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By LEON WILLIAMS VK2DOB ACTIVE FILTER cleans up weak CW reception Dig out those weak CW signals from the noise and inter­ference with this simple filter unit. It easily connects between any receiver and an external speaker. Unlike other designs that use a fixed narrow filter this unit has a variable filter control for obtaining optimum reception. Most radio receivers are only intended to receive voice signals and are required to have an audio bandwidth of several kilohertz. A typical amateur band receiver fitted with a SSB filter has an audio frequency response from 300Hz to 2700Hz, giving a bandwidth of 2400Hz. By contrast, the frequency compon­ ents of a CW signal only occupy a bandwidth of 100Hz or so, depending on the sending speed. It is quite obvious that there is plenty of room in the receiver bandwidth 24  Silicon Chip to fit a little CW signal. This situation is quite OK until another CW signal or some interfering signal is received perhaps only a few hundred Hertz away. We are now in the situation of trying to decipher a CW signal amongst a whole lot of other sounds. It’s a bit like trying to listen to a small voice in a noisy crowd of people. The solution is to narrow the audio frequency response so that the interfering signal is filtered out, leaving the wanted signal on its own. There are two main ways of doing this. First, a narrow crystal filter can be switched in to the intermediate frequency (IF) circuits of a superheterodyne receiver. A typical bandwidth might be 500Hz. When normal voice signals are to be received, the narrow filter can be switched out and a wider (2400Hz) filter switched in. Filters such as this are quite expensive and you would have to be a keen CW operator to consider this. The second alternative is to switch in and out a narrow audio bandpass filter somewhere in the audio stages of the receiver. This technique can be used in both superheterodyne and direct conversion receivers. Direct conversion receivers do not have IF stages. Once again, the filter needs to be switched out when voice signals are received, because the narrow filter would eliminate too many frequency components of the voice and probably make it unintelligible. A recent innovation is the use of out­ board DSP processors which digitise the voice signals and through digital manipulation result in various filter responses. These units are expensive, costing hundreds of dollars. This Adjustable CW Filter has a number of advantages over the methods mentioned above. Firstly, it is self-contained, is powered from a DC plugpack and no modifications need to be done to the receiver. Also it is inexpensive to build, uses standard parts and it simply connects between the speaker socket or audio output of the receiver and an external speaker. The front panel has two controls, Volume and Filter. The filter control can be adjusted to give any bandwidth between wide open (no filtering) and a very narrow band pass response centred on 800Hz. The filter control can also be used to good effect with voice signals by acting to reduce the level of high frequency noise and interference. If 800Hz is not your favourite frequency this can be changed, as explained later on. Fig.2 shows the range of filter responses provided by the unit. A special feature is a LED on the front panel that is turned on when 800Hz is detected. This gives an indication that you are tuned correctly to the CW signal and also it is an oppor­tunity to learn to “read” CW by decoding the flashes with the volume turned down. Circuit description The complete circuit is shown in Fig.1. Audio signals from the receiver are fed to the 10kΩ trimpot VR1. The minimum input level that the tone decoder will function correctly is about 70mV RMS or 200mV peak-peak. Following VR1 is op amp IC1a which has a gain of two. A 560pF capacitor connected across the 100kΩ feedback resistor from pin 2 to pin 1 provides some low pass filtering. As we are not using separate positive and negative power supply rails, the non-inverting input (pin 3) is biased Fig.1 (right): the circuit is a variable bandpass filter centred on 800Hz and includes a tone decoder (IC2) to indicate when the receiver is correctly tuned for CW transmissions. December 1996  25 PARTS LIST 1 metal case, 102 x 62mm x 148mm (W x H x D) 1 PC board, code 06112961, 97 x 68mm 1 9-12V DC plugpack 1 DC panel socket 1 6.5mm mono jack socket 1 RCA panel socket 2 20mm knobs 1 LED bezel clip 1 10kΩ log potentiometer (VR4) 1 10kΩ dual linear potentiometer (VR2a,VR2b) 2 10kΩ horizontal trimpots (VR1,VR3) 17 PC pins Semiconductors 1 TL074, LF347 quad op amp (IC1) 1 LM567 PLL tone decoder (IC2) 1 LM386 audio amplifier (IC3) 1 1N4004 diode (D1) 1 5.6V 1W zener diode (ZD1) 1 5mm green LED (LED1) Capacitors 1 1000µF 25VW electrolytic 1 470µF 25VW electrolytic 3 100µF 25V electrolytic 6 2.2µF 63V electrolytic 5 0.1µF MKT polyester 1 .047µF MKT polyester 4 .022µF MKT polyester 2 560pF ceramic Resistors (0.25W, 1%) 6 100kΩ 2 820Ω 4 47kΩ 1 180Ω 2 10kΩ 1 100Ω 1 5.6kΩ 2 10Ω 1 1kΩ Miscellaneous Screws, nuts, spacers, hook-up wire to half the supply voltage by two 10kΩ resistors. A 100µF capacitor helps filter out noise. This half supply voltage point is also connected to the non-inverting inputs of the other three op amps in IC1. The output of IC1a is split into two paths. First, it is applied to the input of an audio bandpass filter using IC1b. This stage has been designed for a centre frequency of 800Hz, unity gain in the passband and a -3dB bandwidth of 150Hz. 26  Silicon Chip AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 5.0000 01 NOV 96 13:53:59 0.0 -5.000 -10.00 -15.00 -20.00 -25.00 -30.00 -35.00 -40.00 -45.00 20 100 1k 10k 20k Fig.2: this diagram shows some of the bandpass respons­es available from a filter, ranging from a deep notch to quite wide. When calculations are done for any audio filter circuit, it’s almost certain that non-standard value components will be called for. The components specified in the parts list are close enough to the calculated values without greatly affecting the performance. The output of this stage is connected to an identi­cal stage using IC1c. The combination of these two bandpass stages provides a narrow response with high attenuation of fre­quencies either side of the passband. Fig.3 shows the general filter configuration and the formulas used in the design calcula­tions. The output of IC1c is coupled to one half of a dual-gang potentiometer (VR2b) by a 2.2µF capacitor while the output of IC1a is coupled via another 2.2µF capacitor to the second gang (VR2a). The mixer stage uses IC1d and has unity gain. The wipers of VR2a and VR2b are each connected to the mixer stage by a 100kΩ resistor and a 0.1µF capacitor in series. The point to note is that when VR2 is fully anticlockwise, no signal is passed from the bandpass filter output to the mixer, while the full output of the unfiltered signal from IC1a is fed through. When VR2 is rotated fully clockwise, the reverse occurs and only the output of the bandpass filter is fed to the mixer. By varying the position of VR2 we can obtain any degree of filtering between narrow in the clockwise position and wide in the anticlockwise position. The output of IC1d is coupled to the Volume control (VR4) via a 2.2µF capacitor. VR4 varies the signal level applied to the audio power amplifier IC3, an LM386. This drives an external speaker. The 2.2µF capacitor from pin 7 to 0V is included to reduce the level of hum to an acceptable level when a plugpack power supply is used. The 10Ω resistor and .047µF capacitor help keep the amplifier stable at high frequencies. 800Hz indicator IC2 is an LM567 tone decoder, which turns on the LED (LED1) when 800Hz is applied to its input. The LM567 is actually a phase locked loop circuit which compares an internal oscillator to an external signal at pin 3. When they are within a few hundred Hertz of each other, the open collector output at pin 8 switches to 0V and lights LED1. The frequency of the internal oscillator is determined by the resistance between pins 5 & 6 and the capacitor from pin 6 to ground. With the values shown on the circuit the frequency can be varied between about 600Hz and 1500Hz by VR3. The capacitors from pins 1 & 2 to ground provide filtering for the internal circuits and also affect Fig.3: the general filter configuration & the formulas used in the design calculations. the detection bandwidth. The values shown on the circuit were arrived at through experimentation by monitoring LED1 for correct operation under various signal condi­tions. A 180Ω resistor, a 5.6V zener diode ZD1 and a 100µF capaci­tor provide a regulated 5.6V power supply for IC2. The full circuit is powered by a 9-15V 500mA DC plugpack. Typically this will be a 12V 500mA plugpack. Diode D1 provides protection against the power supply being connected with reverse polarity. A 10Ω resistor and a 1000µF capacitor decouple the power supply line and reduce the level of hum. Construction The prototype Adjustable CW Filter was housed in a standard metal case measuring 102mm wide, 62mm high and 148mm deep. The parts are mounted on a PC board measuring 97 x 68mm and coded 06112961. The PC component overlay and wiring diagram is shown in Fig.4. Start construction by assembling the Fig.4: the PC component overlay and wiring diagram for the CW filter. Check your work carefully before applying power. PC board. Inspect the board for shorts between tracks and correct hole sizes. The holes for the trimpots, PC pins and diodes may need enlarging. Use the component overlay as a guide and solder in the resistors, trim­pots and diodes first. In some cases, a low impedance termination will be required by the amplifier in the receiver; eg, 8Ω or 10Ω. If this is the case, an 8Ω or 10Ω 0.5W resistor can be connected across VR1; provision has been made for this on the PC board, Install the PC pins next, followed by the capacitors. Double check the polarity of the electrolytic capacitors to make sure they are installed correctly. Finally, solder in the three integrated circuits, again checking that they are in the correct positions. The parts list details the components associated with the bandpass filters for a centre frequency of 800Hz. If you want to change this, use the equations in Fig.3 to calculate the new values. When it is complete, put the PC board aside and mark out and drill December 1996  27 The completed CW filter is easy to set up and adjust, provided you have access to a multimeter and an audio signal generator. Use cable ties to keep the wiring neat and tidy. the holes in the metal case. There are three holes required on the front and back panels and four mounting holes in the base. Install the sockets on the rear panel and the pots and LED on the front panel. The LED is held in place with a plastic bezel clip. Mount the PC board in the case with 3mm screws and nuts and 6mm spacers. This done, connect the sockets, pots and LED to the PC board with hook-up wire. The prototype used separated cores from rainbow ribbon cable which makes the job of tracing wires easy. If you have trouble identi­ fying the tags of the DC socket, plug the plugpack into the socket, turn on the power and check the voltage and polarity with a multimeter. Finally, fit the two knobs and adjust them so that when the pointer is vertical the pots are at mid-position. Fig.5: actual size artwork for the PC board. Testing To test and adjust the filter you should have a multimeter and an audio generator. If you don’t have an audio generator, try to borrow one as it makes the setting up process easy. Turn the filter and volume pots fully anticlockwise and the input trimpot VR1 and the decoder trimpot VR3 fully clockwise. Plug a speaker into the speaker socket, connect the DC plug­ pack and connect the audio generator to the receiver socket. Power up the CW filter and measure the voltage between pin 6 of IC3 and 0V. If you are using a 12V plugpack 28  Silicon Chip Fig.6: actual size artwork for the front panel. this reading will probably be around +14V. Most plugpacks are unregulated and only really get to their rated voltage at full load, so don’t get too alarmed if the reading seems high. If the reading is 0V or close to 0V, check to see if the plugpack polar­ity is reversed or that diode D1 is around the wrong way. If these check out OK there may be a short circuit in the PC board. Turn off the power straight away and check the PC board for problems. There could be a component in the wrong way, shorts between tracks or a wiring error. If everything appears OK, set the audio generator to 800Hz (or the alternate frequency chosen for the bandpass filters) with an output of around 100mV RMS. Adjust the volume pot so that you can hear the tone in the speaker. Now, using a small screwdriver, turn the decoder trimpot VR4 until the CW LED turns on and note VR4’s position. Keep rotating VR4 until the CW LED turns off, again noting the position. This done, return VR4 to the midpoint between these two positions, disconnect the audio generator and check that the CW LED turns off. That’s all there is to setting up the CW filter. The lid can now be fitted to the base using four self-tapping screws. Using the filter The CW filter is designed to sit alongside your receiver and be permanently connected. The method of connection to the receiver will depend on the specific receiver. Probably it will have an external speaker socket. When using this option you will need to check that the internal speaker is disconnected otherwise you will hear both the unfiltered and filtered signal at the same time. Some receivers have an output that is not directly from the speaker but from a low level audio signal point before the speak­er. If you use this option you will need to once again make sure the speaker in the receiver is turned off. As mentioned previous­ly, if the output of your receiver overdrives the CW filter, adjust the input trimpot VR1 until the sound from the speaker is undistorted. If you don’t have an external speaker for your receiver, now is the time to get one. There are commercially available speaker boxes specifically suited for this purpose or you could buy yourself a cheap speaker and build a box to mount it in. The cheapest solution could be to use a small speak­er box from a discarded mini stereo system. In any event, the sound from an external speaker will almost always be better than that obtained from the little speak­ers found in most receivers. To operate the CW filter, turn the filter control to the wide position and adjust the volume control to suit. Tune in a CW signal on the receiver and watch the CW LED until it flashes in time with the bursts of tone. Turn the filter control clockwise until the CW signal is clear of any interference and easy to listen to. If the interfering signal is wideband it won’t be possible to completely filter it out as some of it will lie in the filter passband but a significant improvement will be heard anyway. You will be amazed how effective the CW filter can be when listening to a noisy crowded band. A small CW signal can sometimes hardly be heard but when the filter control is turned to narrow, the CW signal seems to jump out from the noise. Of course, if the band conditions are good the filter con­ SC trol can be left in the wide position. If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.winradio.com/ December 1996  29