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AMATEUR RADIO BY GARRY CRATT, VK2YBX Build this active filter & improve your CW reception One parameter of receiver performance that is difficult to achieve is good selectivity. For years, designers have tried a variety of techniques including stagger tuned IF stages, LC filters, cascaded crystal filters, mechanical filters and multipole quartz filters in the quest for the perfect (rectangular) response curve. The compromise required to ensure stable, reproducible filters has always been poor shape factor. The result of imperfect filters in shortwave receivers, in practical terms, is the inability to resolve signals on adjacent channels without interference. Although amateur texts are full of comp lex LC audio filters, which are capable of improving reception by tailoring the audio response, very few simple filters which will do the job have been published. Our design provides a dramatic improvement in the resolution of CW signals and is also quite useful when resolving SSB. The heart of the circuit is the commonly available 741 op amp, which is configured as a multiple feedback bandpass filter. Circuit details Fig. 1 shows the circuit. Bandpass filters provide significant attenuation TP +5V 15k ~-------+--',Mh---0+9V 10 + 16VWl Cl 0.1 o--1------1,.-----1 . 001 I AUDIO INPUT + I I/ TO I HEADPHONES CW FILTER Fig.1 : the circuit consists of a bandpass filter based on a 741 op amp (ICl). Note the unusual bias arrangement. This has been employed so that the op amp runs in starved current mode. 88 SILICON CHIP at very high and very low frequencies and much less attenuation to frequencies within the passband. Some explanation of terms is now appropriate. The "bandwidth" of a filter is defined as the difference between the upper and lower points where the filter response falls and remains below the 3dB peak amplitude. The "centre frequency" is the geometric mean of the upper and lower 3dB cutoff points. The "resonant frequency" of a single pole bandpass filter is the centre frequency of the filter, but this is not half the difference between the upper and lower cutoff frequencies; rather it is the square root of the product of the upper and lower cutoff frequencies. The "fractional bandwidth" is the ratio between the bandwidth and the centre frequency. Our simple 741 op amp bandpass filter has a Q of 20, a gain of 15, and a bandwidth of 35Hz or so. Purists will immediately note that the 741 is not biased in the conventional manner using a voltage divider to obtain a ½Vee reference from the single supply. Instead, the non-inverting input, pin 3, is biased from the OV rail so that the 7 41 is throttled right back to the point where it consumes a lot less than its normal current . This starved current mode means that the cheap old 741 can be economical to run from batteries if required. (Note: this circuit is only suitable for tone signals; it would be no good for speech or music). Current dr~in is around 300µA which can be easily supplied by a 9V battery (Eveready type 216 or equivalent). 15k .------------.--------------u+9V PARTS LIST 10!1 1 PC board, code SC06106911 , 50 x 48mm 1741 opamp(IC1) 1 100Q trimpot (VR1) 100 25VW!_ 0.1 o--jl--WA-+--11....__-=-t AUDIO INPUT 220 .0471 t6V)W r o, SPEAKER 10ll .,. Capacitors 1 10µF 16VW electrolytic 2 0.1 µF metallised polyester or ceramic 1 .001 µF metallised polyester or ceramic Fig.2: a simple audio amplifier based on an LM386 IC can be added to the active filter if you don't wish to use headphones. Resistors (0.25W, 5%) 1 82kQ 1 2.7kQ 1 15kQ 1 47Q VRl is used to adjust the centre frequency of the filter, which should be tuned to the offset frequency of the receiver. Most CW operators seem to use 700-B00Hz. There is sufficient range in this control to vary the centre frequency from 600Hz to 1.ZkHz. This is even useful when copying SSB signals , as the filter is capable of rejecting adjacent channel heterodynes, whistles and noise. The circuit values for our filter were determined by solving the three simple equations below: Miscellaneous Battery, zippy box, hookup wire, solder, etc. = R3 X 7t X fc X C = 84.9kQ (2) Gain A = R3/ZR 1 ; therefore R1 = R3/ZA = 2.83kQ (1) Q (3) VRl = Q/(ZQ2-A)27t x fc x C = 54Q where C2 = C3 = C = 0.1 x 10- 6 and fc = 750Hz As can be seen, we selected 0. lµF as the value for CZ and C3, merely because it is a convenient value. This enables us to calculate R3, as we already know the target centre frequency and Q. Simple multiplication and division allows us to calculate the Rl and VRl values. We used 0. lµF as the input coupling capacitor, and .00luF as the output capacitor, which ensures a good match to a pair of high impedance headphones, or external audio amplifier. Of course, the unit could be expanded to include a simple audio amplifier as shown in the circuit of Fig.2. This configuration would require an external power supply connection to run the audio amplifier, anq obviously a larger enclosure to house the sp eaker. The values for Rl, RZ, and R3 have been rounded off to the closest preferred value resistors but, to be precise, two discrete values could be used to make up the exact value calculated, if you wish. Provision has been made on the PC board to use two resistors in each position. RZ is selected as having a value of twice the Q, to ensure minimum offset. Building it The PC board measures just 50 x 48mm (code SC06106911) and is small enough to be built into either a plastic zippy box or inside the actual receiver with which it is to be used. An easy way to mount the PC board is with a piece of double-sided tape, either inside the plastic box or the receiver itself. Fig.3 shows the parts layout on the PC board. You can install the parts in any order you wish but take care with the polarity of the IC and the lOµF electrolytic capacitor. Alignment of the filter is simple. You just connect the audio output of the receiver to the filter and, while listening to a CW or SSB signal, carefully adjust VRl for the clearest sigcontinued on page 93 The book of facts! FERROMAGNEIIC CORF.s • Materials • Design • Applications ill Amidon I Ie-[Due ~ __.:- o., AUDIO J INPUT_....--. & I .__.,.-- ~Ou .E o1 _!- HEADPHONES ....---- n 0,~ Mo, GDe ~·~ ~t;J \ -r GND +9V OUTPUTTO Fig.3: here's how to install the parts on the PC board. Note that Rl, R2 & R3 have been rounded off to the nearest preferred values but you can use two discrete resistors to make up the exact value of each if you wish. The second resistor simply replaces a wire link in each case. Complete data on over 500 types of Iron Powder and Ferrite cores and assemblies, all available in Australia!$,~ paid from: 5•n U. Upost STEWART ELECTRONIC COMPONENTS Pty. Ltd. ACN 004 518 898 P.O. Box 281 Huntingdale 3166 PH (03)543-3733 FAX (03)543-7238 ]UNE 1991 89 Moveable targets for a pistol range I wish to set up a pistol range. What is required is a set-up of five turning targets. There are systems on the market which have a target of about 600 sq cm for shooting from 25 metres. What I need is a system using a target of about 150 sq cm to allow me to shoot from 10 metres with an air pistol. The system needs to allow all five targets to turn together and be controlled from 10 metres away. Targets should turn in 0.2 seconds and need to be faced at the shooter for variable times, to suit rapid Notes & Errata Garage Door Controller, March & April 1991: two lOµF capacitors associated with IC7 need to be changed to lµF to make sure the controller always comes on in the Down mode when power is reconnected. They are the capacitors connected to pins 3 and pin 5 of IC7. The designer has advised us that the timing resistor between pins 15 and 16 ofICl in the transmitter should now be lMQ while the equivalent resistor between pins 15 and 16 ofIC2 in the receiver should be 1.2MQ. The higher resulting transmitter code frequency allows for the inevitable drop in battery voltage. IC2 can cope with the different code frequency. A 4.7kQ supply decoupling resis- fire, standard gun and centre fire shooting. Do you have any suggestions and some idea of costs? (J. S., Casuarina, NT). • The project you propose is a little beyond what we would normally publish in the magazine. The timing and control circuit could be fairly simple though and you could probably use standard servodrives, as used in radio controlled cars and planes, for turning the targets. These are relatively cheap and can be used in conjunction with a radio controller which would suit your purpose for remote control. tor was not shown on the UHF receiver circuit but was included on the PC overlay diagram although it is shown unlabelled. A 4 70kQ resistor from pin 11 of IC la to OV was omitted from both the circuit and PC overlay diagram. It will need to be soldered onto the copper side of the board. A small section of PC track is missing from the pattern near TPl. It joins the 4.7MQ resistor and 15pF capacitor together. Only one of the two diodes associated with the paralleled relays RLAl and RLA2 is needed; the other one can be omitted. The same applies for the diodes associated with relays RLA3 and RLA4. Also, for best operation, roller doors should have a 2kg to 3kg weight added adjacent to their bottom edge. SENDINYOUR APPLICATION For your share of the prizes In the March and April issues of Silicon Chip we published the RemoteControllerfor Garage Doors. The feedback from this project has been enormous. You have to send in your application of how else this kit canbeused.Plusacoupleofphotos if you actually put the application into practice. In conjunction with Oatley Electronics this is your opportunity to share your application and possibly pick up a prize as well. Even if you have not built the Garage Remote you can still send in an entry. Entries close on the 30th ofAugust 1991, and will be judged by the technical staff at Silicon Chip. What you could win? 1st Prize. 7mW HE-NE Laser and Inverter. All you need to power · this unit is 12-14VDC. This is a red Laser and is valued at: $580.00 2nd Prize. Infra Red Night Viewer.See in the dark with this great night viewer. In this kit you will get a NEW 6032 Image converter tube, Case and the components kit. This prize is valued at: $339.00 Amateur Radio - continued from page 89 nal. In the case of a CW signal, as the centre frequency of the filter is adjusted to match the receiver offset frequency (700-800Hz), the level of recovered audio will dramatically increase as the audio signal enters the bandpass of the filter. This will make signals far easier to copy, even in the presence of strong adjacent signals or noise. References Active Filter Cookbook, by Don Lancaster. Published 1975 by Howard W. Sams & Co. Inc; The ARRL Handbook; Ham Radio Magazine, January 1990. 3rdPrize. lmWLaser head with a commercial inverter. All you need is a 12V supply. This prize is valued at: $309.00 SC06106911 I Fig.4: here is the full-size pattern for the PC board. Alternatively, the circuit could be built on Veroboard. Plus 10 Ultrasonic movement alarm kits valued at $50.00 ea. · SEND ENTRIES TO: GARAGE DOOR COMPETITION P.O. BOX 139 COLLAROY BEACH NSW 2097 JUN E 1991 93