Various arrangements can be used as diplexers. Here it takes the form of a simple Semi-Reflecting sheet of dielectric. This has a reflectivity of, say, 10%. As a result 90% of the signal power can pass through the sheet to reach the mixer and 10% of the local oscillator power is reflected into the mixer. This particular arrangement isn't a very good one as some signal and local oscillator power is always wasted. The wasted power is absorbed by a sheet of absorbing material which acts as a matched resistive load (sometimes called a Beam Dump). By increasing the sheet's reflectivity we could make more efficient use of the available local oscillator power, but this would also reduce the fraction of the signal power reaching the mixer. The waste of signal power tends to reduce the sensitivity of the receiver and may make it difficult to obtain enough local oscillator power to obtain optimum mixer performance. As a result, receiver's which use this simple diplexing method tend to have a noise temperature which is higher than would otherwise be possible.
This diplexing arrangement has been used in order to keep the illustration as simple as possible. In many cases more efficient systems can be employed which waste virtually none of the available signal and local oscillator power. Broadly speaking, these improved arrangements are of two types: balanced mixers, and frequency-sensitive diplexers.
Figure 3.5 illustrates the use of a balanced pair of mixers. Here the semi-reflector is chosen to have a reflectivity of 50% and the absorbing load is replaced with a second mixer similar to the first. In this arrangement each mixer sees half of the signal power and half of the local oscillator power. As a result all of the available power is used. (Assuming, of course, that the lenses, etc, are perfect!) The IF outputs from the two mixers can be combined to produce an output which behaves as if all the available signal and local oscillator power had been directed onto a single mixer. The main practical disadvantage of this arrangement is that it requires two (possibly expensive) mixers and enough local oscillator power to operate them both. However, once assembled, systems like this can provide very high levels of performance.
The alternative approach is to replace the semi-reflector with a device whose behaviour is frequency sensitive. This exploits the fact that the signal and local oscillator frequencies are different. Hence we can imagine replacing the diplexing sheet in figure 3.4 with something which is essentially transparent at the signal frequency but perfectly reflecting at the local oscillator frequency. Using a diplexer of this sort we can hope to simultaneously couple all of the available signal and local oscillator power onto a single mixer. Various types of device or circuit can be used to perform this task. For example, we can imagine replacing the semi-reflecting sheet with a Fabry-Perot Resonator.
The sensitivity of the system shown in figure 3.4 can be calibrated using a pair of Black Body radiators of known temperature. In the illustrated system a pair of moveable mirror ‘switches’ can be used to fill the mixer's field of view with radiation from one or the other of these instead of the signal coming from the source. In mm-wave heterodyne receivers these black bodies are usually referred to as Hot & Cold Loads. The term ‘load’ is often used to refer to black body radiators and absorbers since their properties are equivalent to resistors wired into an electrical system. In each case thermal power can be generated and absorbed in a way determined by the temperatures.
The cold radiator or load is usually cooled with liquid nitrogen to a temperature around 77 Kelvins. The ‘hot’ load is quite often at the ambient ‘room’ (although, of course, the telescope may be on top of a cold mountain top, effectively out in the open) temperature. Hence it is only hot in comparison to the liquid nitrogen cooled load. In some cases the hot load is warmed to some standard temperature, say the boiling point of water.
The precise choice of the temperatures of the two loads isn't important provided that both values are well known and differ by enough to have a clearly observable effect upon the receiver system. By comparing the output they produce with that coming from the distant source we can determine the receiver's noise level and measure the source's brightness or temperature.
You should now understand how an actual heterodyne system can be operated at ‘optical’ frequencies. Although this example is for THz frequencies it should be clear that similar results can be obtained for other signal frequency ranges.
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