The superhet gets around this problem by dividing the task of tuning into two parts. To see how this works consider again figure 16.5b. This uses a local oscillator and mixer to frequency down-convert whatever input signal we're interested in into an IF at a fixed frequency. Most commercial AM receivers use a standard IF frequency of 455 kHz. So when we want to pick up Virgin on 1251 kHz we set the LO frequency to 1706 kHz. When we want to pick up BBC Radio 1 on 1089 kHz we adjust the LO frequency to 1544 kHz. etc,... No matter what MW/AM radio station we're trying to receive the IF part of the set always sees it as a 455 kHz carrier with AM sidebands on either side covering up to ±4·5 kHz. This means we can regard the IF section as if it were a TRF AM receiver which never has to be retuned because we only ever ask it to respond to 455 kHz. The IF filters and amplifiers can now be optimised to provide the rejection and selection behaviour we want without having to both about being tunable.
Given the ability to tune the LO frequency and convert any input to the chosen IF (455 kHz in this example) it is reasonable to wonder why the system requires the RF filter between the antenna and the mixer. Unlike the IF filters, the RF FILTER does have to be tuned when we switch from one input frequency to another. So why is it there?... The purpose of the RF filter can be understood by remembering the problem of sideband ambiguity which arises when we use a mixer. When the LO is oscillating at 1706 kHz an RF input at 1251 kHz will produce an IF at 455 kHz which is amplified and filtered by the IF arrangement. However, an RF input at 2161 kHz — if it reaches the mixer — will also produce an IF output from the mixer at 455 kHz. If the system didn't include an RF filter we might therefore find that a local transmitter at 2161 kHz would produce a mixer output which got into the IF system and stopped us from receiving the 1251 kHz signal we're interested in.
More generally a system with an IF permanently tuned to fIF will, when using a local oscillator set to oscillate at fLO will mean that the system is willing to respond to RF signals at both fLO+fIF and fLO-fIF . One of these frequencies will be the signal we're interested in. The other is said to be an unwanted image frequency. (As in ‘mirror image’.) The task of the RF filter is to pass the signals we're interested in and stop and signals at the image frequencies from reaching the mixer. In the example we're considering we've assumed that the RF frequency we want to receive is always fLO-fIF . We therefore require an RF filter which always passes frequencies around this value but which strongly attenuates or rejects fLO+fIF .
The task of the RF filter is complicated by the need for it to be adjustable in sympathy with any changes in the LO frequency. (The LO and RF filter tuning have to ‘track’ together correctly, always keeping fIF apart.) However, the RF filter can have a much broader response than the IF filter. The IF has to pass 455±4·5 kHz almost unaltered but reject everything above 460 kHz and below 450 kHz with a rejection ratio of at least 60 dB if we are to avoid adjacent/alternate channel interference problems. The RF filter can leave adjacent/alternate channel filtering to the IF system, hence it's response doesn't have to strongly reject frequencies this close to the wanted signal. All the RF filter need do is strongly reject signals which are 2fIF away from the wanted signal — i.e. 900 kHz away in the AM/MW case.
By using a superhet we can divide the filtering task into two easier jobs. The IF filters deal with the ‘near-in’ problem, but we can use a fixed filter. The RF filter has to be tunable, but it only has to suppress unwanted signals which are comparatively far away from the wanted channel. In this way we can make a tunable receiver which offers good performance over a wide tuning range. In manufacturer's specifications the quality of the IF filters is indicated by stating the receivers adjacent/alternate channel rejection ratio. The quality of the RF filters is indicated by quoting the receiver's Image rejection ratio. This, as we'd expect, specifies the relative gain/loss of the filter at the wanted signal and image frequencies.
16.3 Broadcasting and ERP.
If you look at a list of broadcasting transmitters you'll usually be given the transmitter power, it's location, and it's carrier frequency. From earlier lectures we already know that, to calculate the power arriving at a reciever we also need to know the effective gains of the transmit and receive antennas. We usually know something about the receiver's antenna, but what about the transmitter? Most lists of broadcast transmitters don't say anything about the actual antennas used. Instead, the listed transmitted power isn't normally the actual total power transmitted, it's the transmitter's Effective Radiated Power (ERP).
The ERP is the product PtGt for the transmitter. In effect, it gives us the power level we'd need to make an omnidirectional transmitting antenna work like the actual antenna. It is so common for broadcasting engineers to use ERP's that they often don't even bother to say this is what they mean by the ‘transmitter power’. So when you see a list like this you can assume that the quoted power is actually PtGt unless you're told something different! This means you can work out the actual received power using a modified version of the link gain equation
where Perp is the quoted transmitter power or ERP — the product of the actual radiated power and the gain of the transmitting antenna.
In fact, most terrestrial (ground based) broadcast transmitter antennas are directional and hence have a gain greater than unity. They redirect power which would be wasted ‘upwards’ into the horizontal plane. However, the usually have a gain which is uniform in all horizontal directions. Hence for receivers on the earth's surface they appear just like an omnidirectional antenna which has been driven with more power than was actually used. Fortunately, when trying to compute a received power we don't need to know anything about this once we know the ERP!
At ‘low’ frequencies (typically the long and medium wave bands up to a few megahertz) it is usual to simplify things even more and assume that the receiving antenna has a gain of unity. This is becuase, at these long wavelengths, it is difficult to make an antenna big enough to have any actual gain. Hence, unless told otherwise, broadcast engineers would assume a receiver antenna gain of unity to simplify the above equation still further. In practice this usually isn't quite correct, but it is good enough to assess the behaviour of a typical broadcast receiver.
Summary
You should now see how the use of modulated carriers allows us to overcome the ‘crowded party problem’ and use suitable antennas to transmit information efficiently from place to place. That, for AM, restricting the maximum modulation frequency to fmax means we can use channels 2fmax wide to communicate signals without interference. You should also see that one of the main advantages of a superheterodyne receiver over a TRF receiver is that it can relatively easily provide the required filter performance to select wanted signals and reject others over a wide tuning range. That the quality of a receiver's IF filters can be specified in terms of the adjacent channel rejection ratio or the alternate channel rejection ratio. The quality of the tunable RF filter being specified by the receiver's image rejection ratio. Finally, you should now understand how it is common to assume that broadcasting uses transmitters which effectively use an ‘omnidirectional’ antenna driven by an Effective Power Level. That we can also often assume MW/LW receivers have unity-gain (omni again) receiving antennas.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.