Fullscreen HTML5Med Res (??MB)HTML4

By MAURO GRASSI Minispot 455kHz modulated oscillator The Minispot produces a 455kHz carrier waveform which is amplitude-modulated with a 500Hz tone. You can use it to align the intermediate frequency (IF) stages of any AM broadcast or shortwave radio. T Fig.1: the circuit consists of a multivibrator (transistors Q1 & Q2) running at 500Hz and this modulates a 455kHz oscillator based on transistor Q3 and a ceramic resonator. 72  Silicon Chip HIS PROJECT GENERATES an amplitude modulated 455kHz RF signal. It can be used to accurately align the intermediate frequency stages of heterodyne AM receivers. If you are going to build the Aussie 3-Valve Radio described in this issue or if you are involved in restoring vintage radios, you will want this Minispot 455kHz modulated oscillator to accurately align the IF stages. For those readers with long memories, it is very similar to the Minispot circuit published in the February 1981 issue of “Electronics Australia” magazine. The objectives of IF alignment are to ensure that all tuned circuits in the IF stages are tuned to the same frequency and that this frequency is the correct frequency, usually 455kHz. If various parts of the IF stages are tuned to different frequencies, the sensitivity of the receiver will be poor. It may also be plagued with unwanted audible whistles appearing in the audio output. Therefore, corsiliconchip.com.au rect IF alignment is essential to good performance. There are various ways in which IF alignment can be achieved. The simplest is to align your receiver “by ear”. This involves tuning to a broadcast signal and adjusting the IF stages until the maximum output from the loudspeaker is obtained. However, this method will almost certainly not give the best results. Not only is it likely to result in having all stages aligned to the wrong frequency but there is also a difficulty in judging where the maximum output is obtained. The ideal method is to have an RF signal generator set precisely to 455kHz and fed into the first IF stage (ie, after the mixer). As the alignment proceeds and the sensitivity improves, the output from the signal generator can be progressively reduced, to avoid activating the AGC (automatic gain control) circuit of the radio (which would otherwise act to reduce the receiver’s sensitivity). Ah, you say, “I don’t have an RF signal generator”. This is where this 455kHz modulated oscillator comes into play. It will do the same job but costs only a few dollars. Circuit description The circuit of Fig.1 can be divided into two parts. The first part consists of a 2-transistor multivibrator (Q1 & Q2) which generates a square wave at around 500Hz. The second part is a phase-shift oscillator (Q3) with a 455kHz ceramic resonator connected between the collector and base of the transistor. This would normally be referred to as a Pierce oscillator. We use the multivibrator to “modulate” the 455kHz oscillator by varying its supply voltage. This is done simply by connecting R7, the 22kW collector load resistor for Q3, to the voltage divider resistors driven by Q2 (R4 & R5). But wait: we are getting a long way ahead of ourselves in describing how the circuit works. Let’s just back up a bit and describe the operation of Q1 &Q2, the astable (free-running) multivibrator. In essence, a multivibrator consists of two transistors which alternately switch on and off. In fact, the way that the transistors are biased ensures that only one transistor can be on at any time. The frequency of the alternate switching is determined by resistors siliconchip.com.au Parts List 1 PC board, code 06101081, 72mm x 32mm 1 9V battery 1 9V battery clip 1 cable tie 1 SPDT toggle switch (Altronics S1325) 1 300mm length of wire for antenna 1 ZTB455 455kHz ceramic resonator Semiconductors 3 BC548 NPN transistor (Q1-Q3) 1 1N4004 diode (D1) 1 3mm green LED (LED1) Capacitors 1 220mF 16V electrolytic 2 47nF MKT polyester 2 68pF ceramic 1 27pF ceramic Resistors (0.25W, 1%) 1 10MW 1 1.5kW 2 33kW 2 1kW 1 22kW 1 470W R2 & R3 and capacitors C1 & C2. To describe the operation, suppose Q1 is initially on while Q2 is off. Since Q1 is on, the collector end of C1 is near ground (0V) and so is the collector end of R1. Now C1 begins to charge through resistor R2 to 0.6V, eventually turning on Q2. When Q2 turns on, its collector goes to 0V, pulling C2 down with it, causing the base of Q1 to be pulled below ground. So Q1 turns off. Now C2 is charged via R3 to 0.6V which then turns off Q2 and Q1 is turned back on. This process repeats continually and the resulting output at either the collector of Q1 or Q2 is a square wave at a frequency dependent on the RC time constant formed by C1 and R2 or equivalently, C2 and R3. The frequency of the square wave produced is given by the equation: f = 1/(0.693(R2C1 + R3C2)) (approx.) = 1/(2 x 0.693R2C1) With the values used in this project (R2 = R3 = 33kW and C1 = C2 = 47nF), the expected frequency is approximately 465Hz. This will vary slightly according to the actual values of R2, R3, C1 and C2. In particular, if R2*C1 and R3*C2 are not exactly equal, the January 2008  73 ON 1k 22k 470 R5 R4 R7 C2 47nF 33k 33k S1 C1 47nF 1S 1k OFF 1.5k D1 R6 POWER + C3 220 F R1 27pF ANT C6 R8 10M 455kHz RES. R2 R3 A K LED1 Q1 +9V GND ANTENNA WIRE (RF OUTPUT) 68pF 68pF CS O D O M z Hk 5 5 4 Q3 1 8 0 1C4 0 1 6 0 C5 Q2 CABLE TIE SECURING BATTERY SNAP LEAD TO BOARD Fig.2: use this diagram to assemble the Minispot PC board. The ceramic resonator is not polarised and can go in either way around. 9V BATTERY Compare this fully assembled PC board with the above wiring diagram when installing the parts. The antenna wire should be about 300mm long. duty cycle will not be exactly 50%. As noted above, the astable multivibrator is used to power the 455kHz oscillator via resistor R7. As we have seen, the collectors of Q1 and Q2 continually switch high and low. R7 is fed from the voltage divider formed by resistors R4 & R5 and since the Capacitor Codes Value 47nF 68pF 27pF mF Code .047mF    NA      NA IEC Code EIA Code   47n 473   68p   68   27p   27 collector of Q2 switches between about +0.2V and +8.4V (nominal), the junction of R4 & R5 will therefore be switched between about +8.4V and +5.5V (without allowing for the slight loading effect of R7). Hence the supply voltage to the 455kHz oscillator is varied over these limits and so the amplitude of the output signal from the collector of Q3 will vary in direct proportion to the supply voltage; ie, it will be “amplitude modulated” at 455kHz. The modulated output signal is AC-coupled by capacitor C6 to a length of wire which functions as an antenna. A 9V battery is used to power the circuit via power switch S1. Diode D1 protects the circuit against reverse battery polarity. Construction The PC board for this project is coded 06101081 and measures 72mm x 31mm. The component overlay diagram is shown in Fig.2 while the samesize PC artwork can be downloaded from our website. Start construction by soldering in the eight resistors. Make sure that the correct values are used, either by referring to the colour code table or better still, measuring the resistors with a multimeter before soldering them. Diode D1 can then go in, making sure that it is oriented correctly. The capacitors are next on the list. Only the 220mF electrolytic (C3) is polarised, with its negative terminal connecting to the ground plane. The ceramic resonator can then be installed, followed by the three transistors and the LED. Make sure that the transistors go in the right way around. The LED is soldered in with its cathode (shorter lead) connected to the ground plane. Next, connect the battery clip, making sure that the red wire connects to the positive supply terminal and the black lead connects to the ground plane. Secure the leads of the battery clip with a cable tie. Two holes have been provided on the PC board to do this. You may now solder the toggle switch. Finally, cut a length of insulated wire about 300mm long. This forms the antenna. Solder one end of the wire to the antenna pad on the PC board. That completes the construction of the Minispot oscillator. Testing and troubleshooting Applying power and flicking the toggle switch to the on position should result in the LED lighting up. If it does Resistor Colour Codes o o o o o o o No. 1 2 1 1 2 1 74  Silicon Chip   Value 10MW 33kW 22kW 1.5kW 1kW 470W 4-Band Code (1%) brown black blue brown orange orange orange brown red red orange brown brown green red brown brown black red brown yellow violet brown brown 5-Band Code (1%) brown black black green brown orange orange black red brown red red black red brown brown green black brown brown brown black black brown brown yellow violet black black brown siliconchip.com.au Fig 3: this oscilloscope screen shot shows the signal at the collector of transistor Q1. It is a square wave at 449Hz with an approximate duty cycle of 50%. Small variations in the values of resistors R2 & R3 and capacitors C1 & C2 account for the small deviations in the duty cycle and frequency from theoretical values. not, it’s possible that either diode D1 or the LED (or both) is reversed. That’s not likely though, because you have carefully followed the preceding assembly instructions, haven’t you? Once power is applied and the LED is lit, the circuit should be producing a modulated 455kHz signal. You should be able to listen to it using an AM radio tuned to either 910kHz or 1365kHz, which are the second and third harmonics of the fundamental frequency. If it is working, you should hear a tone of around 500Hz when the antenna is close to the radio. If you have an oscilloscope, you can check the waveforms which we have included with this article. The collectors of Q1 & Q2 should have a square wave around 500Hz, as shown in Fig 3. The collector of Q3 should be an approximate sinewave at 455kHz, whose amplitude should fluctuate – see Fig 4. Conclusion This simple project is easy to build and cost effective. It will greatly aid in the alignment of the IF stages of any SC AM radio. Fig 4: this oscilloscope screen grab shows the signal that appears at the collector of transistor Q3. At the relatively high timebase speed being used, the waveform appears as an approximate sinewave at 455kHz but slower timebase speeds will in fact show the amplitude as varying – see Fig.5. Fig.5: in this screen shot, the lower trace (green) is the audio waveform at the collector of Q1 while the top trace (cyan) is the resulting amplitude modulated 455kHz output at the collector of Q3. As shown, the modulation is not very clean but it is OK for the intended application. Looking for real performance? Completely NEW projects – the result of two years research • • • • 160 PAGES From the publ ishe rs of 23 CHAPTE Learn how engine management systems work RS Build projects to control nitrous, fuel injection and turbo boost systems Switch devices on and off on the basis of signal frequency, temperature and voltage Build test instruments to check fuel injector duty cycle, fuel mixture and brake and coolant temperatures Mail order prices: Aust. $A22.50 (incl. GST & P&P); Overseas $A26.00 via airmail. Order by phoning (02) 9939 3295 & quoting your credit card number; or fax the details to (02) 9939 2648; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. siliconchip.com.au Intelligent turbo timer I SBN 09585 2294 9 780958 -4 522946 $19.80 (inc GST) NZ $22.00 (inc GST) TURBO BOOS T & nitrous fue l controllers How engin e management works January 2008  75