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Multi-Function Active Filter Module Versatile design can be configured as a low-pass, high-pass or bandpass filter just by moving a few jumper links By JOHN CLARKE This versatile Active Filter is ideal for use as an active crossover in loudspeaker systems but has lots of other uses as well. It can be configured as a low-pass filter (for driving sub-woofer amplifiers), as a high-pass filter or as a bandpass filter, simply by moving a few on-board jumper links. A CTIVE FILTERS ARE used in many analog circuits to tailor the frequency response. For example, an active filter could be used to prevent signals below 20Hz from passing through to the next stage (eg, to an amplifier). In this case, the filter allows the higher audio frequencies to pass through but blocks the sub-audio signals (including DC). This type of filter is called a “highpass” (HP) filter. If a HP filter is incorporated into an audio amplifier, it will prevent the woofer in a loudspeaker system from being driven at very low frequencies. In fact, it could be used as a turntable rumble filter to follow a 58  Silicon Chip magnetic cartridge preamplifier. Preventing a loudspeaker from being driven at very low frequencies is important because such frequencies would cause audible distortion in the sound due to excessive cone movement. In addition, excessive cone movement at or below the loudspeaker’s resonance frequency could damage the loudspeaker. Similarly, an active filter could also be used to limit signals above 20kHz. This will prevent supersonic signals from driving the loudspeaker and protect the tweeter(s) from damage. This type of filter is called a low-pass (LP) filter; it allows frequencies below a certain frequency to pass through but blocks higher frequencies. Bandpass filter Cascading a high-pass filter and a low-pass filter produces a bandpass filter. So if a 20Hz high-pass filter and a 20kHz low-pass filter are cascaded, we end up with a bandpass ranging from 20Hz to 20kHz. This means that the signal is attenuated both below 20Hz and above 20kHz, while those frequencies between 20Hz and 20kHz are basically left un-attenuated. However, some attenuation (or re­ duction) in level does occur as the signal frequency approaches 20Hz siliconchip.com.au PASSIVE FILTERS ACTIVE FILTERS TWEETER HP PREAMPLIFIER SIGNAL MIDRANGE BP TWEETER HP (HIGH PASS) AMPLIFIER AMPLIFIER AMPLIFIER SIGNAL MIDRANGE BP (BAND PASS) AMPLIFIER WOOFER LP (USUALLY IN LOUDSPEAKER ENCLOSURE) Fig.1: a single power amplifier is usually used to drive a passive crossover network in a loudspeaker box. and 20kHz, ie, the so-called corner or “roll-off” frequencies. Additional filters can also be used to split the 20Hz-20kHz audio frequency range into separate frequency ranges or bands. This might be done to produce a 2-way or 3-way active crossover for two or three drivers in a loudspeaker system. In greater detail, many loudspeaker systems include woofer, mid-range and tweeter drivers in the same box – see Fig.1. This is called a 3-way system, while a 2-way system includes just a woofer and a tweeter. The separate drivers are used because no single driver can faithfully reproduce the whole audible range from 20Hz to 20kHz. So the audio band of frequencies is divided up and each driver is fed with its own “ideal” range of frequencies. In a 3-way system, for example, the woofer could be provided with signals ranging from 20Hz to say 150Hz, while the midrange would handle signals ranging from 150Hz to 2kHz. The tweeter would then cover the remainder of the audio range, ie, from 2-20kHz. Passive crossovers In most loudspeaker systems, the incoming audio signal is divided into separate frequency bands using passive filters. These “crossover filters” are located inside the loudspeaker box itself and are made up using inductors, capacitors and resistors. Basically, a well-designed crossover network gives outputs to match the particular drivers used. This ensures that each driver (ie, woofer, mid-range and tweeter) is fed only with a frequency band it can effectively reproduce. siliconchip.com.au WOOFER LP (LOW PASS) Fig.2: the arrangement for an active crossover filter system. The filters go before the power amplifiers and a separate amplifier is required for each loudspeaker driver. In addition, the design must cater for drivers that have different sensitivities and set the signal levels to achieve an overall flat frequency response. For example, the woofer is often less sensitive than the midrange driver and tweeter and so the signals to the latter drivers must be reduced so that the output levels from the three drivers are well matched. This does waste amplifier power, however. Another problem to contend with is non-linearity in the driver impedances and so extra components are often used in the crossover network to correct this, so that the filter appears to drive a purely resistive load. As a result, the crossover networks in highperformance speaker systems are often complex and can be difficult to design and optimise. They also interpose a complex RLC network between the amplifier and the speakers which can mean a loss of damping factor. That particularly affects the lower frequencies where a high damping factor is most needed to achieve tight, clean bass and midrange reproduction. As shown in Fig.1, a single power amplifier usually drives the passive crossover network in a loudspeaker system. However, some loudspeaker systems provide additional connections so that each driver can either be driven independently by its own amplifier (via its passive filter) or by a single amplifier but with separate wiring to each passive filter section. Active crossovers Active crossovers are an alternative to passive filtering. However, for this to work, a separate amplifier is required for each driver – see Fig.2. For a stereo system, that means six power amplifiers (or three stereo amplifiers) to drive 3-way loudspeakers or four amplifiers for 2-way loudspeakers. As shown in Fig.2, the crossover filtering is now placed ahead of each amplifier to set the frequency band Specifications Voltage Gain: adjustable from 0-2; typically set at 1 Frequency Response: filter dependent Filter Attenuation slope: 24dB/octave or 80dB/decade Total Harmonic Distortion: typically .003% at 1V RMS Signal-to-Noise Ratio: >100dB with respect to 1V input and 22Hz to 22kHz unweighted Input Impedance: 47kΩ Supply Voltage: ±15V to ± 60V DC dual rail supply or +12-30V DC single rail supply or 11-43VAC Current Consumption: 40mA maximum July 2009  59 FILTERS SELECTION MATRIX INPUT BUFFER INPUT HPin HP IC2b LPout IN x1 HP IC2a LEVEL OUT HPout IC1a OUTPUT AMPLIFIER x2 LPin FILTERS LP IC3a VR1 OUTPUT IC1b LP IC3b Fig.3: block diagram of the Multi-Function Active Filter. The low-pass and high-pass filter stages each consist of two cascaded op amps and the unit is configured by installing jumper links on the pins of the “selection matrix”. applied to its driver. There are two advantages to this scheme: (1) better control of the driver and (2) the inductive load presented by the driver does not affect the filter response (as it does in a passive system). So our Multi-Function Active Filter module is designed to be used ahead of each amplifier. Basically, you need to build and configure one module for each driver (and amplifier) in the system. For a woofer, the module would be configured as a low-pass (LP) filter, while a bandpass (BP) filter would be used ahead of the mid-range amplifier. The tweeter driver amplifier would have a high-pass (HP) filter ahead of it. Supply options In operation, the Multi-Function Active Filter would typically be powered from the supply rails of the amplifier. Options are available to power the module from supply rails ranging from ±60V down to ±15V or from an 11-43V AC source. AMPLITUDE CUTOFF ROLLOFF SLOPE TRANSITION BAND A HIGH PASS (HP) Block diagram Fig.3 shows the block diagram of the Multi-Function Active Filter (minus the power supply). It uses an input buffer stage (IC1a), four op amps to form the filter stages (IC2a,b & IC3a,b) and an output amplifier stage (IC1b) IC1a is configured with a gain of one and can be connected to drive either the HP or LP filter stages, depending on the jumper options on the PASS BAND FREQUENCY Rolloff slope Note that the signal is not fully attenuated at the cutoff points but instead gradually decreases at a rate determined by the rolloff slope. In this case, each 2-pole filter stage has a rolloff of 40dB per decade or 12dB per octave. However, because the filter stages are cascaded, this rolloff increases to 80dB per decade or 24dB per octave and the signal level is actually 6dB down at the cutoff (crossover) points. For a high-pass filter, the output from IC2b is fed through to level con- HIGH PASS CUTOFF CUTOFF PASS BAND STOP BAND The Multi-Function Active Filter can also be powered from a single supply rail, such as +25V, +15V or +12V. The 12V option enables it to be used in cars. On-board jumper links are used to configure the module for LP, BP or HP operation. The roll-off frequencies are set by selecting the appropriate resistor and capacitor values in the filter feedback networks. These filter component calculations are made easy by using freely available software from the Internet. “Selection Matrix” block. If we want a HP filter, then terminal “IN” is connected to “HPin” on the matrix block. Alternatively, for an LP filter, “IN” is connected to terminal “LPin”. As shown, the high-pass filter uses two 2-pole HP filters based on IC2a & IC2b. These are connected in series (or “cascaded”). Similarly, the low-pass filter stage consists of 2-pole LP filters IC3a & IC3b. Fig.4a shows the response for a HP filter and the way the filter response is described. As indicated, the region where frequencies pass through unattenuated is called the passband. Below the cutoff frequency, the response begins to rolloff (or is reduced) in level. This rolloff region is called the stopband. An LP filter is similar except that it allows low-frequency signals to pass through and blocks signals above the cutoff point (Fig.4b). Finally, the bandpass filter rolls off both the low and high-frequency signals and the pass band is between the high-pass and low-pass cutoff frequencies (Fig.4c). ROLLOFF SLOPE ROLLOFF SLOPE LOW PASS CUTOFF PASS BAND ROLLOFF SLOPE STOP BAND TRANSITION BAND B LOW PASS (LP) C BAND PASS (BP) Fig.4: the high-pass filter (A), low-pass filter (B) and bandpass filter (C) response characteristics. Because the op amp filter stages are cascaded, the rolloff slope in each case is 24dB per octave and the signal is actually 6dB down at the cutoff (crossover) points. 60  Silicon Chip siliconchip.com.au Parts List Amplifiers For Active Crossover Systems T HE AUDIO AMPLIFIER requirements for active crossover loudspeaker systems depend on the power hand­ ling rating for each loudspeaker. Typically, a woofer (or subwoofer) amplifier should have twice the power of the midrange and treble amplifiers. For example, a 100W power amplifier could be used for the woofer, and 50W amplifiers used for the midrange and treble drivers. One problem is that the output from a preamplifier will only have a single RCA output for each left and right channel. However, you will need to connect the preamp signal to two or three active filters, depending on how many drivers are in the loudspeaker. This problem is easily overcome by using an RCA Plug to 2 x RCA Socket such as the Jaycar Cat. PA-3560. Two such adaptors will be required for each channel if you want to drive three active filter modules (ie, if you have a 3-way loudspeaker system). Alternatively, you could use RCA plug-to-plug leads with piggyback RCA sockets (eg, Jaycar WA-7090/1/2/3 or Altronics P-7260) or you could make up your own 2-way or 3-way RCA socket panels. trol VR1 by connecting point “HPout” to “OUT” in the selection matrix. Alternatively, for a low-pass filter, the output of IC3b at “LPout” is connected to the “OUT” terminal. Bandpass filter connections Bandpass filtering is achieved by cascading the high-pass and low-pass filter stages, ie, by connecting the output of the high-pass stages to the input of the low-pass stages or vice versa. However, it is normal to feed the signal to a HP filter first and then use this to drive the LP filter, rather than placing the LP filter first. This will result in less noise due to the final low-pass filtering. However, you can connect the LP filters first if that’s what you want to do. Normally, to configure a bandpass filter, the signal is first fed to HP filter stage IC2a by linking “IN” to “HPin”. The output from IC2b is then fed to the input of low-pass stage IC3a by connecting “HPout” to “LPin” in the Selection Matrix. The resulting bandsiliconchip.com.au pass filtered signal at the output of IC3b is then fed to VR1 by connecting “LPout” to “OUT”. Level control The signal on VR1’s wiper is fed to IC1b. This is configured as a noninverting amplifier with a gain of two. As a result, VR1 can be adjusted to vary the signal at its output between zero and x2. This level adjustment allow the sound levels from the woofer, midrange and tweeter drivers to be adjusted when multiple filter modules are used. By the way, the recommended design for each 2-pole stage is for a Butterworth response. When connected in series, the result of cascading two Butterworth filters is a Linkwitz-Riley (L-R) response. This is ideal because at the crossover region, where one filter takes over from another, the overall L-R frequency response is flat. Note that the HP and LP filters must be set for same crossover frequency for this to happen. 1 UB3 plastic utility case 130 x 68 x 44mm (optional) 1 PC board, code 01107091, 123 x 63mm 1 3-way PC-mount screw terminal block with 5.08mm pin spacing (CON1) 4 DIP8 IC sockets 1 3-way DIL pin header with 2.54mm pin spacings 2 3-way SIL pin header with 2.54mm pin spacings 5 jumper plugs to suit pin headers 1 100mm length of 0.8mm tinned copper wire or four 0Ω links 4 PC stakes Semiconductors 3 LM833 dual op amps (IC1-IC3) 1 TL071, LF351 single op amp (IC4) 2 1N4744 15V 1W zener diodes (ZD1,ZD2) 2 1N4004 1A 400V diodes (D1,D2) Capacitors 2 470µF 16V PC electrolytic 1 100µF 16V PC electrolytic 2 4.7µF non-polarised (NP) electrolytic 2 100nF MKT polyester 1 10nF MKT polyester 1 220pF ceramic C1,C2,C3 to suit application (use MKT polyester) (see text & tables) Resistors (0.25W, 1%) 1 47kΩ 2 150Ω 4 10kΩ 3 10Ω Ra, Rb, R1, R2 & R3 to suit power supply & filter type (use 1% 0.25W for R1, R2 & R3) (see text & tables) As indicated previously, the MultiFunction Active Filter board can only produce a single LP, HP or BP filter output. This means that it can only provide signal to one loudspeaker driver – it is not designed to provide for two (or more) outputs. This in turn means that if you want separate LP, BP and HP filter outputs, then three Multi-Function Active Filter modules must be built (or six for a stereo system). Basically, a different filter is required for each amplifier and it can be installed inside its associated amplifier’s case. July 2009  61 Fig.5: this screen grab shows the frequency response for the low-pass filter configuration with a nominal corner frequency of 1kHz. The attenuation slope is 24dB per octave. arrangement. This was used in preference to the unity gain Sallen-Key style of filter because the MFB response is less affected by component value variations due to manufacturing tolerances. Note that 10Ω stopper resistors are included in series with the HP filter inputs. This is done in each case to prevent instability (oscillation) in the preceding stage. IC2a’s output is fed to the second HP filter stage IC2b (ie, the stages are cascaded), while IC3a drives the second LP filter stage IC3b. For a HP filter, IC2b’s output is fed to level potentiometer VR1 by linking “HPout” to “OUT” in the Selection Matrix. Alternatively, for a LP filter, the output from IC3b is connected to level potentiometer VR1 using a jumper to link “LPout” to “OUT”. Again, this functions exactly as described for block diagram Fig.3. Finally, for a bandpass arrangement, HP filter IC2b’s output is fed to LP filter IC3a via a jumper link between “HPout” and “LPin”. IC3b’s output is then fed to VR1 level via a jumper link between “LPout” and “Out”. Minimising noise Fig.6: the frequency response for a high-pass filter configuration with a nominal corner frequency of 1kHz. Once again, the attenuation slope is 24dB per octave. The inputs of the various active filter modules are then all driven in parallel by the preamplifier. Circuit details OK, let’s now take a look at the full circuit details – see Fig.7. It comprises three dual op amps (IC1-IC3) plus a single op amp (IC4) in the power supply section. The first thing to note here is that the designations for the op amps used in the input buffer, filter and output stages match those shown on the block diagram of Fig.3. So if you’ve followed the description for Fig.3, understanding how the full circuit works should be a snack. As shown, the incoming audio signal is applied to unity gain buffer 62  Silicon Chip stage IC1a via a 4.7µF non-polarised capacitor and a 10Ω stopper resistor. The capacitor is there to block any DC voltage, while the stopper resistor blocks any stray RF signals that may have been picked up by the leads. IC1a is biased to Earth 2 via the associated 47kΩ resistor. This earth is at 0V for plus and minus supply rails and at half-supply (0.5Vcc) for a single supply. IC1a’s output is fed to either HP filter IC2a or to LP filter IC3a, depending on the input jumper location in the Selection Matrix. This works exactly as indicated previously in the description for the block diagram (Fig.3). Both the high-pass and low-pass filter stages (IC2a, IC2b, IC3a & IC3b) use a multiple feedback (MFB) 2-pole As stated earlier, the signal from IC1a is normally fed to the HP filter stages first (“IN” linked to “HPin”), so that the LP filter stages can then minimise noise. Alternatively, the LP stages can be placed first by linking “IN” to “LPin”, “LPout” to “HPin” and “HPout” to “OUT”. The resulting audio signal on VR1’s wiper is fed directly to the non-inverting input (pin 5) of IC1b. As previously stated, this amplifier has a gain of 2 but this gain reduces to 1 for frequencies above 72kHz due to the 220pF capacitor across the feedback resistor. IC1b’s output appears at pin 7 and is coupled to the output terminals via a 150Ω isolating resistor and a 4.7µF NP (non-polarised) capacitor and 150Ω isolating resistor. Power Supply In operation, the Multi-Function Active Filter would typically be powered from the supply rails of the amplifier. As stated previously, options are available to power the module from dual DC supply rails or from an AC source. The unit can also be powered from a single supply rail, such as +25V, +15V or +12V. The 12V option enables it to be used in a car. siliconchip.com.au siliconchip.com.au July 2009  63 47k 10 K 2 3 A V– 4 IC1a 8 ZD2 15V 1W INPUT BUFFER K D2 1N4004 A D1 1N4004 1 100nF K A LK1 10 1 2 LPin IN R1c HPout OUT SELECTION MATRIX HPin LPout 470 F 16V R3c C2c R2a C1a V– C3a ZD1 15V 1W MULTI-FUNCTION ACTIVE FILTER 4.7 F NP Rb Ra 3 2 3 2 R2c A K R1a LP FILTER IC3a C1c HP FILTER IC2a C2a 470 F 16V 1 1 10k 10k 10 R1d V– 2 C2d R3d R2b C1b 100 F 16V 3 A 8 4 8 7 HP FILTER 4 IC2b C1d IC3b 5 6 R1b 150 C2b 6 LP FILTER K D1, D2 5 6 R2d C3b 4 IC4 7 7 V– 100nF V+ A 6 5 10k K ZD1, ZD2 10k LEVEL VR1 EARTH 1 10nF EARTH 2 1 LK2 2 OUTPUT AMPLIFIER 10k 220pF IC1b V– V+ 7 8 4.7 F NP 1 IC1 – IC4 150 IC1 – IC3: LM833 IC4: TL071 Fig.7: the complete circuit for the Multi-Function Active Filter. IC1a serves as an input buffer stage while op amp IC1b is the output amplifier. Cascaded op amp stages IC2a & IC2b together form the high-pass filter, while IC3a & IC3b make up the low pass filter. IC4 is used to provide a half-supply reference if the unit is powered from a single-rail power supply. 2009 SC  INPUT – 0V + SUPPLY INPUT 4 OUTPUT ± SUPPLIES: LK1=1, LK2 =1 19070110 Rb D2 4004 HPout C2c C1c 1 100nF Table 1: Capacitor Codes Value µF Value IEC Code 100nF 0.1µF 100n 10nF 0.01µF   10n 220pF NA 220p EIA Code    104    103    221 150 C1d C2d Fig.8: follow this parts layout diagram to build the PC board. The various tables show the values for resistors Ra & Rb and for the filter components (R1-R3 & C1-C2), while the linking options for the selection matrix are shown at right. Links LK1 & LK2 go in position 1 for a dual-rail supply (or for an AC supply) but must be moved to position 2 for a single-rail supply. In summary, the three options for powering the module are as follows: (1) A dual-rail (plus & minus) supply of between ±15V and ±60V (this connects to the “+” and “-’ supply inputs of the terminal block); (2) A single DC supply rail ranging from 12-60V (this connects between the “+” and “0V” supply inputs); and (3) An AC supply ranging from 1243VAC (in this case, the “+” and “-” inputs are tied together and the AC supply is connected between these IN 220pF 10k EVIT CA LK1 10k 1 R1d 15V CON1 LPin VR1 GND 10k R1c 2 1 OUT NI HPin IN LPin TU O 4.7 F NP SIGNAL OUTPUT LPout OUT HPout LOW PASS FILTER commoned inputs and the 0V input). In the case of a dual supply, diodes D1 and D2 (1N4004) protect the circuit against reverse polarity connection. Zener diodes ZD1 and ZD2 then regulate the supply to provide ±15V rails which are then used to power op amps IC1-IC3. Two two 470µF capacitors decouple the ±15V supply rails. Resistors Ra & Rb are used to limit the current into ZD1 and ZD2. The values of these two resistors depend on the input voltage (see Table 4 for the required values). In addition, for a dual supply, Earth 1 and Earth 2 are connected together by installing jumper link LK2 in position 1 (LK1 must also be in position 1 or left out). With no signal, this sets op amps IC1, IC2 & IC3 so that their outputs sit at 0V. For a single supply, ICs1-3 need to GND OUT 2 IN R2d 470 F 1 100nF 47k HPin LPout LEVEL R3d 100 F 10nF IC3 LM833 10k ZD2 LK2 –– 150 V0 – 10k 1 IC4 TL072 SUPPLY 0V INPUT 470 F R2c + R3c + C3b C2b IC1 LM833 C2a C3a 15V SIGNAL INPUT 4.7 F NP 10 1 IC2 LM833 RETLIF R1a R2a 10 Ra ZD1 C1b 10 C1a R2b 4004 R1b D1 SINGLE SUPPLY: LK1=2, LK2 = 2 HPin IN LPin LPout OUT HPout HIGH PASS FILTER HPin IN LPin LPout OUT HPout BANDPASS FILTER be biased at half-supply so that the signal can swing symmetrically without clipping. This half-supply rail is provided by op amp IC4. As shown, a half-supply voltage is derived using two 10kΩ resistors in series across the positive supply rail. This is decoupled by a 100µF capacitor and then buffered by IC4 to drive Earth 2 when LK2 is in the “2” position. In addition, for a single supply, the negative supply pins for ICs1-3 are connected to the 0V supply rail by placing link LK1 in position 2. Note that when LK2 is in position 2, the half-supply output from IC4 is bypassed to earth (0V) via a 10nF capacitor. This prevents oscillation in the filter op amps. The 150Ω resistor at pin 6 of IC4 isolates the op amp’s output from the capacitance in the shielded output leads. Table 2: Resistor Colour Codes o o o o o o o o o o o No.   1   1   1   1   4   2   2   2   2   3 64  Silicon Chip Value 47kΩ 15kΩ 13kΩ 12kΩ 10kΩ 6.2kΩ 5.6kΩ 4.7kΩ 150Ω 10Ω 4-Band Code (1%) yellow violet orange brown brown green orange brown brown orange orange brown brown red orange brown brown black orange brown blue red red brown green blue red brown yellow violet red brown brown green brown brown brown black black brown 5-Band Code (1%) yellow violet black red brown brown green black red brown brown orange black red brown brown red black red brown brown black black red brown blue red black brown brown green blue black brown brown yellow violet black brown brown brown green black black brown brown black black gold brown siliconchip.com.au into RF? There’s something to suit every radio frequency fan in the SILICON CHIP reference bookshop RF Circuit Design – by Chris Bowick A new edition of this classic RF design text - tells how to design and integrate RF components into virtually any circuitry. $ 75 Practical RF H’book – by Ian Hickman A reference work for technicians, engineers, students and the more specialised enthusiast. Covers all the key topics in RF that you $ need to understand 90 Table 3: Filter Type Configuration Low-Pass Filter Link IN to LPin; Link LPout to OUT High-Pass Filter Link IN to HPin; Link HPout to OUT Bandpass Filter Link IN to HPin; Link HPout to LPin; Link LPout to OUT Table 4: Power Supply Configuration Input Voltage ±60VDC, 43VAC ±55VDC, 40VAC ±50VDC, 35VAC ±45VDC, 30VAC ±40VDC, 28VAC ±35VDC, 25VAC ±30VDC, 20VAC ±25VDC, 18VAC ±20VDC, 15VAC ±15VDC, 11VAC +30V +25V +20V +15V +12V Ra 1.2kΩ 5W 1kΩ 5W 820Ω 5W 680Ω 5W 560Ω 5W 470Ω 5W 390Ω 5W 270Ω 5W 120Ω 1W 10Ω 0.5W 390Ω 5W 270Ω 5W 120Ω 1W 10Ω 1/2W 10Ω 1/2W Rb 1.2kΩ 5W 1kΩ 5W 820Ω 5W 680Ω 5W 560Ω 5W 470Ω 5W 390Ω 5W 270Ω 5W 120Ω 1W 10Ω 0.5W NA NA NA NA NA Finally, for an AC supply, D1 & D2 function as half-wave rectifiers to derive positive and negative supply rails. The circuit then functions exactly the same as for a dual-rail DC supply. Construction All parts for the Multi-Function Active Filter are mounted on a PC board coded 01107091 and measuring 123 x 63mm. This can either be housed inside a UB3 plastic utility case measuring 130 x 68 x 44mm or installed within an amplifier case. siliconchip.com.au Links LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 1, LK2 position 1 LK1 position 2, LK2 position 2 LK1 position 2, LK2 position 2 LK1 position 2, LK2 position 2 LK1 position 2, LK2 position 2 LK1 position 2, LK2 position 2 Note that corner cutouts will be required if mounting the board in a utility case, to clear the integral mounting posts. Fig.8 shows the parts layout on the PC board. However, before starting the assembly, you have to decide on the power supply to be used, the type of filter arrangement and the cutoff frequency. Table 4 shows the resistors (Ra & Rb) required for various power supply voltages, plus the LK1 & LK2 linking options. The filter component values Practical Guide To Satellite TV – by Garry Cratt The reference written by an Aussie for Aussie conditions.Everything you need to know. $ 49 You’ll find many more technical titles in the SILICON CHIP reference bookshop – see elsewhere in this issue into ? S O R C I M There’s something to suit every microcontroller maestro in the SILICON CHIP reference bookshop Microcontroller LNEW W Projects in C wPRO IC as $ E! – by Dogan Ibrahim 81 Graded projects introduce microelectronics, the 8051 and programming in C. $ 60 Hands-On Zigbee – by Fred Eady An in-depth look at the clever little chip that’s starting to be found in a wide range of equipment from consumer to 50 $ industrial. 96 Programming 16-Bit Microcontrollers in C – by Luci Di Jasio Learning to fly the PIC24. Includes a CD ROM with source code in C, Microchip C30 complier $ and MPLAB SIM. 90 You’ll find many more technical titles in the SILICON CHIP reference bookshop – see elsewhere in this issue July 2009  65 Using The FilterPro Software From TI Fig.9: this is how FilterPro should look when set up to calculate values for a low-pass 2-pole Butterworth filter. The first step here is to download the 2.848MB zipped file (available from http: //focus.ti.com/docs/toolsw/folders/print/ filterpro.html) and run the FilterPro­Setup. exe file. That done, navigate to C:\ProgramFiles\Ti Analog Design Centre\Filterpro and create a shortcut on your desktop for FilterPro.exe. When you launch FilterPro, the program will show a screen with a graph, the filter circuit and various settings (see Fig.9). The graph shows the frequency response of the filter using an amplitude versus frequency plot. The actual rolloff can be seen as well as any excursions in the response across the passband or at the cutoff frequency. Calculating The Filter Component Values C HOOSING THE CROSSOVER FREQUENCIES for loudspeaker drivers requires careful consideration.You will need the data sheet for each driver in order to make a decision as to where the crossover frequency should lie. Ideally, the crossover frequency should be well away from the driver’s resonance frequency and the adjacent drivers should be a good match to ensure a smooth frequency response across the audio band. Many books have been written on the subject and a good reference is “The Loudspeaker Speaker Design Cookbook” by Vance Dickason. This is available from Jaycar, Cat. BA-1400. Once you have decided on the crossover frequencies, the filter component values can be calculated. Tables 5 and 6 show the recommended values for a range of common frequencies. For other frequencies, you can download software off the net to make the calculations easier. Our recommendation is to use “Filter Pro” from Texas Instruments. You can download it from http://focus.ti.com/docs/toolsw/ folders/print/filterpro.html If this site becomes unavailable, do a search for “Ti filter software” or for “FilterPro”. Information on how to use FilterPro and other useful information on filters is available at http://focus.ti.com/lit/an/sbfa001a/sbfa001a.pdf An alternative on-line program is also available from Okawa Electric – see the section entitled “Using the FilterPro Software From TI”. 66  Silicon Chip Two other responses are also shown on the graph: the phase response and the group delay. The phase response plots the phase variations in the filter output as a function of frequency. By contrast, the group delay shows the slope (or rate of change) in the phase response and is ideal for displaying the filter response to a pulse signal. Several different filter types can also be selected – ie, Bessel, Butterworth and Chebychev. Each has a different “Q” value and so the filter response differs from one to the other. Each filter type has its own advantages and disadvantages. For example, a Bessel filter has a Q of 0.577 (1/√3) and has a smooth but drooping amplitude response across the passband. It has very little pulse response overshoot and its rolloff is not as steep as for a Butterworth filter. Butterworth filters have a “Q” of 0.7071 (1/√2) and have the flattest possible (max­ imally flat) amplitude response in the passband and a moderate pulse response rise (or overshoot) at the cutoff frequency. A Chebychev filter has a higher Q again. This filter has ripple in the passband, a steeper cutoff rate and higher pulse response overshoot compared to the two lower Q filters. The Q value depends on the amount of ripple that can be tolerated and is 0.956 for a 1dB passband ripple and 0.863 for a 0.5dB passband ripple. A filter with a “Q” of 0.5 is critically damped and shows no pulse response overshoot. The Bessel, Butterworth and Chebychev filters are all under-damped and so each show some degree of overshoot in its response. An over-damped filter would have a “Q” of less than 0.5. Butterworth filters For audio work, the best compromise filter type is the Butterworth, especially when two filters are cascaded as in our Multi-Function Active Filter. So in FilterPro, select “Butter- are selected from Tables 5 & 6 (see also the panel titled “Calculating The Filter Component Values”). Note that for the single supply option, Rb, D2, ZD2 & C5 can be omitted. However, it does not matter if they are installed. Alternatively, for a dual rail supply option, IC4, R4, R5 & C6 are not required. Note also that either 5W or 0.5W resistors can be used for Ra & Rb, as the PC board accepts both types. For a LP filter only, there is no need to install the HP components. These siliconchip.com.au FilterPro provides values for the resistors and capacitors using R1, R2 & R3 and C1, C2 & C3 component designations. These are easily equated with the component designations on the circuit diagram (Fig.7) and parts layout diagram (Fig.7). Note: the a, b, c & d designations on Fig.7 are there simply to distinguish one filter circuit from another. Bandpass filter A bandpass filter is made by designing two separate cascaded HP and LP circuits. For example, if you want a bandpass filter with rolloffs at 500Hz and 2kHz, you simply use FilterPro to design independent 500Hz high-pass and 2kHz low-pass stages. Do not select a bandpass design in FilterPro – the calculations are not applicable to the Multi-Function Active Filter module described here. Alternative software worth” as the filter type and select “2” for the number of poles. The circuit type should be set to “MFB single ended” and the set display value should be “component values”. For components, select “E24” series for the resistors and either “E6” or “E12” for the capacitors (these “E” series values select the number of values available in a decade range). The relevant resistor and capacitor values will then be calculated based on readily available components. Note: some component suppliers may not have the full E12 capacitor series. In that case, a recalculation may have to be made using the E6 series instead if using the E12 series gives components values that are unavailable. The next step is to enter the cutoff frequency, select either LP or HP and then click on an unused section of the screen to start calculating the values. Note that the circuit for the multiple feedback 2-pole filter shows the values for a single 2-pole filter section. These same values are also used in the second 2-pole filter stage of the Multi-Function Active Filter. If you want to use an alternative program to FilterPro or if you want to check the predicted response of your filter using the values given by FilterPro, a good on-line program is one from Okawa Electric. For the low-pass filter, go to http://sim.okawa-denshi.jp/en/ OPtazyuLowkeisan.htm For the high-pass filter navigate to http://sim.okawa-denshi. jp/en/OPtazyuHikeisan.htm These sites not only allow you to calculate filter components but also allow you to input component values. The program will then show the actual cutoff frequency, filter Q and other features. These calculations can sometimes give a better result (ie, closer to the required Q and cutoff frequency) than FilterPro. Note, however, that the R1, R2, R3, C1, C2 & C3 labelling is a little different to that of the FilterPro and our circuit, so make sure you transpose the labelling correctly. Also, do not forget to tick the Q value field at 0.707 rather than using the ticked damping ratio field of 1 for the calculation. include IC2, R1a, R2a, C1a, C2a, C3a, R1b, R2b, C1b, C2b & C3b. The two 10Ω stopper resistors can also be left out (but not the one on pin 3 of IC1a). Similarly, for a HP filter, you can leave out LP components IC3, R1c, R2c, R3c, C1c, C2c, R1d, R2d, R3d, C1d & C2d. Start the assembly by carefully inspecting the board for any defects, then install the four wire links. Alternatively, 0Ω resistors can be used instead of the wire links. These look similar to a 0.25W resistor but have just one single black band around the centre of the body. Next, install four PC stakes at the input and output positions, then install the resistors and trimpot VR1. Table 2 shows the resistor colour codes but a digital multimeter should also be used to check values, just to make sure. Follow these with the diodes, zener diodes and the ICs. These parts must all be installed with the correct orientation. Note that IC4 is a different type to IC1, IC2 & IC3, so don’t get it mixed up. We used IC sockets for the ICs and these sockets also have an orientation notch at one end – see Fig.8. The electrolytic capacitors are next on the list and these must also be oriented correctly. The only exceptions here are the two 4.7µF NP (nonpolarised) types which can go in either way around. Once these parts are in, install the two 3-way SIL (Single In-Line) headers for links LK1 & LK2. The two jumpers Fig.10: the low-pass filter design software from Okawa Electric shows the circuit values and filter responses in a similar way to FilterPro. A high-pass filter design tool is also available from Okawa Electric – see text. siliconchip.com.au July 2009  67 Table 5: High-Pass Filter Component Values (Butterworth Response) Frequency C1 (IEC Code) (EIA Code) C2 (IEC Code) (EIA Code) C3 (IEC Code) (EIA Code) R1 R2 50Hz 100Hz 120Hz 150Hz 200Hz 300Hz 500Hz 1kHz 1.5kHz 2kHz 3kHz 5kHz 10kHz 20kHz 330nF (334) 150nF (154) 150nF (154) 100nF (104) 68nF (683) 47nF (473) 33nF (333) 15nF (153) 10nF (103) 6.8nF (6n8) (682) 6.8nF (6n8) (682) 3.3nF (3n3) (332) 1.5nF (1n5) (152) 680pF (681) 330nF (334) 150nF (154) 150nF (154) 100nF (104) 68nF (683) 47nF (473) 33nF (333) 15nF (153) 10nF (103) 6.8nF (6n8) (682) 6.8nF (6n8) (682) 3.3nF (3n3) (332) 1.5nF (1n5) (152) 680pF (681) 330nF (334) 150nF (154) 100nF (104) 100nF (104) 100nF (104) 68nF (683) 33nF (333) 15nF (153) 10nF (103) 10nF (103) 6.8nF (6n8) (682) 3.3nF (3n3) (332) 1.5nF (1n5) (152) 1nF (102) 20kΩ 22kΩ 24kΩ 22kΩ 20kΩ 20kΩ 20kΩ 22kΩ 22kΩ 20kΩ 20kΩ 20kΩ 22kΩ 20kΩ 4.3kΩ 5.1kΩ 4.7kΩ 5.1kΩ 4.7kΩ 4.7kΩ 4.3kΩ 5.1kΩ 5.1kΩ 4.7kΩ 4.7kΩ 4.3kΩ 5.1kΩ 4.7kΩ Table 6: Low-Pass Filter Component Values (Butterworth Response) Frequency R1 R2 R3 50Hz 100Hz 120Hz 150Hz 200Hz 300Hz 500Hz 1kHz 1.5kHz 2kHz 3kHz 5kHz 10kHz 20kHz 5.6kΩ 5.6kΩ 4.7kΩ 5.6kΩ 6.2kΩ 6.2kΩ 5.6kΩ 5.6kΩ 5.6kΩ 6.2kΩ 6.2kΩ 5.6kΩ 5.6kΩ 6.2kΩ 5.6kΩ 5.6kΩ 4.7kΩ 5.6kΩ 6.2kΩ 6.2kΩ 5.6kΩ 5.6kΩ 5.6kΩ 6.2kΩ 6.2kΩ 5.6kΩ 5.6kΩ 6.2kΩ 12kΩ 15kΩ 12kΩ 13kΩ 15kΩ 13kΩ 12kΩ 15kΩ 13kΩ 15kΩ 13kΩ 12kΩ 15kΩ 15kΩ C1 (IEC Code) (EIA Code) C2 (IEC Code) (EIA Code) 150n (154) 68nF (683) 68nF (683) 47nF (473) 33nF (33) 22nF (223) 15n (153) 6.8nF (6n8) (682) 4.7nF (4n7) (472) 3.3nF (3n3) (332) 2.2nF (2n2) (222) 1.5n (1n5) (152) 680pF (681) 330pF (331) 1µF (105) 470nF (474) 470nF (474) 330nF (334) 220nF (224) 150nF (154) 100nF (104) 47nF (473) 33nF (333) 22nF (223) 15nF (153) 10nF (103) 4.7nF (4n7) (472) 2.2nF (2n2) (222) Be sure to choose the correct filter component values when building the PC board – see Tables 5 & 6. In this case, the board has been configured as a highpass filter and is set up to accept dual supply rails. 68  Silicon Chip can then be fitted to these headers. They both go in position 1 for a dualrail supply (or if you are using an AC supply) – see Table 4. Alternatively, install them both in position 2 if you intend using a single rail supply. The selection matrix requires a 3-way DIL (Dual In-Line) pin header and this should now be installed – it goes in just to the left of trimpot VR1. Once it’s in, install the jumpers on this header to select your filter type (ie, LP, HP or bandpass). The assembly can now be completed by installing the 3-way screw terminal block. Power supply checks Before applying power, check that the supply link options are correct (see Table 4) and that the correct values have been installed for resistors Ra & Rb. Check also that you’ve installed the correct link options for the filter type. Next, connect one probe of your DMM to the 0V supply input, apply power and use the other probe to measure the supply voltages on the ICs. For a dual (±) or AC supply arrangement, check that there is +15V on pin 8 of ICs1-4. Similarly, there should be -15V on pin 4 of ICs1-3, while pin 4 of IC4 (if installed) should be at 0V. For the single supply arrangement, check for +15V on pin 8 of ICs1-3 and on pin 7 of IC4 (if installed). Note that the measured voltage will be lower if the supply voltage is less than 15V. Pin 6 of IC4 should be at half-supply SC (eg, 7.5V for a 15V supply). siliconchip.com.au