Wireless Technology – the world of AM, FM and Wi-Fi
Back to our Roots
Although many of us see Wi-Fi as the be all and end-all of wire free communication we could not be further from the truth. This article does encompass trends over the last hundred years which has seen a trend to move to UHF bands and use very sophisticated forms of modulation, the rule is always ‘the simpler the better’. Yes, I was in the telecom business many years back and yes, this article may lean towards older techniques and yes, we aren’t living in the dark ages but sometimes we lose sight of the skills that our forefathers (and mothers) had. Modern trends are extremely reliable but when they do fail we are miserable, from your local power grid to an internet connection.
Over the last two hundred years there has been, in my book, four great strides to bring us modern radio telecommunications:
- Firstly, the discovery of electromagnetic waves – Maxwell, Hertz and Hughes.
- Secondly, the discovery of the vacuum tube principle, thereafter it’s implementation in amplitude modulated circuits.
- Thirdly, the design and implementation of FM or Frequency Modulation.
- Fourthly, the strides taken to reduce bandwidth with increased power to deliver a rock solid signal, the implementation of SSB. (Single Side Band).
One thing that stands out, head and shoulders above the achievements of commercial enterprises such as Bell Labs whom have been credited with almost anything related to telecommunications is the perseverance and incredible work done by the ARRL (American Radio Relay League).
Although most youngsters would never have heard of SSB, like the challenges facing RCA to develop a broadcast picture in colour on the same frequency and bandwidth of a monochrome signal, SSB was indeed an incredible leap forward – nearly 100 years ago.
The SSB Signal
As the inner workings of a SSB transmission system is out of the scope of this article it is worthy to note that a DSB (Double Sideband) signal is essentially an amplitude modulated carrier wave, modulation taking place in the output stage of the transmitter caused by deliberate fluctuations in the supply rail voltages. If the transmitter was 1kW, the modulator was in the region of 500W to get 50% modulation depth. I hope this makes sense.
In the SSB transmitter however, besides the modulation taking place in the early stages, developers found a unique trick to broadcast the carrier only when it is needed – in other words, the carrier wave is only present when audio (if audio is the ‘intelligence’ signal) is present. There is a sideband filter removing either the lower or upper sideband, the carrier wave is transmitted only when a modulating component is present, this being done through a ring-bridge mixer (in my time). Of course the downside was that if you did not have a dedicated receiver for SSB (one with a Beat Frequency Oscillator) we had Donald Duck on the receiving end. Huge power gains were had. All the marine transmitters I worked on could transmit with suppressed, reduced or full carrier. From what I can recall their transmitter power on suppressed carrier was in the region of 1,500 watts Peak Envelope Power. The downside was that the transmitters had to run in class A which put them at about 50% efficiency. If the cooling blower stopped working you’d know about it within 15 seconds.
Wireless Technology: Other clever tricks used to modulate a signal.
Angular modulation, the most common being FM or frequency modulation techniques have been around for ages, mostly credited to Edwin Armstrong, an American electrical engineer. Here was a brilliant guy, also credited with the design and build of the superheterodyne receiver.
Frequency modulation has a very large advantage over AM and that being the rejection of static. It’s primary disadvantage is that it is chiefly line of sight. FM uses wide bandwidth so the frequencies are much higher – VHF in the marine band is above 150MHz. Signals are clearer because of the wider bandwidth allocated and of course, stereo broadcasts are commonplace. AM broadcasts for long range, FM for short range (terrestrial).
FM receivers use a discriminator to detect the ‘intelligence’ signal.
Phase modulation is another form of angular modulation.
Vestigial Sideband Modulation (VSB, or VSB-AM) and Quadrature Amplitude Modulation (QAM) are both used in television broadcasts (see our digital brother below, ADSL). Yes, your television signal is also single sideband, with more than one carrier. If you are interested in television broadcasts (who isn’t) read up on the very clever techniques used to modulate the carriers to get colour, luminescence and sound. We take television broadcasts for granted but some extremely innovative techniques were used to get our modern NTSC, SECAM and PAL broadcasts to be as accurate as possible on the receiving end. PAL of Phase Alternating Line cleared up a lot of the colour issues with NTSC caused by phase shift of the received signal.
Digital Wireless Technology: Move over Analogue, here comes your Digital brother on cable.
Our digital world makes a lot of sense – very much like morse code where the user keys his transmitter on and off to create a short tone or dot, a long tone or dash or off, digital makes it easier – it’s either a dot (a one) or a space (a zero). Our DSL modem uses the DMT (not the drug but Discrete Multi Tone) system, one which we will focus on and not CAP (Carrier-less Amplitude, nearly redundant after 1996 or MVL Multiple Virtual Line) as these systems are not compatible, the DMT system more popular. In the DMT system the line is broken up into 256 channels of 4.3125kHz each and uses an algorithm to modulate/demodulate the signal processing through QAM (see above: Quadrature Amplitude Modulation).From Cisco’s website comes a very neat description: Discrete Multi-Tone (DMT) describes a version of multicarrier DSL modulation in which incoming data is collected and then distributed over a large number of small individual carriers, each of which uses a form of QAM modulation. DMT creates these channels using a digital technique known as Discrete Fast-Fourier Transform. By varying the number of bits per symbol within a channel, DMT carriers can also be rate-adaptive.What makes ADSL so unique as well is that the voice signal is separated from the data signal via means of a bandpass filter – the low pass filter is used on the telephone line side. Voice telephony only uses one channel, that being the low end or up to 3.4kHz. In some circles we talk about 300Hz to 3.4kHz or 3.7kHz as being the voice bandwidth. This does not include Celine Dion’s vocal range by the way. (on topic as well – note how poor a signal from a live performance sounds when broadcast over a land or cell phone line).There’s many other forms of modulation and demodulation techniques used in in our digital world, each day improvements being made especially around increasing performance, reducing power consumption on copper and in general, working on extremely limited bandwidths. Compare this to the accepted practice of using spark-gap transmitters in the early 1900’s. Yet, this was state of the art. Wireless Technology: Fidelity and Bluetooth[GARD]The two largest growth areas in the telecommunications industry has been in the satellite and cellular network sectors. On a consumer level Bluetooth and Wi-Fi are both very popular, with Ericsson being the ‘inventor’ of Bluetooth back in 1994. Wi-Fi is accredited to the Australian CSIRO or the Commonwealth Scientific and Industrial Research Organisation.- Bluetooth operates in the 2.4GHz band, Wi-Fi, 2.4~5GHz bands.
- Bluetooth uses the GFSK (Gaussian Frequency Shift Keying) modulation technique.
- Wi-Fi radio links use either a direct sequence spread spectrum technique called CCK or Complimentary Coded Keying or OFDM, Orthogonal Frequency Division Multiplexing. CCK modulation is done through QPSK or Quadrature Phase Shift Keying and OFDM, Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM).
Do we really need to know all of this? Not at all, unless you are an engineer and are working on a project. Just like most things in life, engineers dedicated to a certain task specialise in a certain field – Wireless Fidelity is just such a specialised field. I am not an engineers tasked with project managing such a task but the fundamentals are important.
Major Obstacles
Many interesting discoveries over the years were never put into practice until many years later. Nowadays, thanks to the internet and high speed communication we can gloat over our achievements but truth be told, most of the stuff we take for granted was known about and put on paper already by the mid 1900’s. Like Lego blocks, we build on a foundation and move upwards hopefully. Major stumbling blocks were frequency drift, high frequency amplification, interference and of course, power consumption. As we have evolved our main focus has been on making things smaller and more efficient. Many scientists were not even aware of the dangerous environment they had created for themselves, a typical example would be the Curies.
Although we can look on at Wi-Fi in awe I define the NTSC colour system as being more advanced, chiefly because of the fact that it had to be backward compatible to the monochrome television system already in place. I don’t look at the radar system as being a phenomenal achievement, I look at the cavity magnetron and then radar (plus the detection processes of course) as being a major achievement.
The Danger Zone – ControlBoth satellite and the cellular network has one huge inherent weakness: Control. We have absolutely no control over modern telecommunications, just like our national power grid. Batteries and SSB radio yes.The Where To syndrome