Module 7 - TYPES OF RADIOS & RADIO CIRCUITS
- Reading Assignment
- Radio Communications
- Simple CW Transmitter
- Superheterodyne Receiver
- Demodulation Types
- Quiz 7
- Recommended Reading
Read Chapter 3 pages 3-15 to 3-19 in your text before you continue.
If you have questions after reading the assignment, consult the recommended reading.
Back in Module 2 we briefly covered demodulation & equipment basics. In this module we will look deeper into radios and what makes them work.
It might help to review modulation and demodulation in Module 2 before continuing this module.
A slight review: A basic radio station consists of a receiver, transmitter, antenna switch, power supply & antenna.
If the station antenna is shared between the transmitter and receiver, the TRW switch allows the antenna to be switched to the transmitter when sending and to the receiver when receiving. In a transceiver, this TR switch is inside the unit and requires no attention by the operator.
Just what happens during communication via a radio? When transmitting (sending a signal), information (voice, data, video, commands, etc.) is converted to electronic form. The information in electronic form is attached or embedded (modulate) on a radio wave (a carrier), & the radio wave is sent out from the station antenna into space.
At the Receiving end: The radio wave (modulated carrier) with the information is intercepted by the receiving station antenna. The receiver amplifies the weak signal and eventually extracts the information (modulation) from the modulated carrier wave.
Then the information is then presented to the user in a format that can be understood (sound, picture, words on a computer screen, response to a command).
All this sounds pretty simple, but it in reality is pretty complex. This complexity is one thing that makes ham radio fun, learning all about how radios work, but don't be intimidated. You will be required to only know the basics at this point, but you are free to go on and learn as much about the 'art and science' of radio as you want.
Simple CW Transmitter
An RF signal is developed by the oscillator. This is connected to the driver (actually a low level transmitter) which amplifies the oscillators RF signal. It is called a driver because it is being used to amplify the signal and then apply that signal to another stage (apply called the amplifer) to amplify the signal further. In reality you might use several driver stages before going to the final amplifer stage. It is more stable & less expensive to amplify the signal in smaller steps than to try to attain final output power in just one stage.
This being a CW (Morse Code)transmitter, there is no power going to the antenna until the telegraph key is depressed. Then the full power of the transmitter is applied to the antenna, usually through a T/R switch.
The oscillator determines the frequency the transmitter will be transmitting on and the antenna is designed to work on the band of frequencies the oscillator is capable of being tuned to transmit on.
In electronics, a superheterodyne receiver uses frequency mixing (heterodyning) to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio use the superheterodyne model.
The hetrodyning takes place in a mixer, which is a non-linear amplifer. Whenever you mix two frequencies together you get four results: The two original frequencies, the sum of the frequencies & the difference of the frequencies.
An example would be a cw signal at 50 Hhz. The 50 Mhz signal is mixed with a frequency generated by the local oscillator (tuned by your dial setting) at 50.455 Mhz. The difference of these mixed frequencies is .455 Mhz or 455 Khz. This IF (intermediate frequency) is now amplified by the IF amplifer in preparation for further processing.
If you should like to receive a 51 Mhz signal, you would simply tune your local oscillator to 51.455 Mhz and the difference would again be 455 Khz.
This is known as a single conversion superheterodyne receiver, since you have only performed a 'single' conversion down to a single IF. See figure M7-2 for the block diagram.
If you were to add an second local oscillator and second IF amp, you would have a dual conversion superheterodyne receiver. See Figure M7-3 below. You should be able to do the simple math using the frequencies shown in the figure if you want to actually prove the conversions yourself. Note that the second local oscillator is fixed.
Receiver sensitivity indicates how faint an input signal can be to be successfully received by the receiver. The lower the rated sensitivity level, the better. This means a lower sensitivity rating means better. The sensitivity is expressed in uv (microvolts). A receiver with a sensitivity of .15 uv is better than a receiver with a sensitivity of .35 uv. This the receiver with the sensitivity of .15 uv can hear weaker signals than the one with a sensitivity of only .35 uv.
Receive sensitivity is very important to be able to adequately receive the weak signals associated with amateur radio. We are not looking for perfect readability in ham radio most of the time. We just need enough signal so we can understand what is being said above the noise levels.
The sensitivity of a receiver may be improved by inserting a preamplifier between the receiver antenna output and the receiver antenna input itself, in the above examples it would go before the antenna and the first mixer. If you see a test question the preamplifier simply goes between the antenna and the receiver. The preamplifier cannot amplify all signals no matter how weak they are because if the signal is weaker than the noise that is present, you will not be able to hear or process the signal.
If you were only performing receiving you could place the pre-amplifer outside up at the antenna itself. This way the preamplifier could boost the signal when it was stronger and compensate for the loss of the feedline between the antenna and the receiver. (as in a commercial TV station signal) Most of the time this is not practical for a Ham, because we are also transmitting so the preamplifier would be damaged when you transmitted.
Selectivity is a measure of the performance of a radio receiver to respond only to the radio signal it is tuned to and reject other unwanted signals nearby in frequency.
Selectivity is required to ensure that only the wanted signal is received. Front end tuning (at the mixer or preamplifier stages) or selectivity tends to be wideband in nature, or variable to enable the receiver to cover the complete band of desired frequencies.
In a superheterodyne receiver, the main selectivity is provided within the fixed frequency IF stages. Here tunable band pass filters are able to provide very high degrees of selectivity ranging from what is required for wideband transmissions including wideband FM, down to very narrow band transmissions occupying a few kilohertz including single sideband or even a few Hertz like CW & PSK31.
Therefore dependent upon the type of transmission the receiver will be used for, the bandwidth required, and hence its selectivity, different types of filter may be used.
Obviously when signals occupy the same frequency there is little that can be done, but by having a good filter it is possible to ensure that you have the best chance or receiving and being able to copy the signal that is desired.
Another advantage of having the narrowest filter necessary with just enough bandwidth to only allow the type of signal you desire to receive is that less noise is received. This improves the signal to noise ratio or how much more signal than noise you are receiving. This is like only opening a wide enough door to let what you want into the garage. For example opening a small door to enter your garage instead of raising the wide garage door used to let in vehicles. If it was raining (noise) and the wind was blowing (more noise) less would be let in by the small door.
Well we have sent intelligence via several different methods but we have not received the modulation and demodulated it yet.
First let us discuss AM (Amplitude Modulation). AM is demodulated by an 'Envelope Detector' which is simply a diode which strips the RF off the carrier and just leaves the difference in amplitude. It is then is amplified and applied to a speaker or headphones.
CW (Code) is a little different. Since the intelligence is just the carrier being switched on and off there will be no audio present unless we do something different. Therefore we use a Beat Frequency Oscillator and a Product Detector. The beat frequency oscillator is slightly off the carrier frequency. These two frequencies 'beat' together and produce the difference which is an audio tone. We can now easily hear the code being received as a tone which goes on and off just like it was sent. Consult Fig M7-2 above for a block diagram of a CW or SSB receiver.
SSB or Single Side Band is very similar to the CW method. But now we adjust the 'BFO' to exactly the incoming carrier frequency (600 Khz in the SSB modulation example to the left).
We hear the difference between the sideband and this
re-injected carrier as an audio reproduction of the
original modulating signal. (300 Hz to 3.3 Khz in Fig M2-7)
Pretty slick, Right?
Last but not least is FM (Frequency Modulation).
FM is the least susceptible to noise because most noise is amplitude modulated. When we receive the FM waveform it has noise riding on the top and bottom of the signal. However, we don't need any amplitude information. So we simply chop off the top and bottom of the signal. This is called limiting. Limiting eliminates the amplitude noise. Now we can get on with the demodulation process.If you recall we change (modulate) the frequency +/- 5 Khz (or +/- 15 Khz depending upon the format used) to send the intelligence. This is called deviation, which refers to deviation from the center frequency of the signal.
Now we use a circuit that will change that frequency deviation back to an audio signal. Actually we use two circuits. One tuned to just above the carrier frequency and the other just below the carrier frequency. The output of these combined circuits (called a discriminator) is representative of the original modulating signal back at the transmitter.
As was mentioned earlier in this module, a driver is just a lower power amplifer "driving" a higher power amplifer.
Sometimes a ham might decide he needs more transmit power to be able to communicate. Especially with a hand held radio (HT), when used in a mobile setting it has several disadvantages. One is the relatively low rf output power, typically only 5 watts. Also just using the 'rubber ducky' antenna inside the vehicle greatly reduces both the practical receive and transmit range or ability.
The Mirage BD35 mobile amplifer pictured address both of these disadvantages. First it hooks the receiver and transmitter to an outside antenna mounted on the vehicle. Secondly it increased the transmit power from 5 watts to 35 watts output power, almost +8 dbm.
The Mirage BD35 is designed to work with today's 2 Meter band / 440 meter band handhelds. It has automatic frequency band selection, so you will never forget to switch bands. A single input connector and single output connector for both bands makes it easy to use with dual band radios and antennas.
The BD-35 has an automatic RF sense transmit/receive switch for easy operation. You even get a free mobile mounting bracket. The amp is just 5 x 1Â¾ x 5 inches and requires 12 VDC at 6 amps.
Sometimes you want much more power for your base transmitter. Especially when you are using simplex operation. Simplex operation is transmitting and receiving on the same frequency. You do not have the advantage of using a repeater to increase your range. You may just want to communicate with other hams in far-way places on VHF.
The amplifer shown above will increase your output power to 350 watts!
Additionally the amplifer contains a receiver pre-amplifer to increase the receiver sensitivity.
Amplifiers are an easy add-on to transceivers bought with cost containment in mind at the time of purchase. At a later date, you can add these amplifiers to upgrade your mobile or base station to a much higher power rig with out having to purchase a new transceiver at a much greater cost.
As you know a superhetrodyne receiver uses a local oscillator and a mixer to create an intermediate frequency, which is then amplifed and processed to retrieve whatever intellegence was originally sent by a distant transmitter.
In the past before 50 Mhz and 144 Mhz were incorporated in readily available transceivers, a device was available to use an HF transceiver as the basic radio for VHF and higher freqency communications.
A transceiver used in this fashion is referred to as an IF radio, indicating its role as the "intermediate frequency" stage in the chain of radio electronics.
By the use of that new device, the lower frequency of the HF transceiver is
This device called a Transverter, is a radio frequency device that consists of a device that houses an upconverters and a downconverter in one unit.Transverters are used in conjunction with transceivers to change the range of frequencies over which the transceiver can communicate.
Present transceivers commonly cover up to 440 MHz, so transverters are used in amateur radio to convert radio transceivers designed for use on the VHF bands to operate on even higher frequency (microwave) bands. Common transceiver/transverter combinations include transverters 902 MHz, 1296 MHz, 2304 MHz, 3456 MHz, 5706 MHz, and 10368 MHz designed for use with 144 MHz IF radios.
Some transverter units include transmit/receive switching built into the design, whereas other units require external switching. The use of external switching is popular in applications where preamps and amplifiers are included.
Many transverters are built into waterproof enclosures for installation on a radio tower or other antenna support structure to get the device as close as possible to the antenna so as to prevent signal loss in a long transmission line down to the transverter. Then just the 144 MHz signal has to be feried back and forth from the antenna to the IF Radio.
Again the best way to learn & is to do some problems and learn from them by repetition. So, lets do some practice.
You can take the quiz as often as you wish.
In this module you have been introduced to different types of radio circuits & how they work.
In subsequent modules we will study how to operate these different types of receivers.
View the complete Technician Question Pool in Chapter 11 of your Ham Radio License Manual.
Review the Term Glossary in Chapter 10 of your Ham Radio License Manual
Practice Exams are available at www.arrl.org/exam-practice and there are links to many other test practice sites also.