# UK Amateur Radio ## Foundation Licence 2002
### Transmitters and Receivers Part A
### Radio Transmitter
Simple block schematic diagram or "concept" diagrams of a transmitter.
This diagram will be used for all assessment questions. The blocks are not intended to relate directly to any particular Transmitter but indicate the basic functions that a transmitter need to perform. You need to know the various relative positions and what each past is named. 1= Audio Stage 2=Modulator stage 3=Frequency generator stage 4= RF power amplifier stage. The term stage is used to indicate a complete section of the transmitter.
This is the symbol used to indicate a microphone also know as a mic. |
This is the symbol used to indicate an aerial or antenna (the two words mean the same) |
Technical requirement of radio transmitters Inside a transmitter as can be seen from the block or concept diagram above is a Frequency generator stage. This part may in fact be one or more Frequency generator stages or Oscillators. It is the oscillator(s) that provides the overall stability of the generated frequency at which the Transmitter will radiate its signal to the aerial. By looking up in the Band Plan you can find out the limits of the operating frequency of the amateur bands so that you do not tune the transmitter outside those limits and cause interference to others. For the Foundation Licence you are only allowed to operate commercially available equipment or commercial kits built strictly in accordance with the instructions. At Foundation Licence level the truely "Homebrew" transmitter is not permitted to avoid the clear risk of accidental (inadvertent) operation out of amateur bands and also to prevent EMC problems. In the concept diagram above the microphone is use to provide the signal to be "modulated" onto the Frequency "Carrier" (the frequency carrier is generated in the Frequency Generator stage mentioned above) in the Modulator stage.
### Modulation modes |
| In the picture to the left the top wavy line represents you talking into the microphone. Infact it would only look like that if it were a single tone from an audio oscillator. This is the Audio Waveform Below is the Carrier Waveform generated by the frequency generator or carrier oscillator. It is many times higher in frequency than the audio signal The bottom one is your voice superimposed upon the carrier and is called the Modulated Waveform. Note that in this case the carrier and the modulated carrier look very similar. What has been altered is the frequency of the modulation and is called Frequency Modulation or FM
| Can you spot the difference in the two sets of diagram. The top two and the middle two are the same only the bottom ones vary. The one looking a bit like a fish has had the amplitude of the carrier modulated and is called Amplitude Modulation AM.
You will need to be able to distinguish between these two diagrams of FM and AM as possible written assessment questions will use the diagrams
For voice transmission we can use FM and AM and a new one to you SSB (Single Side Band). On your radio at home that plays music you will already have heard FM, and AM, but SSB is used by amateurs and sounds a bit like a distorted voice.
### What do Amateur Radio communications sound like ? The next section has clickable icons to play sounds to you. You must have a suitable sound card and speakers attached and turned on to hear the sounds.
| Click on the buttons to hear typical sounds on the amateur bands. The files take a short while to load so be patient. The first FSK is RTTY whilst the second is Amtor which about halfway through has a second signal appear close by.
Data transmissions can be made by CW (continuous wave telegraphy) and FSK (Frequency shift Keying). A data transmission is any other type of transmission that is not speaking whether it is Morse code: Packet: RTTY: PSK31: Fast scan TV: Slow scan TV:
| So let us recap:- Do you know what stage 1, stage 2, stage 3, and stage 4, are individually called? If not then check out the top of the page as we are going on to discuss the transmitter a bit more. What is this the symbol for ?? What is this the symbol for ???
Ok - so you now know all the parts in the block or concept diagram or you have them written down on a sheet of paper. We have dealt with stages 1,2,3, and now we come onto stage 4. This stage is where the signal is amplified it what is called the RF Power Amplifier Stage and is also know as the Final Stage as it is the last point of the Transmitter before the aerial. Before we transmit a modulated signal (speech for instance) into the aerial we must know that the transmitter is properly connected to the aerial and that the aerial is the correct one for the frequency that the transmitter is set to. We need to know that the transmitter is seeing the correct load at the end of the feeder. The aerial is the load just as is a dummy load. At this point if you have not already looked at the section on Feeder and Antenna then now is the time to do it else you will not understand what I am talk about. Click here to be taken to the section. Ok so now you are aware of the terms aerial and dummy load. To check that the correct aerial load is "shown" to the transmitter a very small amount of power is fed to the aerial via an SWR meter and ATU, for sufficient time to judge the SWR (more about this in the section on Feeders and Antennas). If the match is good then no adjustment of the ATU is required. Else adjustment is made and another check is made and so on until the "best match" is achieved. When you are satisfied that the aerial is correctly matched for the transmitter frequency it is safe to operate. Never operate if you think things are wrong. The use of the wrong antenna can result in damage to the transmitter and also you could cause interference to others. Whether you are using AM or FM driving the microphone too hard (that is speaking too loudly into the microphone) may cause a distorted signal due to OVER MODULATION on AM or OVER DEVIATION on FM. The microphone gain control ( where fitted) need to be correctly adjusted. The effect will be to cause interference to adjacent channels. This correct use of the microphone drive control is particularly important when using a packet controller (TNC); or when a computer sound card is used to drive the microphone input for say RTTY PSK31 SSTV etc.

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