The most basic component of radio wave transmission is the oscillator. An oscillator is an amplifier circuit with a controlled feed back path. It can be said that an oscillator is a regenerative device though it will need a power source to continue oscillating due to power losses in ineffecient Xsistor and tube devices as well as resistors and other lossy components.
The oscillator has a resonant portion of it's circuit which is used to determine the frequency of the oscillator. These components can be what is known as an LC tank circuit or a crystal resonator which has properties of both a capacitor and inductor and a vary small resistance We will cover that topic later.

Once an oscillator begins to oscillate or feeds back on its self than it has become a transmitter. That's right it can be detected by a radio receiver. An oscillator will fit into the catagory of what is know as a CW (continuous wave) device. It Xmits a single frequency while it is on.
But what constitutes a Xmitter? I mean in actual practice you want to send a signal further than your neighbors house. Simple, a series of amplifiers in a string each multiplying the power from the previous stage.

There are some rules to adhere to in developing these amplifiers. First off, remember the oscillator? The oscillator is the single source of frequency generation, therefore it must be handled with care. The LC tank circuit discussed previously is subject to all sorts of conditions that can alter it's frequency. This is not so substantial in a Xtal oscillator but still exist none the less.
So how do we protect an oscillator from drifting or being pulled off frequency? We use buffer stages. Buffer stages are extremely important in radio design. What is a buffer? The best buffer amplifiers have less than unity voltage gain. Ah huh? We are talking about an emitter follower or common collector amplifier.
Why are they the best? Because they have a very high impeadance input therefore not loading the oscillator to heavily. The input impeadance is basically R1/R2. It also has a low output impeadance of roughly RE which is a good match to a CE amplifier which has a high voltage gain. Another advantage of an emmiter follower amplifier is that it is a current amplifier and therefore is a good current source. Now if you are familiar wth a darlington pair which is easily made using two Xsistors, we find that the current is beta X beta of the two Xsistors. The net result is you will obtain good isolation between the oscillator and amplifier stages. This also helps to keep the oscillator from feeling phase changes from the output stages which can most definately effect the oscillator frequency.
Below are some examples of buffer amplifiers

Let's step back a minute and discuss the differences between an LC tank circuit and a crystal which will be abbreviate Xtal. Assuming we understand that a C (capacitor) stores an electro-static charge and an L (inductor) stores an electromagnetic charge, without going into too much detail we find that these opposing fields transfer energy back and forth at a given rate detemined by the value of the components. These LC components can be notorious for drifting off frequency with temperature being the primary culprit. Now an Xtal is less subject to these conditions and has a much higher Q (quality factor). The Q is determined by X/R or the amount of reactance to resistance. I'm not going to delve into the subject of reactance because I want to discuss radio in general. You can look up the subject of reactance.

__
XL= 2 ||  f L                    Inductive reactance

1
XC=  ___________      Capacitive reactance
__
2 ||  f C

1
fo =    ____________      Resonant frequency
__    ______
2 ||  \/  LC

Now to complete what is known as a CW Xmitter you will find that these buffer stages must be added at various parts of the amplifier string.
OK CW is fine but there is no intelligence on the signal.Well if the Cw transmitter is keyed on and off by applying power to the amplifier stages than we have what is known as Morse code Xmission. We will need a special type of receiver to interpret the CW transmissions though. This receiver will have a BFO (beat frequency oscillator) which applies a substitute carrier to beat against the in coming carrier and produce a beat note. We will discuss receivers later. But just so you know, a carrier is the CW signal being transmitted from the transmitter. It is the frequency or unmodulated RF signal.

Ah modulation... Modulation can come in various forms but it usually canotes vioce communication or intelligence. How do we produce modulation? I'm not going to go into high level and low level modulation. The most common method of producing modulation takes us back to the all important oscillator. We can amplitude modulate or frequency modulate the oscillator. We do this by applying an audio signal such as the audio that goes to the speakers of your stereo etc. Now of course we usually input modulation from a microphone. Then we amplify the signal from the microphone to the oscillator.
It is the way that the modulation is applied that distinguishes the type of modulation being used, either AM (amplitude modulation) or FM (frequency modulation). If we apply the amplified audio signal to say the bias of the oscillator circuit or the bias of a successive amplifier, the audio will be super-imposed onto the output by way of amplitude. In other words a loud audio voltage level will cause the amplifier to increase its output respectively.
In FM we input the same audio signal but, instead of changing the amplifier output level we will vary the tank circuit frequency.
This can done by various methods the most common being the use of a varactor diode. Because varactor diodes are used in virtually all PLL circuits it has become the dominate method today. Now some will argue that using a varactor to change the frequency is not true FM and is actually PM (phase modulation) and they will say that true FM sounds better. Well that maybe true but they are detected the same way in a receiver and in narrow band communication services designated by FCC to be 5KHz band width or less, you cannot tell the difference.

There are many techniques used to improve the quality of an FM/PM signal. One such method is know as pre-emphasis/de-emphasis. This has to do with the fact that a varactor tends to be more responsive to higher frequencies of the audio signal. This can make the audio at the receiver sound kind of tinny like talking through a tin can. A simple low pass filter at the Xmitter end and a high pass at the receiver, puts it all back in unison.
Another method commonly used method to increase the modulation level is known as a "compander" (compressor/expander) system. This is sort of an AGC (automatic gain control) circuit that prevents the audio from exceeding a certain band width by controlling the audio level. The receiver than expands the audio back to its original level giving it a fuller sound.

bare with me guys...it's a way from being done and just tryin to clear up a few things here for the group.
?  ?

edit: added some spacing to make it easier to read (nice post :) )
Posted on 2006-08-01 22:03:37 by mrgone
I will have two break this up into sections because I can't keep redoing formulas and indentations etc.

Even though there are really only two types of modulation, AM & FM, they can be broken down into sub catagories. Some of these catagories would be: Spread spectrum, Single Side Band (SSB), double side band and various sub carrier techniques.
Since we have recently discussed AM I would like to continue with a form of AM known as single sideband. Before we can grasp the subject of SSB we must understand the basic components of an AM signal.

As you you now know that a carrier is simply the oscillator frequency being Xmitted through the amplifier string and is the basic rf signal with no modulation. So what happens when we apply audio AM modulation to the oscillator carrier. Well the basic components of an AM transmission are:
1. The carrier
2. An upper side band signal and
3. A lower side band.
The upper and lower side bands ride on the carrier and are the upper and lower frequencies outside the carrier. Typically a carrier will be approx. 5KHz wide with the upper sideband being another 5KHZ and the lower sideband being another 5KHz bringing the total bandwidth of an AM signal to 15 KHz.
These signals can actually be observed on a spectrum analyzer as a large peak in the middle with two smaller peaks on either side of the carrier. When 100% modulation is used than 50% of the energy will be in the carrier and the other 50% will be in the two sidebands. So on a spectrum analyzer the two sidebands will be half the amplitude of the carrier.

An interesting thing to note here is that the carrier its self still has no intelligence. The intellegence is in the upper and lower sidebands. One day a very smart ham radio operator said to himself, "Ya know, the same intelligence that is in the upper sideband is also contained in the lower sideband." What if we could shave off the upper or lower sideband and send only one sideband. (Today this is called SSB reduced carrier)

But let's not get distracted here. Well if we can eliminate one of the sidebands and the carrier which contains no intelligence, than why not get rid of it too. So we will now send only the upper or the lower sideband which contains all of the intelligence of the original audio signal. Presto! SSB.

What is the advantage of SSB? Well obviously we have reduced band width which allows room in the radio spectrum for more communication services. If the AM signal was 15KHz than it is now 5 KHz or less.
Another very interesting point to ponder is that it is extremely power efficient. Why is that you say? Because the carreir is always putting out RF energy, but the sidebands only exist when there is modulation. If the radio announcer stops speaking, than there are no sidebands. Only a dead carrier. So if we are sending only one sideband out of our transmitter, there will be no power out until voice modulation is applied.
This can be observed on a simple power output meter. You will actually see the needle bounce up and down according to the level of modulation. If the operator stops speaking than there will be zero output from the Xmitter. That's efficiency!

Posted on 2006-08-02 13:31:39 by mrgone
Pat of the reason I wanted to do this tutorial is so we can group some of this info that  is spread around into one location. So I'm sure you have seen some of this recently.

Antenna Basics:

Granted a "long wire" which is a term used for random length of wire will receive all freqs., an antenna will be resonant at some frequency depending on it's length. This is not crucial in receiving a radio signal but is crucial in a transmitter application. The antenna must be cut to resonance or you will have very high SWR (Standing Wave Ratio) which simply put you will have a bad impeadance match and the antenna will reject the RF frequency energy and will send it back to the transmitter amplifier which can quite easily burn up the final amplifier. A resonant antenna will also improve reception at a particular band of frequencies.
Radio waves & light travel at a speed of 300 million meters per second. From this you can calculate the length of the antenna by dividing the frequency in megahertz in to 300 to abtain the length of a full wave antenna. Example 300/7 = approx. 41 which is why the ham radio guys refer to 7MHz as the 40 meter band. The transmissin line also refered to as coax cable "if using an industry standard cable" is always usually 50 ohms. It's been found that a half wave antenna is a very good resonator and if the antenna is fed at the center, this will be it's lowest impeadance point giving a close match to the 50 ohm transmission line. The voltage swing of the rf signal is lowest at center.
A cheap and dirty method of calculating a half wave antenna length also known as a DIPOLE antenna, is to divide the frequeny in megahertz into the magic number of 468 to get the length in feet. The 468 number is the 1/2 wave length inside a wire. The actual 1/2 wave length in free space is obtained by the number 492 for foot calculation. A frequency wave length is actually shorter inside a wire.
All of these concepts can be applied to HF,VHF & UHF antennas. Many times at these frequencies we are concerned with even more filtration as well as amplification in the antenna its self. These type antennas are called beam antennas. A beam antenna consists of a driven element which is the dipole that we have described and is the element which is fed directly by the Xmission line. It is always a half wave length though often times you might see loaded coils on the ends or near the ends which effectively simulate a full half wave antenna even though physically it is much shorter than a half wave length. An inductor will appear elecrically to add to the length, and a capacitor will effectively shorten the length. The capacitors are a little different than a conventional capacitor. They are usually air dielectric and can be easily made. I have taken beer cans and smashed them into a round flat disc. Then drill holes in the center of the discs and slide them over the ends of the wire and seperate them with some tape so they don't move. The loading coils can be some heavy gauge mabe no 12 or thiscker wire wound around a PVC pipe or a toilet paper roll and attached to the ends of the wire.
Now why is it necessary to add loading coils you ask? Well if your antenna is 40 meters long it may be a little difficult to rotate it. Thi is the point of a beam antenna. A simple beam antenna wil be 2 or 3 elements. We already discussed the driven element. You will also need a reflector. The reflector is placed behind the driven element perpendicularly and is slightly larger the the driven element. It can be at some division dimention of distance behind. There are many theories to this but the key factor is that the energy either Xmitted or Rcvd will be reflected back toward the driven element. Now we have signal adding or aiding each other. Any other elements will go in front of the driven element and they are refered to as directors. So if we add more directors we will sharpen the radiation lobes coming from the antenna.
Now at SHF and above, everyone is famaliar with the common dish antenna. The dish is designed as a mathematica portion of a parabola. That is why they are known as a parabolic dish. We find that at a gven distance fron the center of the partial parabola, we have an optimal focal point. That means that the energy bouncing off the inside of the dish will all converge at a single point. This in a sense is a form of amplification although a dish antenna usually has an active amplifier in the focal point. This is know as an LNA or Low Noise Amplifier. It is designed to have an extremely high signal-to-noise ratio to pump very week signals from outer space into the transmission line.

Posted on 2006-08-06 21:42:33 by mrgone

?  ? Now why did we go through all this stuff about SSB when we already covered most of it earlier and what does this have to do with receivers? Well first of all we discussed the efficiency of SSB. A typical SSB transmitter will have a 9 DB gain over an AM transmitter with the same peak envelope power. So we want to make sure that we know how to receive this very important type of transmission.
?  ? So let's now go back to our receiver. If we stick with our most basic single conversion receiver, we remember there is a local oscillator and the RF signal coming in from the antenna being heterodyned in the mixer. Well remember the SSB Xmission has no carrier so we must insert it back in, inside the receiver. So one good place to do this would be in the mixer. We will choose a frequency of 1.5 KHz or higher (audio frequency) than the frequency of the Xtal filter for upper sideband and 1.5KHz or lower than the Xtal filter for lower sideband. With a sharp bandpass Xtal filter of say of say 3 MHz we will receive only the desired sideband which is now intelligeable do to the inserted carrier. The carrier oscillator is known as the BFO (Beat Frequency Oscillator).?  A BFO is also required for CW reception because if you remember, CW (Continuous Wave) is only a carrier so we need to ave the BFO to create a heterodyne beat note that we can hear. By varying either the 1st local oscillator or the BFO we can change the beat note for both CW and SSB. If you zero beat the BFO to the carrier frequency than you will hear neither.
Posted on 2006-08-07 00:35:54 by mrgone

Posted on 2006-08-10 21:47:12 by mrgone
Broad Band amplifiers & RF Power AMPS

A word about Linear Power Amplifiers

Getting back to broad band ampifiers in general. If we look at the 2nd diagram below we see that the rf signal is capacitively coupled to the first stage. Notice the collector now has a step down transformer. This is what must done when we begin to develope significant power in a primarily transmitter amplifier string. When the power approaches values of 100 milliwatts or less you will need to couple using Xformers especially in broad band application. The Xformer here could be an Amidon ferrite toroid core for example at hf frequency you might use an FT-50-43. The turns ratio will depend upon the impeadance match between stages. Notice it is a step down Xformer. The windings will be say no. 30 guage enameled wire with for example 30 turns on the primary and the secondary is usually wrapped right over top of the primary and may be say 10 turns. We want to make sure that the polarity is the same for both primary & secondary. That means that the end of the primary that goes to the power source will be the same end that the secondary uses for the ground because in AC as we remember the positive and negetive are both effectively ground connections. We also notice in the two Xformers that there are no capacitors in the collector leg because we want to keep the amplifiers broad band. The ferrite toriods are capable of offereing the high impeadance needed without forming a frequency selective LC tank circuit. Once again the toriod by the nature of its shape offers very resonalble self shielding charateristics therefore giving lea way to non sheilded design if proper PC board techniques are applied.
Posted on 2006-08-17 09:24:57 by mrgone
To the Ineffable All,

Speaking of oscillators, my favorite is the Wien-Bridge oscillator.  It is easy to build and needs no special components like split coils or ganged capacitors.  If fact it only needs capacitors and resistors for its frequency determining network.  And the frequency is easily variable.  Good distortion specs also.  There's lots of links and literature on this oscillator.  Ratch

http://www.zen22142.zen.co.uk/Design/wien_osc/Wien-Bridge%20Oscillator.htm
Posted on 2006-08-19 08:23:36 by Ratch
Well the link you provided shows that it used to produce audio frequency. You want to stick with coils in the RF spectrum. I personally would not use an OP Amp in any radio frequency oscillator applications. The key to a good stable oscillator is low power consumption. This also applies to VCOs, and the same priciples should be adhered to though a PLL is more forgiving.
Posted on 2006-08-20 20:24:55 by mrgone
mrgone,

Yes, coils become more effective and smaller at higher frequencies.  I submit that the key to a stable oscillator in not power consumption, but a stable feedback loop that controls the oscillation frequency.  For instance, microwaves and radars consume and emit large amount of electrical energy in a short time, but they are stable.  Ratch
Posted on 2006-08-21 06:53:46 by Ratch

Posted on 2006-08-21 08:43:27 by mrgone