Bredhurst Receiving and Transmitting Society
4.Transmitters and Receivers
|Transmitters and Receivers
4a Simple block or "concept" diagrams of transmitter and receiver. See Table 2 below.
4a.1 Identify the items in a simple transmitter block diagram and recall their order of interconnection:
Microphone, audio (microphone) amplifier stage, frequency generation stage, modulator stage, RF power amplifier stage, feeder and antenna.
Below is shown a Simple block schematic diagram or "concept" diagrams of a transmitter.
This diagram is from the tutor notes for the course and would be used for any assessment questions on this topic. The diagram is not intended to relate directly to any particular transmitter it is intended only to show the basic functions that are performed within a transmitter.
Each part of the transmitter is called a "stage" hence the Mic is connected to the Audio stage and so on.
1= Audio Stage. The very low signal from the microphone (mic) is amplified to the required level and the passed onto the next stage.
2= Modulator stage mixes the signal from the Audio stage with the Frequency generator to give the signal that the last stage amplifies.
3= Frequency generation Stage. This generates the frequency to be transmitted and is mixed in the modulator with the audio stage in readiness for the amplifier stage.
4= Power Amplifier stage. This amplifies the small signal from the modulator and send it via the feeder (usually coax but not always) to the antenna.
|4.b Technical requirements of radio
4b.1 Recall that the frequency generation stage(s) (e.g. oscillator(s)) in a transmitter defines the frequency on which the transmitter operates.
Recall that incorrect setting of these stages can result in operation outside the amateur band and interference to other users.
From above Stage 3 = Frequency generator stage. This is the stage that you have control of as the operator as it is the part of the rig that is associated with the tuning dial and display of the transmitter. The frequency is created by an oscillator or series of oscillators. It is these oscillators which define the working frequency which ever mode is being used.
If this stage creates frequencies incorrectly, possibly by being badly built then the output from the transmitter will also be wrong. It is this stage that you should check when you test that the transmitter is on the right frequency with say a frequency counter or general coverage receiver.
The frequency generated could be so wrong that Operation out of band of designated frequencies for amateur use could occur, and such operation could cause interference to other users of the radio spectrum.
Thus operating telephony (speech) near to the band edges even with a transmitter working fully to specification could result in you operating out of band.
4b.2 Recall that the audio (or data) signal is modulated onto the radio frequency 'carrier' in the modulation stage of the transmitter
2=Modulator stage. This stage mixes together the signal from the Audio stage and the Frequency generator stage in preparation for amplification in the 4th Stage.
Recall that modulation is by varying the amplitude or frequency of the carrier, resulting in AM or FM modulation modes.
The modulation stage determines whether the output signal will be AM or FM depending whether the amplitude is changing hence Amplitude modulation or the Frequency is changing hence Frequency Modulation.
Recall that speech can be carried by AM/SSB or FM, and that data may be transmitted by means of suitable audio tones generated in a radio modem or TNC (terminal node controller)
Depending upon whether the modulator varies the amplitude or the frequency of the carrier will the output be AM or FM
AM is the variation of the Amplitude where as FM is the variation of the Frequency.
Speech can be carried on AM, SSB or FM.
DATA can also be carried on AM, SSB or FM but requires that the data is passed through a radio modem ( modem = modulator / demodulator) to change the 0 or 1 of the data code into two different tones.
A particular type of data mode is called "Packet" and this requires a TNC or terminal node controller which converts the typed letters from a computer into data - 0s and 1s and then put them together into "packets" which are sent at high speed as bursts of data.
4b.3 Identify drawings of an RF carrier and amplitude modulated, frequency modulated and CW radio signals. Understand the terms carrier, audio waveform and modulated waveform.
Before we look at the composite drawings of both AM and FM let's look as something that is a little simpler.
Whilst the wave form shown is in fact a sine wave we are using is here as a simple representation of an audio signal. An Audio voice signal is in fact a very complex wave form which does not need to be covered in this course.
This audio signal is the same signal input notwithstanding whether the final output being AM of FM.
As with the Audio signal this is a representation of an RF carrier wave and is the same whether the output is AM of FM
Below are two diagrams note the each have the Audio Wave and Carrier Waves which are the same it is only when the carrier is mixed with the audio does the resultant wave form of the RF signal become either FM or AM.
The images of the modulated signals (bottom diagrams) on the left hand side above represent FM and on the right hand side represent the AM.
The diagram below represents a CW radio signal of the letter "A". CW is just the on and off keying of the carrier wave without any audio or modulation.
4b.4 Recall that the power amplifier of the radio signal is carried out in the final stage of the transmitter (RF power amplifier).
4= RF power amplifier stage. This is the final stage and it amplifies the modulated signal and is connected to the antenna connection on the rig. From here usually coaxial connector linked with a coaxial feeder delivers the signal to the antenna.
This is the stage that also controls how much amplification is given to the signal before it is sent to the antenna for radiation.
4b.5 Recall that the RF power amplifier output must be connected to a correctly matched antenna to work properly and that use of the wrong antenna can result in damage to the transmitter.
In the section on antennas and feeder you will learn (or have learned) that an antenna has to be matched to the transmitter frequency. It is this power stage of the transmitter that could be damaged if not connected to a properly matched antenna. This is where the ATU comes in to properly match the antenna and feeder to the transmitter output impedance which is usually 50 ohms.
If you used the wrong antenna or no antenna at all then damage to the transmitter could be the result.
4b.6 Understand that excessive amplitude modulation causes distorted output and interference to adjacent channels.
An excessively high level of input audio signal to the modulator results in an excessive amplitude modulated signal, for instance if you shout into the microphone, then instead of a louder sound of your voice in the receiver being heard in fact only a much distorted voice will be heard and depending upon the extent of distortion may nor may not be understood at all. The additional ADVERSE effect of an excessive input, thus overmodulation is that interference to adjacent channels / frequencies may well occur in an Amplitude modulated transmitter.
The problem of excessive input can be prevented by the CORRECT use of the Microphone gain control where fitted to the transmitter.
Understand that excessive frequency deviation will cause interference to adjacent channels.
With regard to FM much the same occurs but the an excessive frequency deviation which will cause interference to adjacent channels.
Recall the need to ensure the microphone gain (where fitted) is correctly adjusted.
Another item which can cause a problem with excessive input is a different microphone than supplied with the rig as it might have higher output than the original.
4c Simple block or "concept" diagrams of a receiver.
4c.1 Identify the items in a simple receiver block diagram and recall their order of interconnection:
antenna, feeder, radio tuning and RF amplification, detection/demodulation, audio amplification and loudspeaker or headphones.
You need to be able to readily distinguish the various sections of the diagram below
The aerial feeds the signal into the receiver via a feeder.
4d Technical requirements of radio receivers
4d.1 Recall that tuning of receiver is carried out in first stages of the receiver.
The tuning and RF amplification stage selects the wanted signal
The first stage selects (or tunes) to the signal. Just as with the Transmitter the tuning dial is linked to this section and like with tuning a portable radio into the station the same has to occur when you want to listen to amateur radio stations.
This first stage is known as the tuning and RF amplification stage for in addition to tuning into the signal as the incoming signal is very weak it has to amplify it before passing it onto the next section.
4d.2 Recall that detection/demodulation (recovery of the original modulating signal) is carried out in the second stage of the block diagram and audio amplification is in the third stage of a receiver.
The Detection / Demodulation stage recovers the original modulating signal. The detection / demodulation stage of the receiver would need to be correctly set to the mode being received, CW, AM, USB, LSB, FM, RTTY etc so that the signal would be correctly detected.
So we understand that the previous stage can tune and amplify an RF signal so next we need a section that can recover the original modulating signal or DETECT the signal.
Let's put that another way. You may recall that in a transmitter the audio signal ;was changing the RF signal or as it is called modulating the signal so detection is to recover this original audio signal.
You may be able to remember detection by thinking of a policemen who is a detective and the graphic below !
The signal even when detected is still to small to hear so as with a CD player or tape deck the signal is sent to an Audio amplifier and then delivered so that you can hear it to a speaker of ear piece / head phones.
The origin of some of the text and drawings on this page is from the RSGB with additions by the web master