Fullscreen HTML5Med Res (??MB)HTML4

An FM Radio Tuner Card for your PC Fancy an FM radio receiver inside your PC? This simple circuit plugs into a spare slot on your PC’s mother­board and is tuned using an on-screen display. Why on earth would you want to fit an FM receiver into your PC? Well, why not? If you’re the type who enjoys music while you work or while you take a break, this FM radio receiver is only a couple of mouse clicks away. It’s a mono-only design but when you’re working you’re not likely to notice the difference, espe­ cially when using low-cost multimedia loudspeakers. A PC-controlled FM receiver has several advantages. It’s convenient to use, there are no batteries to go flat and, because it fits inside the PC and is software controlled, you don’t have to worry about a case, a tuning knob or an external power supply. Unlike most other PC-controlled tuners, this circuit in­cludes its own on-board audio amplifier and this is capable of driving a set of external speakers to good volume. In other words, this circuit operates independently of the sound card. This means that the sound card can be Design by MARK ROBERTS 18  Silicon Chip FEATURES • • • • • • Full 88-108MHz FM band coverage Fully self-contained on one ISA card Computer controlled via your screen 3 preset memories for your favourite FM stations Slider volume control Does not need a sound card – fully independent operation. used for other purposes while the FM receiver is operating. The circuitry for the PC FM Tuner is The circuit is built on an ISA card which plugs into a spare slot on your PC’s motherboard. Note that several changes were made to the PC board layout after this photo was taken. memory presets (1, 2 or 3). The receiver also “remembers” the last station it was tuned to when it was turned off. built on an expansion card and plugs into a standard ISA slot on the your PC’s motherboard. A single on-board link sets the address to either 300H or 301H. Note that the card isn’t Plug and Play (PnP), so if you have other non-PnP cards in your system you may have to manually juggle the resources to suit. The screen grab on the facing page shows the display that’s used to “drive” the FM tuner. There’s really not much to it! The top half con­sists of a linear dial scale to show the tuned frequency (88-108MHz), while the bottom half carries the controls – an On/ Off button, a Mute button, a volume control slider, three memory preset buttons and a tuning knob. You tune the unit by dragging the tuning “knob” with the mouse, or you can click anywhere on the circumference of the knob to tune to that spot. Alternatively, you can tune the unit Block diagram by clicking the up and down arrows. In addition, up to three stations can be stored in memory by clicking the ‘Memory’ button (so that it displays ‘WR’) and then clicking one of the Fig.1 shows the block diagram of the PC FM Tuner. It’s built around a Philips TDA7000 FM radio IC, which is virtually a complete FM radio on a Fig.1: the block diagram of the PC FM Tuner. Most of the hard work is done by the Philips TDA7000 IC. JUNE 1999  19 lator so that the received deviation is always less than ±15kHz. In effect, the recovered audio signal is compressed to reduce its dynamic range. Although this isn’t desirable in a hifi FM tuner, the results are still quite good and this technique considerably simplifies the filtering circuitry that would otherwise be required. Basically, the technique trades dynamic range for lower audio distortion. In fact, the distortion is typically less than 2.3% at ±75kHz deviation, so your favourite FM station will still come in loud and clear. Circuit details Fig.2: inside the Philips TDA7000 FM receiver IC. This device is virtually a complete FM tuner on a single chip. Also shown are external components to make a full working FM receiver (we did just that in the November 1992 issue of SILICON CHIP). This time, though, it is teamed with other components to make the PC-based tuner. single chip. It drives an LM386 audio amplifier stage via an 8-step analog multiplexer, the latter providing the volume control function. The multiplexer is controlled by the data on the PC bus and this in turn is controlled by the software. The PC bus also controls a D/A converter stage to provide the tuning voltage to the TDA7000 chip. This tuning voltage is applied to a varicap diode. In addition, the PC bus controls a muting switch which connects to the muting circuit of the TDA7000. This allows the receiver to be muted when tuning between stations by setting the Muting button on the control panel to the ‘on’ position. The TDA7000 chip Fig.2 shows the various circuit blocks inside the TDA7000, as well as the external parts required to make a complete FM tuner. Unlike many other FM tuners, this design is easy to align since only the local oscillator (ie, the VCO) requires adjust­ment. This is done by ‘tweaking’ the coil across the VCO, so that the 20  Silicon Chip tuner covers the desired frequency range. The TDA7000 IC more or less functions as a conventional superheterodyne tuner. This means that the incoming FM signal is mixed with a local oscillator signal (from the VCO) to produce an intermediate frequency (IF). This IF signal is then filtered to remove any mixer artefacts and then demodulated to recover the desired audio signal. There’s just one deviation from normal practice. Virtually all FM broadcast receivers use an IF of 10.7MHz whereas the TDA7000 uses an IF of just 70kHz. So why does it do this? The answer is that an IF of 70kHz can be filtered using standard active op amp circuits instead of coils and ceramic filters. Normally though, a low IF results in high distortion levels when used with wideband deviation FM (broadcast band FM has a maximum deviation of ±75kHz). The TDA7000 overcomes the problem by employing a clever trick. What happens is that the recovered audio is also used to modulate the local oscil- Take a look now at the complete circuit diagram shown in Fig.3. You can easily discover the functions of the main ICs by relating them back to the block diagram. As mentioned above, the TDA7000 IC (IC5) is really the heart of this circuit. The incoming RF signal is picked up by the antenna and fed to the TDA7000’s internal mixer (pins 13 & 14) via a bandpass filter, consisting of two 27pF capacitors (C4 & C5) and inductor L1. Its job is to filter sign­als that lie outside the desired tuning range and thus eliminate interference. Varicap diode D1 and inductor L2 are used to tune IC5’s internal voltage controlled oscillator (VCO) so that the receiver covers the FM broadcast band. The tuned frequency in turn depends on the voltage applied to D1, which varies its capacitance ac­ cordingly. This tuning voltage is derived from the D/A converter (IC4). The recovered audio signal appears on pin 2 of IC5 and is fed via a lowpass filter (R5 & C8) to the top of a resistive divider network (R15-R22). The filter stage, in conjunction with the divider resistance, provides the necessary 50µs de-emphasis for the recovered audio signal. The eight steps in this divider are in turn fed to the X0-X7 inputs of IC3, the 4051 analog multiplexer. This IC is controlled by the signals on its three binary control inputs, designated A, B & C (pins 11, 10 & 9). In operation, the three binary control signals select which of the eight input channels is switched through to the output at pin 3. In essence, IC3 functions as a single-pole 8-position switch. It selects one of eight input levels and applies Fig.3: the circuit might look complex but it is based on just a few ICs and a sprinkling of other components. Best of all, it's easy to build and get going! JUNE 1999  21 Parts List 1 double-sided PC board, code 06106991 1 3.5mm mono PC-mount mono speaker socket 1 PC jumper link 1 100mm length 0.7mm enamelled copper wire 1 backplane bracket plus rightangle brackets (see text) 2 750mm length light-duty insulat-­ ed hookup wire (for antenna) 1 PC FM Tuner software utility (download Pcfmtune.zip from www.siliconchip.com.au) 1 2.2µF 16VW electrolytic (C28) 11 0.1µF MKT polyester (C2, C3, C11, C12, C17, C20, C21, C23, C25, C26, C27) 1 0.1µF ceramic (C31) 1 .0047µF ceramic (C6) 1 .0039µF ceramic (C1,C13) 1 .001µF ceramic (C8) 2 330pF ceramic (C15,C18) 1 270pF ceramic (C9) 1 220pF ceramic (C19) 2 100pF ceramic (C14, C16) 2 27pF ceramic (C4, C5) 1 4.7pF ceramic (C10) Semiconductors 1 74LS273 octal D-type flipflop (IC1) 1 LM386 audio amplifier (IC2) 1 4051 4-channel analog multiplexer (IC3) 1 MAX504 D/A converter (IC4) 1 TDA7000 FM receiver (IC5) 1 LM7805 5V regulator (IC6) 2 74LS05 hex inverters (IC7,IC8) 1 74LS32 quad 2-input OR gate (IC9) 2 BC548 NPN transistors (Q1,Q4) 2 BC327 PNP transistors (Q2,Q3) 1 BB809 varicap diode (D1) Resistors (0.25W, 1%) 4 82kΩ (R1,R2,R10,R11) 1 47kΩ (R7) 2 33kΩ (R4,R8) 1 22kΩ (R5) 2 10kΩ (R12,R15) 1 6.8kΩ (R16) 1 5.6kΩ (R17) 5 2.7kΩ (R3,R9,R14,R18,R20) 1 2kΩ (R19) 2 1.5kΩ (R6,R21) 1 1.2kΩ (R22) 1 1kΩ (R13) 1 10Ω Capacitors 1 470µF 10VW electrolytic (C24) 1 220µF 16VW electrolytic (C22) 3 10µF 16VW electrolytic (C7,C29, C30) the switched audio output to pin 3 of the LM386 audio amplifier stage (IC2). IC2 operates with an AC gain of 20 by virtue of its internal feedback components. The amplified output appears at pin 5 and is coupled to the loudspeaker via a 470µF capacitor. Control circuitry IC1 and IC4 are the main control circuits for the tuner. The data on the ISA bus is generated by the software and is applied to data inputs D0-D7 of IC1, a 74LS273 octal D-type flipflop. This device contains eight identical D-type flipflops and functions as a buffer stage for the data lines. Inverter stages IC7 & IC8, together with OR gates IC9a & IC9b, form a hardware decoder which sets the I/O address of the card. 22  Silicon Chip Note: a kit of parts for this project is available from Jaycar Electronics. The kit includes all parts including a PC board with plated-through holes, but does not include the backplane connector or the software. This decoder monitors the A0-A9 address lines of the ISA bus and, when the correct address (either 0300H or 0301H) is present, pulls pin 9 of IC7d high. This in turn switches pin 9 of OR gate IC9c low, which means that signals on the IOR (Input/Output Read) line are applied to pin 12 of IC9d. Provided that the AEN (address enable) line is low, this signal also appears on pin 11 and is used to clock IC1. In other words, the AEN and IOR lines decide when the address is accessed. Each time pin 11 of IC9d goes high, the data on the D0-D7 inputs is latched into IC1 and appears at the Q0-Q7 outputs. Outputs Q0-Q2 of IC1 are used to control the D/A converter (IC4), which in turn produces the tuning control voltage for the varicap diode (D1). IC4 is a MAX504 10-bit D/A converter. The serial data on Q0 of IC1 (as generated by the software) is fed into pin 2 (DIN), while Q2 and Q1 drive the clock (CLK) and chip select (CS-bar) inputs respectively. The analog voltage output appears at pin 12 (VOUT) of IC4 and is applied to the varicap diode via a 47kΩ resistor. The next three ‘Q’ outputs from IC1 (Q3, Q4 & Q5) are fed to the binary control inputs (pins 11, 10 & 9) of IC3. These lines switch the multiplexer to select one of eight volume levels, as described previously. Transistors Q1 and Q2 form the muting switch and are con­trolled by the Q6 output of IC1. When Q6 of IC1 is high, both transistors are on and pin 1 (Mute) of the TDA7000 is connected to the +5V rail (Vcc) via a 10kΩ resistor. This turns the muting circuit in IC5 off. Conversely, when Q6 is low, transistors Q1 & Q2 are off and the muting circuit turns on. Output Q7 of IC1 controls transistors Q4 & Q3 to provide on/off switching. When Q7 is high, Q4 turns on and provides base current for Q3. Thus, Q3 also turns on and connects the +12V line from the ISA bus to the input of 3-terminal regulator IC6. IC6 in turn provides a regulated 5V rail to power the circuit. If Q7 subsequently goes low (ie, if the on/off button on the software-generated control panel is clicked to ‘off’), Q4 and Q3 both turn off. As a result, no power is applied to the input of the regulator and so the circuit shuts down. Construction All of the parts for the FM radio (except for the loud­speaker), are fitted to a PC board coded 06106991. Fig.4 shows how the parts are fitted. The prototype was built on a double-sided board with plat­ed-through holes. If your board doesn’t have plated-through holes, it’s simply a matter of soldering all component leads on both sides of the board. You will also have to fit vias (links) to the unused holes, to connect tracks on one side of the board to their corresponding tracks on the other. But more on this in a moment. Before starting construction, inspect the board carefully to ensure that it has been correctly etched. This done, start the assembly by installing all the Capacitor Codes           Fig.4: the parts layout for the FM Tuner Card. This is a double-sided PC board – the component side is shown in grey and the underside in blue. If you don’t have a plated-through board, the points marked solely with a dot must be fitted with “pin throughs” (or vias) and you must solder the component leads on both sides of the board (see text). resistors, the capacitors and the ICs. Table 1 shows the resistor colour codes, while Table 2 shows the codes for the MKT polyester and ceramic capacitors. It’s also a good idea to check each resistor on a digital multimeter, just to make sure of its value. Note particularly that a 0.1µF ceramic capacitor is in­ stalled on the copper side of the board, directly beneath IC1. Keep all capacitor leads as short as possible and don’t forget to solder all component leads on both sides if the board doesn’t have plated-through holes (this includes the ICs). All the ICs can be directly soldered to the PC board. Take care to ensure that they are all oriented correctly and don’t get them mixed up. Next, install the varicap diode (D1), the four transistors and the 3-terminal Value IEC Code EIA Code 0.1µF 100n 104 .0047µF 4n7 472 .0039µF 3n3 392 .001µF 1n0 102 330pF 330p 331 270pF 270p 271 220pF 220p 221 100pF 100p 101 27pF 27p   27 4.7pF 4p7 4.7 regulator. Once again, take care not to get the transistors mixed up and watch their orientation. In particu­lar, note that Q3 and Q4 face in opposite directions. The regula­ tor is mounted with its leads bent at rightangles, as shown in the photo. Now for the two inductors (L1 and L2). These are both made by winding 0.7mm enamelled copper wire (ECW) onto a 4mm former (eg, a 4mm drill bit). L1 consists of six closely-spaced turns, while L2 consists of five turns, evenly spaced to form a coil 8mm long (this coil is later adjusted during the alignment procedure). After winding each coil, slide it off the drill bit, scrape away the enamel from its leads and push it all the way down onto the PC board before soldering. The 3.5mm audio socket and the backplane bracket can now be installed. You can either make up a couple of rightangle brackets to attach the backplane bracket, or you can salvage a backplane bracket with integral attaching points from an old Table 1: Resistor Colour Codes               No. 4 1 2 1 2 1 1 5 1 2 1 1 1 Value 82kΩ 47kΩ 33kΩ 22kΩ 10kΩ 6.8kΩ 5.6kΩ 2.7kΩ 2kΩ 1.5kΩ 1.2kΩ 1kΩ 10Ω 4-Band Code (1%) grey red orange brown yellow violet orange brown orange orange orange brown red red orange brown brown black orange brown blue grey red brown green blue red brown red violet red brown red black red brown brown green red brown brown red red brown brown black red brown brown black black brown 5-Band Code (1%) grey red black red brown yellow violet black red brown orange orange black red brown red red black red brown brown black black red brown blue grey black brown brown green blue black brown brown red violet black brown brown red black black brown brown brown green black brown brown brown red black brown brown brown black black brown brown brown black black gold brown JUNE 1999  23 Sorting out I/O and resource problems . . . Fig.5: click on Start>Control Panel>System>Device Manager to bring up this window, showing which devices are installed and any problems (indicated by a yellow question mark). Fig.6: if you double-click the Computer icon in Fig.5 above, then select Input/output (I/O), you’ll get a complete listing of all I/O addresses being used and the hardware that’s using them. By selecting the other buttons at the top of the window, you can also find which IRQs are being used and by what, which memory the devices are using and also which devices are using direct memory access (DMA) channels. The Reserve Resources tab allows you to allocate resources for legacy cards if necessary (ie, non-PnP cards) to avoid conflicts. 24  Silicon Chip Fig.7: from Fig.5, double click the device you're interested in, then click the Resources tab and it will tell you which interrupt request (IRQ) and I/O range is being used by that device and, most importantly, if there are any device conflicts. In this case, we’re in the clear. Fig.8: if necessary, you can manually change the resources allocated to existing cards – just select the setting you wish to change in Fig.7, then click the Change Setting button and enter in the new values . One bonus is that you get to see immediately if you have entered values which conflict with other devices. If so, change the values to something that doesn’t cause problems! Fig.9: this is the “receiver” that pops up on your screen when you load the software. At this stage it is not turned on – clicking the “power” button will do that for you. The other controls are a slide volume control, three memory preset buttons, a “rotary” tuning knob and a pair of “click and hold” tuning buttons. expansion card. A hole will have to be drilled in the bracket, to align with the audio output socket. Once all the parts have been fitted, you will notice that there are quite a few vacant holes. If you don’t have a plated-through board, what you have to do now is install “pin-throughs” at each of these hole locations. These can be made from tinned copper wire and are soldered to both sides of the PC board. Now would also be a good time to check that all component leads are soldered to their pads on the top of the PC board. Don’t neglect this step – one missed solder connection on the top of the board is enough to stop the circuit from working. Strictly speaking, you only have to solder those pads on the top of the PC board that have tracks running to them. Howev­er, by soldering all the pads, you can be sure of not missing any. Finally, connect an antenna by soldering a 750mm length of light-duty hook-up wire to the PC board. The antenna lead is then fed through the backplane connector via a small hole drilled adjacent to the antenna connection point. Software The software for this project can be downloaded from the SILICON CHIP website, www.siliconchip.com.au The download is free and the file you want is called Pcfmtune.zip (it will be at or near the end of the downloadable software listing). You’ll find it by clicking the “Software Downloads” link on the home page. If you don’t have Internet access, you can buy the software on two floppy disks from Silicon Chip Publications for $12, including postage). Unzip the file after downloading, then install the software by running setup.exe. Assuming you’re running Windows 95/98, this will install the files in a folder called ‘vhf’ and install the necessary entries in your Start menu. Installing the card You will need to set the I/O address of the card before installing it in the computer. In most cases, the default address of 300H should work just fine. This is set be installing the jumper across pins 2 & 3. If you strike problems, try the alternative 301H address setting (ie, jumper pins 1 & 2). Neither of the available addresses should cause any con­flicts with commercial expansion cards. If you do strike prob­lems, you can check the resources that are being used via the System Properties utility in Windows 95/98. To view these, double click the System icon in Control Panel, click the Device Manager tab and double-click on Computer at the top of the list of devic­es. You can now check the I/O addresses that are in use by selecting the Input/Output (I/O) button – see Fig.6. If you do find a card that occupies the 300H/301H address space, try chang­ ing the resources assigned to that card. To do this, double-click the device in the Device Manager list, click the Resources tab, click on the resource setting you wish to change (in this case, the Input/Output Range) and click the Change button – see Fig.7. Note that if the card isn’t a plug Fig.10: clicking on the little button at bottom left of the dial scale brings up this “about” box which, among other things, tells you the voltage being applied to the varicap diode to tune the station being listened to at that time. and play type, it will also often be necessary to change its configuration using the software setup disk that came with it. Having said all that, we don’t expect too many problems with resource conflicts. In nearly all cases, is should simply be a matter of plugging it in. By the way, don’t forget to connect a loudspeaker. As men­tioned at the start, the audio output from the PC FM tuner isn’t directed through the sound card, so you can’t rely on its loud­speakers. If you have a spare pair of multimedia loudspeakers, try plugging them Fig.11: this screen grab shows the contents of the vhf.ini file which records the preset channels, their volumes and the mute status. If you really wanted to, you could alter the data using a text editor and the FM Receiver would respond next time it is turned on. But why bother when the software does it all for you anyway? JUNE 1999  25 directly into the PC FM tuner’s audio output socket. If you get sound (mono) through both loudspeakers, you’re in business. If not, you will need a suitable mono-to-stereo adapter socket. A word of warning here – do not use a conventional (un­shielded) loudspeaker in close proximity to your computer’s monitor. If you do, it could magnetise the internal shadow mask and cause strange colour patches. Always use properly designed multimedia speakers if you want them on the desk. Test & alignment top Fig.12: two patterns are required for this double-sided PC board. The pattern above is for the top (component) side while the pattern below is the ‘normal’ copper side. If etching your own board for this project, great care will need to be taken to ensure that the two patterns line up correctly on the blank board. The easiest way to do this is with some form of pin registration on the board and through the film patterns. 26  Silicon Chip Now run the software (Start, Programs, FM Receiver, FM Receiver). The first thing you should see is the FM Receiver image on screen (see Fig.9) but you shouldn't hear anything yet, because you haven’t turned the “receiver” on. Move your mouse pointer to the “on” button and click it. The FM receiver “dial” now lights up and the power button illuminates green. Now you should hear some sound coming from your speaker(s). Clicking on the “mute” button should quieten inter-station noise. Click the mute back off and try tuning in some stations. If you’re within about 20-30km of some reasonably strong FM stations, you should be able to pick them up. Sweep through the entire frequency range and keep a record of the stations you hear and their locations on the ‘dial’. You will need to know which stations are on which frequencies – in many cases, FM stations broadcast their frequency as part of their callsign or station promotions. If the indicated station frequencies are higher than they should be, spread the turns on inductor L2 to decrease its induc­tance. Conversely, if the indicated frequencies are too low, push the turns closer together. Basically, it’s just a matter of adjusting L2 so that you can tune right across the FM broadcast band (from 88-108MHz) with the stations in the correct locations on the dial. Be sure to make only small adjustments to L2 at any one time before re-checking the frequency range. Make sure too that the computer is switched off each time you remove and replace the tuner card, to avoid possible damage to this card or to the SC motherboard.