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AMATEUR RADIO By GARRY CRATT, VK2YBX Active CW filter for weak signal reception This simple circuit will clean-up those difficult to read CW signals. It provides active bandpass filtering and tone decoding of the incoming signal, and then regenerates the signal using an oscillator. Considerable concentration is required by both experienced and inexperienced CW operators when called upon to copy a wea~ signal affected by strong background noise. This in turn can lead to fatigue and may result in errors. By using this active filter, the ease of reception of weak CW signals is substantially improved. Active filters have long been used to improve the readability of weak CW signals but they still depend to a large degree upon receiver tuning, as the "beat" signal must fall within the narrow filter passband. This type of filter is most useful when the receiver being used has neither AF (audio frequency) or IF (intermediate frequency) filtering designed for dedicated CW reception. The design presented here is slightly more complicated in that it comprises two stages of active bandpass filtering, each having a slightly different centre frequency. This combination provides an adequate passband (400Hz or so) and rapid rolloff, in this case around 20dB per octave. We could have used a single high Q bandpass filter but this would have given a very narrow bandwidth, making it difficult to use. Fig .1 shows the combine d response of the bandpass filters while Fig.2 is a block diagram of the Active CW Filter. As shown, one bandpass filter is centred on 1.15kHz and the other on 1.35kHz. The output from the filter stages is then fed to a PLL tone detector. When a valid tone is detected, the PLL "keys" (ie, switches on) an audio oscillator to regenerate the signal. Circuit details All the parts for the Active CW Filter are mounted on a small PC board. We used PC stakes to terminate the external wiring connections. 94 SILICON CHIP Now take a look at Fig.3 which shows all the circuit details. The bandpass filters are two stages of an LM1458 op amp (ICla & IClb) and these provide an overall gain of around 40dB. Bias for the non-inverting inputs of the op amps is provided by a voltage divider consisting of two 22k0 resistors connected across the supply, while the 47kn and 470 resistors form an input attenuator which ensures that the filter is not overdriven. The output of the filter (pin 7 of IC1 b) is fed to pin 3 of ICZ via a 47kQ resistor and 1µ,F capacitor. IC2 is an LM567 phase locked loop tone decoder, commonly used in +10 \ -10 i -2U I\ J V / -40 / -50 -60 100 \ I -30 Fig.1: this graph shows the combined response of the two bandpass filters. The response is centred on 1.2kHz and the passband is about 400Hz. / 400 200 600 \ 2k 1k I\ 4k I\ 6k 10k HERTZ INPUT 1.15kHz 1.35kHz BPF BPF PLL TONE DETECTOR LPF AUDIO OSCILLATOR OUTPUT Fig.2: block diagram of the Active CW Filter. The output from the bandpass filter stages is decoded by a PLL tone detector which then switches an audio oscillator to regenerate the signal. telephone circuits for decoding signalling tones. When a valid tone (ie, a 1.ZkHz signal) is received, the PLL locks on and pin 8 switches low. VRl, in conjunction with the 2.7k0 resistor and O.lµF capacitor, sets the centre frequency of the tone decoder. In practice, VRl is set to give a centre frequency of 1.2kHz. The O. lµF capacitor on pin 2 sets the bandwidth (ie, the locking range) of the tone d.:::coder to about 130Hz. This narrow locking range prevents two adjacent signals from activating the unit. The output of the LM567 (pin 8) drives PNP transistor Ql, a BC557 which operates as a DC switch. This in turn controls phase shift oscillator stage IC3. When pin 8 of IC2 switches low, Ql turns on and supplies power to pin 7 of IC3 and also bias to pin 3 via two 33k0 resistors. IC3 is an LF351 BIFET amplifier and this generates a reasonably pure audio tone which the operator can listen to directly with a set of high impedance headphones. VRZ adjusts the oscillator for minimum distortion. Construction The circuit is built up on a small PC board coded SC 06204901 and measuring 106 x 66mm. Fig.4 shows the parts layout. You can install the ---------------------<11-----+9V PARTS LIST 47k 5 1 1 2 1 1 ON/OFF + 9 V - - - - - - - - - - + - - -......-------o>---o+9V 022I. 10 16VW .,. + - 33k 01 BC557 C IC2 LM567 VR1 10k O.ll .,. 2.2 + 16VW+ B 01+ 1~. , 1 "~' "'!"' EOC VIEWED FROM BELOW VR2 470k + F-o OUTPUT ~ 33k .,. -t ACTIVE CW FILTER Fig.3: the circuit uses IC1a & IC1b as the bandpass filters and these feed tone decoder stage IC2. When a valid tone is detected, pin 8 of IC2 goes low and turns on Q1 and phase oscillator stage IC3. PC board, code SC 06204901, 106 x 66mm PC stakes battery snap connector SPOT toggle switch RCA output sockets 1OkQ trim pot (VR 1 ) 4 70kQ trimpot (VR2) Semiconductors 1 LM1458 op amp (IC1) 1 LM567 tone decoder (IC2) 1 LF351 op amp (IC3) 1 BC557 PNP transistor (01) Capacitors 2 1 OµF 16VW electrolytic 1 2.2µF 16VW electrolytic 2 1µF 1 6VW electrolytic 1 0.47µF metallised polyester 2 0 . 1- µF metallised polyester 1 .022µF metallised polyester 3 .01 µF metallised polyester 4 .0033µF metallised polyester Resistors (0 .25W, 1 470k0 3 1 390k0 1 2 47k0 1 3 33k0 1 2 22k0 1 5%) 5.6k0 4. 7k0 3.3kQ 2. 7kQ 470 APRIL 1990 95 D.DAUNER ELECTRONIC COMPONENTS WE STOCK A WIDE RANGE OF ELECTRONIC PARTS • i-_,_______. 0 . 47k --.✓ GND INPUT Fig.4: here's how to install the parts on the PCB. The finished board can be housed in a plastic jiffy box and powered from a 9V DC plugpack or from batteries. -----.----7 rXo z.., - a,a-- 0 ! I oooa.; ~-0? Jo L Fig.5: here is a full-size artwork for the PC board. parts in any order you wish bu~ be sure to install the ICs, transistor and electrolytic capacitors with correct polarity. you will need a digital multimeter (to monitor the voltage on pin 8 of IC2) and either an audio oscillator or a frequency meter. The first job is to set the centre frequency of IC2. Adjustments If you have an audio oscillator, apply a 1.2kHz signal to the input and adjust VRl until pin 8 switches low. Note the setting on VR 1, then continue to rotate the trimpot until pin 8 switches high. Set VRl to the midpoint of these two settings. Alternatively, if you have a frequency meter, you can set the centre frequency of IC2 by monitoring pin 5. Make the measurement with the probe in the xlO position and adjust VRl to give a reading of 1.2kHz. Check the completed PC board carefully for possible errors before applying power. To adjust the unit, CAPACITOR CODES D D D D D 96 IEC No Value 1 2 1 3 4 470n 0.47µ,F 100n 0 .1µ,F .022µ,F 22n .01 µ,F 10n .0033µ,F 3n3 SILICON CHIP EIA 474 104 223 103 332 for Development • Repair • Radio Amateur • Industrial Electronic • Analog and Digital WHILE STOCKS LAST Quartz in filter 10.9MHz 6kHz BW. $12.50 US Filter capacitor 4/.tF 3kV. . $15.00 Electromagnetic Airpump for Aquarium ........ . . . ... . .. $9.00 Timer Motor 240VAC 6RPH. . $6.00 Come and see. Showroom: 51 Georges Crescent, Georges Hall, NSW 2198 (Behind Caltex Service Station In Blrdwood Road) Phone 724 6982 TRADING HOURS: Monday to Friday 9.00 a.m. to 4.00 p.m. Saturday from 9.00 a.m. to 12.00 noon. Now, whenever a 1.2kHz tone is applied to the input, IC3 should generate a new tone at the output. This can be monitored using a pair of headphones. Adjust VR2 so that the output has minimum distortion content. Using the CW filter The entire project could be housed in a plastic jiffy box and run from either a DC plugpack or batteries (9V). There are no controls to operate other than the ON/OFF switch. To use low impedance headphones, it will be necessary to add a simple attenuator to the output of the audio oscillator. This can take the form of a lkO potentiometer wired as a voltage divider. The filter can be driven from the headphone or "record" output of most receivers. In fact by using the record output, which allows the receiver loudspeaker to operate in the normal fashion, it is possible to make an instant comparison between filtered and unfiltered audio. You'll be amazed at the results! ~