of an FM signal, the amplitude of the received signal will increase. If the tuned circuit is suffi­ ciently selective, the changing frequency caused by the modula­tion on the FM signal will cause an amplitude change in the signal across the tuned circuit. This same effect was used in the earliest receivers to detect FM signals. The ‘slope’ of the tuned circuit’s selectivity allowed this change in signal amplitude with changing frequency. This was called “slope detection”. If you tune an FM signal using an AM receiver, the best sounding audio will be received when the FM centre Table 1: Resistor Colour Codes         No. 1 1 1 1 1 1 1 88  Silicon Chip Value 330kΩ 22kΩ 10kΩ 4.7kΩ 3.3kΩ 2.2kΩ 100Ω 4-Band Code (1%) orange orange yellow brown red red orange brown brown black orange brown yellow violet red brown orange orange red brown red red red brown brown black brown brown 5-Band Code (1%) orange orange black orange brown red red black red brown brown black black red brown yellow violet black brown brown orange orange black brown brown red red black brown brown brown black black black brown www.siliconchip.com.au mentarily on each quench cycle, it is possible for the ultra-low microwatt oscillator signal to be detected in nearby conventional receivers. This was a major problem with 1930’s valve versions of these receivers. These operated at many times greater power levels and the loud levels of radiated hash at the quench rate of the receiver could be heard in every nearby receiver - hardly a desirable character­istic! Modern transistor equivalents, including this design, seldom encounter the same problem. In part, this is because they operate at much lower power levels. Instead of 90 to 150V DC power supplies required for a valve, this receiver makes do with just a sniff of current from a pair of AAA cells; ie, just 3V. To further prevent this problem arising with this design, we’ve not made any provision for an external antenna. The receiv­er is highly sensitive and good reception can be achieved without any extra antenna. Also, attaching a short 1m long wire to the circuit is possible, say via a 4.7pF ceramic capacitor to the collector of Q1, it will shift the received frequency a little since it will partially load the tuned circuit, reducing the effectiveness of the detector. Construction The receiver can be built either using the PC board and housed in any convenient case. One of the prototypes was built into a small peppermint tin. (It’s actually something of a little joke. The tin was a marketing giveaway from a manufacturer of one of the most advanced digital mobile radio systems currently produced. It somehow seemed appropriate to recycle it to house one of the oldest types of analog radio circuits.) Begin construction by carefully inspecting the PC board for any unwanted short circuits between tracks or other manufacturing defects. Check for undrilled holes, too. Mount all of the resistors and capacitors first. Then make and install the two coils, L1 and L2. L1 can be made by winding four turns of enamelled copper wire around a convenient 6mm dia­meter former. A drill bit or a pencil work well. L2 is a small RF choke. If one cannot be found, you can make it by winding 30 turns of 36 gauge enamelled copper www.siliconchip.com.au Fig.3: this diagram shows the extra parts that are required in order to use low-impedance stereo headphones (eg, from a portable CD player or a Walkman). This involves adding an extra audio stage based on transistor Q3 and a small audio transformer. wire on a 1MΩ 0.25W resistor. VC1 is miniature plastic trimmer capacitor which is used to tune the receiver to your favourite station. Insert it into the PC board carefully before soldering and don’t use too much heat when soldering this into place. The plastic can melt if the trimmer gets too hot. If you wish to only receive one station or you only want to tune a small range of frequencies, you could replace VC1 with a fixed capacitor. A 22pF ceramic capacitor works to cover the aviation band and 33pF can be used for the FM band. This may require some adjustment of coil L1, depending on the actual capacitor used to precisely tune into the signal you want. You may need to add or subtract a turn or two to L1 to allow the fixed capacitor to be used. Install the two transistors next, making sure that the audio transistor (BC549) is used for Q2 and the RF transistor (BF199) for Q1. The receiver won’t work if they are reversed. Then add the wiring for the switch, the battery and the earphone. Earphone options There are several earphone options. If possible, use a high impedance crystal earphone, although they can be hard to find in some locations. Most large parts suppliers almost always stock them. An alternative is to use a piezo speaker recovered from an old toy or from one of those greeting cards that plays a tune or a few pre-recorded words. However, while these little speakers deliver lots of volume at high audio frequencies, they don’t do so well at mid-to-low audio frequen- Five identical Video and Stereo outputs plus h/phone & monitor out. S-Video & Composite versions available. Professional quality. VGS2 Graphics Splitter NEW! HC-5 hi-res Vid eo Distribution Amplifier DVS5 Video & Audio Distribution Amplifier For broadcast, audiovisual and film industries. Wide bandwidth, high output and unconditional stability with hum-cancelling circuitry, front-panel video gain and cable eq adjustments. 240V AC, 120V AC or 24V DC. High resolution 1in/2out VGA splitter. Comes with 1.5m HQ cable and 12V supply. Custom-length HQ VGA cables also available. Check our NEW website for latest prices and MONTHLY SPECIALS www.questronix.com.au Email: questav<at>questronix.com.au Video Processors, Colour Correctors, Stabilisers, TBC’s, Converters, etc. QUESTRONIX All mail: PO Box 348, Woy Woy NSW 2256 Ph (02) 4343 1970 Fax (02) 4341 2795 Visitors by appointment only December 2002  89 A small peppermint tin was used to house one of the author's early prototypes (and it wasn’t even built on a PC board). cies. However, you can add a simple horn to improve the sound quality somewhat. It can be made by cutting the top 100mm section from a plastic soft drink bottle. A hole around 8mm diameter was drilled into the cap and the piezo speaker was then hot-glued to the cap. The resulting sound is not loud but it is clearly audible from a metre or more in a quiet room. Painted, it gave A piezo speaker scrounged from an old toy or from a greeting card can be used directly with the circuit shown in Fig.1. A simple horn made by cutting the top 100mm section from a plastic soft drink bottle can be used to improve the sound quality of a piezo speaker (see text for details). 90  Silicon Chip a 1930’s look to one of the prototype receivers. By the way, don’t try to use a pair of stereo earphones from a Walkman. Their 32Ω impedance is much too low for this receiver. However, if you still would like to use these, then you’ll need to add a small amplifier stage to the receiver and the current will rise substantially. The required components and changes to the receiver are shown in Fig.3. Suitable transformers include Dick Smith or Altronics Part Number M-0216. If possible, connect the earphones so that the left and right earphones are in series. This helps increase the volume further. Fig.4 shows the earphone connections required. Fig.4: this diagram shows how the connections to a stereo headphone plug are made. Only the ring and tip terminals are used – there is no connection to the sleeve. 06212021 Suitable transistors RF transistors should be used for Q1, while audio transis­tors are suitable for Q2 and Q3, the latter being required if the stereo headphone modification is added. Suitable transistors include: Q1: BF 115, BF184, BF199, BF494, MPSH11, 2N2222, etc (ie, RF transistors with hfe>100 and fT>250 MHz). Q2: BC109, BC549, 2N3904 (ie, almost any high-gain small signal audio NPN transistor is likely to prove suitable). A variety of these transistors were used on the three pro­totypes built, all working almost identically. The main dif­ference was the current drain, with this varying between 0.6 and 1mA, depending on the RF transistor being used. Testing and operation Before proceeding further, carefully check the PC board again and the location and orientation of all parts. Check, especially carefully, the orientation of the two transistors. Are they in the correct location? Are all resistors in the correct location too? Check the underside of the PC board for poorly soldered joints or shorts caused by solder bridges where connec­ tions have been accidentally soldered too closely together. Once you’ve checked the layout again, and with so few parts, testing is as simple as connecting the battery and switch­ing on the power to the receiver. You should hear a loud hiss in the earphone. If that’s the case, adjust the trimmer capacitor until you hear an FM station. Fig.5: this is the full-size etching pattern for the PC board. Check your board for defects before installing any of the parts. If you don’t hear a signal, it’s likely that the coil you’ve wound for L1 is a little too large. The simple solution is to, firstly, turn off the power to the receiver, then spread out the coil’s turns. Spread the turns of the coil apart so that it occupies a length of, say, 12 or 15mm. Then, turn on the power again and try tuning again. If you don’t hear any hiss at all, turn off the power and recheck all your connections, especially those going to the earphone. You can check that the audio amplifier stage is working by pressing your finger lightly on the underside of the PC board with the power on (Don’t worry - The battery voltage is high enough to be dangerous) and press on the base of Q2. You should hear a low hum if it’s operating correctly. If you cannot hear anything, check the battery and the battery holder’s connections carefully. Tuning Once you have the receiver operating, the receiver can be carefully tuned into your favourite station. This should be done with a plastic or insulated adjustment tool. An old plastic knitting needle or discarded piece of plastic rod from a kitset model plane works well. This minimises any frequency shift caused by the body as SC it gets close to the circuit. www.siliconchip.com.au