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MODULE V - FUNDAMENTALS OF ELECTRONICS
MF 300 Khz to 3 Mhz which mainly includes the A.M. radio band of about 530 Khz to 1650 Khz (varies between countries). |
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OHMS LAW This is the first and most fundamental lesson in electronics. See the tutorial or recognized texts. There are four basic electrical units here. They are (1) Power - in watts - [P], (2) Voltage - in volts - [E], (3) Current - in amperes or amps - [I] and (4) Resistance - in ohms - [R]. - again see the tutorial or recognized texts. OSCILLATOR An oscillator is an electronic circuit where some of the amplified output is fed back to the input to maintain a flywheel effect or oscillations. Circuit design, components and layout the frequency of oscillation. For extreme accuracy we might use a crystal to maintain frequency. At its most basic we could design one simple oscillator circuit to operate at 7050 Khz. Although not recommended we could apply and remove power at a morse code rate and we would have a simple yet extremely crude C.W. transmitter operating in the 40m amateur radio band as a Q.R.P. transmitter. Don't even think about it!. RECEIVER A collection of circuits designed to receive signals over one or more bands of interest and covering one or more modes of operation. At its most simplest it could be a crystal set designed to receive the a.m. radio band. At its most complex it could be a very sophisticated surveillance receiver designed to cover anything and everything. Typical receivers are a.m. / f.m. tuners, t.v. receivers or s.w. radio. RTTY or rtty This stands for Radio Teletype where amateurs, amongst others, would transmit signals generated by a keyboard device not unlike you sending email. Instead of an internet connection you have a radio connection. QRP or qrp ( Low Power Transmission ) This is a very much reduced power mode of operation favoured by many amateur radio operators because of the skill and challenges involved in making contacts. Power is limited to 5 watts maximum on C.W. or 10 watts on S.S.B. although of course often less is used. Because of the low power output the equipment can be battery operated and is quite suitable for portable operation. Frequently Q.R.P. activities are conducted in conjunction with other recreational pursuits such as fishing, camping etc. Often the equipmentis home made (home brewed). If you are looking for a real challenge in one of the finest fraternities in the world then start right here. But first go and get a license. S/N RATIO Signal to Noise Ratio. Noise is the ultimate limiting factor in the reception of radio signals. Noise is generally classified as either natural (QRN) or man made (QRM). Natural noise emanates from galactic and atmospheric noise picked up by the antenna as well as thermal noise generated in the antenna itself. Similarly man made noise such a fluorescent lights, motors and a host of other appliances and tools is also picked up by the antenna. These sources of noise are then amplified by the various stages in a receiver. However these amplifying devices create noise of their own also. The noise problem varies with the frequency of reception. Generally the noise figure of a receiver (i.e. allowing for the noise generated by the receiver itself) is not of great importance for frequencies somewhat below 30 Mhz because the external noise combined i.e. natural and man made will always exceed the noise figure of the receiver. Visualise if you can, an arbitrary noise level of 10 uV - these figures are for comparative or illustrative purposes only and bear no resemblance to reality. Now that is 10 one millionths of a volt. If we wished to receive a certain signal that was received on our antenna and had a strength of say 1000 uV or 100 times the noise voltage you can see such a signal would be copied quite readily. On the other hand if a desired signal was only 1 uV (and many often are at this level) then the noise level outweighs it by a ratio of 10:1. Now that is a pretty rough explanation but you should get the general idea. Of importance to reception, the narrower the bandwidth of the receiver, the bigger the improvement to your prospects of recovering the desired signal. Once a desired signal drops in level in comparison to noise in a particular location then all hope of recovery is lost. This is a highly technical topic which is almost a science in itself. I have attempted to do no more than give you a general appreciation of the topic. SSB or ssb ( Single Side Band ) Assuming you have read the section on A.M. you would be aware that two of the disadvantages of a.m. transmission are the twice the bandwidth to convey the same information and only 25% of the power is used in each side band. The remaining 50% of power is expended in the carrier. It makes more sense in terms of economy of bandwidth as well as economy of power to simply transmit only one side band. This is called S.S.B. or Single Side band. If our carrier (initially) in the transmitter is say 9000 Khz or 9 Mhz and we modulate that signal with useful voice frequencies of say 300 Hz to 2400 Hz (this spectrum contains all useful information and indeed is roughly the bandwidth of your telephone system) then dealing with the highest frequency of 2400 Hz (2.4 Khz) we get side bands (including carrier) of 000 Khz to 9002.4 Khz as well as 9000 Khz down to 8997.6 Khz. Fifty per cent of our power is in the 9000 Khz carrier and 25% in each of our side bands. At high power levels this would be both wasteful and inefficient. In fact at this point in our transmitter we are only dealing with very low power levels. What if we introduce a highly specialized and highly accurate filter that will only accept frequencies in the range of 9000.3 Khz to 9002.4 Khz and reject all others. Presto we have a signal which occupies a bandwidth of only 2.1 Khz wide, therefore more channels then can be accommodated in the same band. Further the power amplification formerly available to us can now be devoted exclusively to our narrow band signal in a linear amplifier. Unfortunately at the receiving end things get a lot more complicated and expensive. Firstly our I.F. Amplifier must accept signals no wider than the 2.1 Khz. In practice you use a similar crystal filter or if the receiver is part of a transceiver then you use the same filter as was used in the transmit section. Secondly because no carrier is transmitted with the S.S.B. signal we must provide one locally in the receiver. This is called a B.F.O. or Beat Frequency Oscillator and 9000 Khz is typical but not the only frequency or method. When mixed with the received signal the B.F.O. and Detector (sometimes called Product Mixer) will put out our original audio of 300 Hz to 2400 Hz. All other frequencies are filtered out. In the transmit section the 9000.3 to 9002.4 signal is mixed with a local oscillator signal or frequency to produce a signal at our final frequency of transmission. SWR or swr his is called Standing Wave Ratio (or more correctly V.S.W.R.) and is much beloved by many who like to become paranoid over something. For some unknown reason C.B.'ers seem to excel at this. A transmitter requires a load to deliver power to. This is called an Antenna. If some of the transmitted power is reflected back along the transmission line toward the transmitter then we have a situation where voltage standing wave patterns exist. "The ratio of the maximum voltage on the line to the minimum value (provided the line is longer than a quarter wavelength) is defined as the voltage standing wave ratio or vswr". It is often mistakenly assumed that power reflected from a load is power lost. If there is proper matching at the input end of the line this is only true if there is considerable loss in the line itself. S METER It is presumed you understand both I.F. amplifiers and automatic gain control (A.G.C.) An S-meter is simply a meter set up to measure the current through the agc control line so that on strong signals it shows say S9 + 20 dB while on weak signals it might be at the bottom end of the scale on say S1 or S2. Be aware, S-meters are notoriously inaccurate, can not be compared between various receivers and are only useful for relative measurements applicable to your receiver, using your current antenna, at your present location and on one particular band. They are widely misunderstood and all too often given a level of importance they don't rightly deserve. TRANSCEIVER A unit which contains both the transmitter and receiver. It has the advantage that common electronic circuits are shared rather than duplicated if you operated the two units separately. TRANSISTOR Firstly we had valves and then two bright sparks in Bell Labs. back in 1948 invented the transistor. Almost similar to a valve (triode) the transistor has revolutionized the world. Not only are there types which handle very high voltages there are types capable of very large amounts of power. Because of transistors much equipment has shrunk to a fraction of its former size while extending capabilities almost beyond imagination. By the 1960's transistors began to become integrated together in single packages to perform all sorts of logic blocks. The digital explosion had begun. Today literally millions and millions of transistors are formed on the one thin wafer to produce devices like the Pentium III processor at previously unheard of speeds and power. TRANSMITTER see Oscillator UHF or uhf 300 Mhz to 3 Ghz (that's 3,000 Mhz or 3,000,000,000 cycles per second) - this band is occupied by U.H.F. T.V., some radar installations, mobile phones, two way radios and a heap of other exciting stuff. USB or usb ( Upper Side Band ) Identical but the direct opposite to L.S.B. (Lower Side band) - see also A.M. and S.S.B. VHF or vhf 30 Mhz to 300 Mhz occupied by traditional T.V. stations, some amateur bands, commercial two way radio, maritime and aircraft bands as well as the F.M. radio band of 88 - 108 Mhz. WATT The fundamental unit of power consumed see Ohms Law WAVELENGTH Originally in radio, the frequency of signals was not mentioned. The custom was to refer to the 'wave length'. This is easily computed from: In reality this indicates, from a purely technical standpoint, that a wavelength is determined by dividing the speed of light by the signal frequency in cycles per second. The underlying reason here is that radio waves do travel at the speed of light. This is approximated as 300 million metres-per-second (and no correspondence will be entered into on that point). As frequencies increased by about the 1930's the wavelengths diminished in physical size to the point the term 'short-wave' came into vogue. To understand radio waves etc. visualise a pebble dropped into a pond. At the point where the pebble hits the water the transmitting antenna is situated. Waves then radiate outward from that antenna. |
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