|
|
|
Is the the automatic regulation (electronically) of the gain of a receiver in inverse proportion to the received signal strength. This allows, within certain limits, the audio output of a receiver to remain relatively constant over a range of fading signal conditions.
Where the "phase" of the current amplitude varies with time. One complete cycle occupies 3600 irrespective of amplitude (visualize a circle). The number of these cycles-per-second is the frequency of the signal.
For mathematical reasons this is referred to as a sine wave. A signal may commence at 00 then go to its most positive value at 900 then recede back to zero value at 1800and continue to its most negative value at 2700 and then turn back to zero again at 3600. This is then one complete cycle.
Perhaps the most common frequency around a home is our power mains. In Australia the frequency used for power mains is 50 cycles per second or now referred to as 50 Hz. The abbreviation is an acknowledgement to Heinrich Hertz. In the U.S.A. and other parts of the world the mains frequency is 60 Hz.
With a 50 Hz mains frequency one cycle occupies 1 / 50th of a second or 20 milli-seconds.
Therefore the signal is most positive after 5 milliseconds, back to zero after another 5 milliseconds, down to its most negative after the next 5 milliseconds and finally back to zero after a final 5 milliseconds. This whole cycle occupies 20 milliseconds or 20 mS and repeats 50 times a second.
With a 60 Hz mains frequency of course one cycle occupies 1 / 60th of a second or 16.67 milli-seconds.
A.C. at audio frequencies extends from 20 Hz to about 20,000 Hz or 20 Khz. Depending upon your age you will not actually hear it beyond 15 Khz and older people are unable to hear much beyond 10 Khz. Animals can hear much higher frequencies. The audio A.C. frequencies are referred to as A.F.
Signals beyond those above are referred to as radio frequencies ( R.F. ) and generally cover the spectrum:
L.F. - 30 Khz to 300 Khz although there are signals transmitted well below this region principally the OMEGA navigation network.
M.F. - 300 Khz to 3 Mhz which mainly includes the A.M. radio band of about 530 Khz to 1650 Khz (varies between countries).
H.F. - 3 Mhz to 30 Mhz and comprises amateur radio, short wave broadcasters among a host of others. Largely becoming superseded by satellite transmissions.
V.H.F. - 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.
U.H.F. - 300 Mhz to 3 Ghz this band is occupied by U.H.F. T.V., some radar installations, mobile phones, two way radios and a heap of other stuff.
Beyond 3 Ghz is virtually satellite transmissions.
It is interesting to note by way of numerical comparison that firstly, each band is 10 times the previous band. Secondly the L.F. band spanning 30 to 300 Khz could be duplicated 10,000 times over in the space occupied by the U.H.F. band.
Also at the bottom end of 30 Khz the signal cycle repeats 30,000 times a second. At the top of the U.H.F. band the signal cycle repeats 3,000,000,000 times a second (mind boggling?).
A very important attribute of A.C. (e.g. 50 / 60 Hz) is that it is generally easy to convert voltages with the aid of power transformers.
Now we have learnt above about audio frequencies A.F. and also about radio frequencies R.F.
In the early days of what is now known as early radio transmissions, say about 100 years ago, signals were generated by various means but only up to the L.F. region.
Communication was by way of morse code much in the form that a short transmission denoted a dot (dit) and a longer transmission was a dash (dah). This was the only form of radio transmission until the 1920's and only of use to the military, commercial telegraph companies and amateur experimenters.
Then it was discovered that if the amplitude (voltage levels - plus and minus about zero) could be controlled or varied by a much lower frequency such as A.F. then real intelligence could be conveyed e.g. speech and music. This process could be easily reversed by simple means at the receiving end. This is called modulation and obviously in this case amplitude modulation or A.M.
This discovery spawned whole new industries and revolutionized the world of communications. Industries grew up manufacturing radio parts, receiver manufacturers, radio stations, news agencies, recording industries etc.
There are three distinct disadvantages to A.M. radio however.
Firstly because of the modulation process we generate at least two copies of the intelligence plus the carrier. For example consider a local radio station transmitting on say 900 Khz. This frequency will be very stable and held to a tight tolerance. To suit our discussion and keep it as simple as possible we will have the transmission modulated by a 1000 Hz or 1Khz tone.
At the receiving end 3 frequencies will be available. 900 Khz, 901 Khz and 899 Khz i.e. the original 900 Khz (the carrier) plus and minus the modulating frequency which are called side bands. For very simple receivers such as a cheap transistor radio we only require the original plus either one of the side bands. The other one is a total waste. For sophisticated receivers one side band can be eliminated.
The net effect is A.M. radio stations are spaced 10 Khz apart (9 kHz in Australia) e.g. 530 Khz...540 Khz...550 Khz. This spacing could be reduced and nearly twice as many stations accommodated by deleting one side band. Unfortunately the increased cost of receiver complexity forbids this but it certainly is feasible - see Single Side band.
The second disadvantage is half the transmitted power is in the carrier (900 Khz in our example) and 25% is in each side band of which we only need one. For a commercial radio station transmitting at say 20 Kw of power, about 15 Kw is wasted but for them this is no great burden because availability of cheap and simple receivers for the listener is of far greater importance.
The third disadvantage is that whilst the signal is amplitude modulated, common forms of radio interference are also amplitude in nature. Examples of such interference to radio reception are natural phenomena such as electrical storms etc. (QRN) as well as man made noise (QRM) which can emanate from nearby electrical appliances, lights, electric drills or even the humble electronic calculator and most probably your computer.
To get away from this amplitude affect by noise F.M. Radio was devised.
Ever since radio transmissions first began there were experimenters and tinkerers'. Indeed even today a great many of the advances in radio science continue to come from this band of people.
Most are now called Amateur Radio Operators and to prevent total chaos each would-be operator sits for modest examinations set by the laws of his or her region to gain a licence.
You will find amateurs are courteous, helpful, constructive (in more ways than one) and always have a warm welcome for newcomers. If you need help just politely ask.
Amateur Operators have assigned bands and modes of operation. They also observe certain standards of etiquette and ethics. Amateurs play a significant role in providing communication links when needed, particularly in times of natural disaster. It is a wonderful fraternity which over-rides the boundaries of nationality, politics and religion.
Virtually every country in the world has an umbrella amateur radio organization. In the U.S.A. it is the American Amateur Radio League (A.R.R.L.), Great Britain has the Radio Society of Great Britain (R.S.G.B.) and Australia has the Wireless Institute of Australia (W.I.A.).
If you are a newcomer to radio and are keen to pursue it as a hobby then contact the amateur radio organization for your region. Help is available everywhere.
This is the unit of electrical current flow. The rate at which current flows past a given point. - see recognized texts.
A passive network comprising usually, but not always, resistors that reduces the power or voltage level of a signal without introducing significant distortion.
See amplitude modulation.
A generalized expression meaning morse code transmission.
This is a relative power unit. At audio frequencies a change of one decibel (abbreviated dB) is just detectable as a change in loudness under ideal conditions.
For a given power ratio the decibel change is calculated as:
dB = 10 log P2/P1
If we used voltage or current ratios instead then our formula becomes:
dB = 20 log V2/V1
Direct current is where at all times the voltage polarity remains constant. Unlike a.c. there is no varying cycle.
D.C. may however, particularly where it is rectified from mains a.c., contain residual a.c. superimposed or part of the voltage. This is often referred to as mains hum.
A means or circuit designed to convert amplified R.F. energy into recovered audio which contains the desired intelligence.
A dipole is an antenna. It is a fundamental form of antenna consisting of a single wire whose length is approximately equal to half the transmitting wavelength.
The length of a half wave in space is approximately:
length (metres) = 150 / Freq (Mhz) or length (feet) = 492 / Freq (Mhz)
The actual physical length in practice is slightly different from this owing to other factors.
A most popular dipole known to almost everyone is the folded dipole which forms part of most V.H.F. T.V. Antennas.
D.X. is a short hand way of saying 'long distance'. In the early days of radio a lot of short hand was devised to minimize morse code transmissions.
A dx'er is rather like an angler who has gone fishing. The angler seeks a catch of the biggest fish whilst the avid dx'er seeks the elusive 'long distance' contact.
Whether he/she be an amateur radio operator, short wave listener (s.w.l.'er) or even an a.m.b.c.b. dx'er (a.m. radio broadcast band dx'er}. Absolutely fascinating!.
Frequency modulation was devised to overcome the problem that A.M. reception was susceptible to noise interference.
With F.M. instead of the carrier having its amplitude modulated the signal frequency is varied or controlled by the modulating (audio) frequency.
In the receiver the signal undergoes a great deal of amplification where the tops and bottoms are chopped of the signal - this is called 'limiting'. By limiting the amplitude of the signal all a.m. components (including noise) are thereby removed. This is why F.M. is preferred for quality music transmission. On the downside it tends to occupy greater bandwidth although narrow band F.M. does exist for two-way communication.
Commercial F.M. broadcasts occupy 200 Khz channels throughout the 88 - 108 Mhz band. This compares with the 10 Khz (or 9 Khz) channel spacing in the a.m. radio band or short wave broadcasting.
3 Mhz to 30 Mhz and comprises amateur radio, short wave broadcasters among a host of others. Largely becoming superseded by satellite transmissions.
Tuned circuits in radios have one severe limitation - bandwidth. Without going into a complex explanation let us assume that the best response can be about 2% of the signal frequency. In the early days of a.m. radio, circuits simply tuned straight across the frequency band of interest.
Applying our 2% rule we find at say 540 Khz, the bandwidth is 10.8 Khz. We would be able to receive this signal without a great deal or little interference from adjacent channels. On the downside if we wanted to receive a signal at say 1550 Khz our bandwidth becomes 31 Khz or spanning 3 channels. We would have little hope of satisfactorily receiving a signal because our bandwidth also now includes both adjacent channels.
A method of receiving called the 'superhetrodyne' principle evolved.
Here as part of our receiver we have a 'local oscillator' or mini transmitter where the incoming received signal is mixed with the local oscillator. As a result 4 frequencies become available.
Firstly the original signal, (2) then the original local oscillator signal, (3) then the original signal plus the local oscillator signal and then finally (4) the original signal minus the local oscillator signal.
Confused?. Consider this practical example of your little transistor a.m. radio. It is designed to receive about 540 - 1650 Khz. The local oscillator will always tune in tandem with the input section to produce another signal at 995 - 2105 Khz.
At all times the difference frequency is a constant 455 Khz or what is called the intermediate frequency or I.F. All other frequencies arising from this process are then filtered out.
When you tune your radio you are actually tuning the local oscillator which is more correctly called the 'V.F.O.' or variable frequency oscillator.
Because we always have a constant difference frequency of 455 Khz it is relatively easy to design and construct narrow band circuits to suit our requirements. It is in these circuits (I.F. Amplifier) that the greatest amplification occurs.
30 Khz to 300 Khz, although there are signals transmitted well below this region, principally the OMEGA naval navigation network.
Assuming you had 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.
Depending upon which side band is chosen to transmit one is called upper side band and the other is called lower side band. In amateur radio, conventions exist as to which side band is transmitted in a particular amateur band.
300 Khz to 3 Mhz which mainly includes the A.M. radio band of about 530 Khz to 1650 Khz (varies between countries).
This is what I consider the first and most fundamental lesson in electronics.
I sob every time a licensed electrical contractor contacts me for help over some electronic device and he doesn't know the basics. Do they sleep through the first semester?.
I will keep this dead simple. I won't introduce any complications. Everybody capable of reading will understand it and hopefully never, ever, ever forget it. I will not give highly technical definitions to confuse the newcomer.
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]. Now how basic can you get? Easy to remember!.
Equally easy to remember is this formula:
E = IR which of course is E = I * R
If that causes mental difficulty in remembering then think 'EIR' or "Elizabeth the First Regina"
If you re-arrange that formula algebraically you also get:
R = E / I as well as I = E / R
Now guys and gals is that simple or what?
An Example: A 12V battery has a 6 ohm resistor (rated at 50 watts) put across its terminals. How much current will flow? Well I = E / R which is I = 12 / 6 or 2 amps of current. See dead easy.
Now with power calculations, and this is all at D.C., because other factors become involved in A.C. power calculations if we are not talking about pure resistance e.g. power taken by a motor.
Basic formulae again:
P = E2 / R or P = I2 * R or P = E * I
In the example above if we substitute 12V, 6 ohms or 2 amps we will always get 24 watts of power consumed in the resistor (as heat dissipation).
Very practical examples of this using a.c. power and pure resistance are: electric jugs or kettles, electric radiators for heating and electric toasters.
Piece of cake wasn't it?. Pat yourself on the back
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!.
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.
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. I had only a passing interest in this aspect of the hobby and that was about 25 years ago. As to what is happening today I don't know but I suspect it's a fair bet that computers and packet radio (bit like the www) have overtaken it.
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 equipment is home made (home brewed).
Personally I think it is the last frontier in any hobby in a world that has gone mad with "buy the latest-greatest-all singing-all dancing ready made gizmo's". It takes real talent to be a classy Q.R.P. operator. Unfortunately I wouldn't count in that category. "Those who can do! - those who can't teach!" <grin>
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.
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 below 30 Mhz because the external noise combined i.e. natural and man made will always exceed the noise figure of the receiver.
Visualize 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 rough appreciation of the topic.
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 9000 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.
This 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.
Contrary to popular belief it is not the holy grail. Whilst everyone should strive for technical excellence as well as efficiency there is absolutely no reason to slash your wrists because you can't get an ideal S.W.R.
In fact I can personally 100% guarantee that the sky will not fall in on you. What is acceptable depends on many things including your site, set up and circumstances. Efforts and expense to achieve a perfect S.W.R. are frequently all totally out of proportion.
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 vwsr"
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.
Be technically efficient not paranoid.
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.
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.
I guess the only limit is man's imagination.
see Oscillator
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.
Identical but the direct opposite to L.S.B. (Lower Side band) - see also A.M. and S.S.B.
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.
The fundamental unit of power consumed see Ohms Law
Originally in radio, the frequency of signals was not mentioned. The custom was to refer to the 'wave length'.
This is easily computed from:
300,000,000 / Frequency (cycles) or 300 / Frequency (Mhz)
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).
Just how the custom of talking in wavelength originated I have never been able to establish. I suspect it had a lot to do with transmitting antennas where the calculation is quite critical.
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.
Collin's Home Page Colin's Amateur Radio Page Colin's Snooker Page Colin's Guest Book Contact Me About Me
This Web page was last updated on Friday September 21, 2001
© 2001 Designed by Colin McCord