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AMATEUR RADIO BY GARRY CRATT, VK2YBX Judging receiver performance Prospective purchasers of communications receivers often judge the performance by sensitivity alone. This article sets out to explain the various parameters considered by designers & why they are critical to overall receiver performance. High quality filters This basic “adjacent channel rejection” performance is largely determined by the quality of the IF filters used. As radio spectrum availability is reduced, commercial users are being forced to adopt narrow channel allocations. Commercial channel spacings of 12.5kHz are now common. This means that a receiver must be able to reject strong signals having 60dB or so more amplitude, 12.5kHz away from the operating channel. This kind of selectivity can only be achieved through the use of high quality filters having steep skirts. Such filters should be used at all intermediate frequencies (IF) used in the receiver. Hence, commercial receivers must have adequate sensitivity (typically 0.3µV for 12dB Sinad) whilst maintaining a high level of adjacent 68  Silicon Chip channel selectivity. The Department of Transport and Communications specification for 25kHz spaced equipment is 73dB, and for 12.5kHz the specification is 65dB. This is a good indica­tor of the importance of adjacent channel rejection. In addition to the narrow-band IF filters necessary for tight channel spacing, care must also be given to the specifica­tion of any “mix down” crystals used in IF conversion. Such crystals must have tight frequency tolerance and temperature drift specifications, as the narrower channel spacing makes it far easier for the receiver to drift off the nominal fre­quency. Front end selectivity is also an important parameter. Interfering signals f1 AMPLITUDE Whilst it is certainly true that the ability of a receiver to detect and produce intelligible audio from a weak signal is a very important performance parameter, there are other more important characteristics rarely appreciated by the end user. A receiver must not only have the ability to “hear” minute signals and discriminate against noise, but it must also have the ability to reject adjacent signals having a power level of up to one million times that of the “on channel” signal (+60dB). 3rd f2 3rd 5th 5th 7th 470 7th 480 490 500 510 520 FREQUENCY (kHz) 530 540 Fig.1: this diagram shows 3rd, 5th & 7th order products in a 144MHz receiver. A good receiver should exhibit 60dB of intermodulation immunity to two mathematically related interfering signals within several hundred kilohertz of the wanted input frequency. which can cause spurious responses are the intermediate frequency, the image frequency fc - IF (or fc + IF if the local oscillator is above the input frequency), fc - 2IF (or fc + 2IF), fc ± 455kHz, fc ± 2 x 455kHz. By using a bandpass filter comprising several tuned circuits, correctly matched to the RF amplifier and the mixer stage, up to 50dB of image suppression can be achieved, without compromising receiver sensitivity, or selectivity. Choice of IF Careful choice of IF is also important. By selecting a first IF high in frequency, say 70MHz or so, all images will fall well outside the passband of the receiver, increasing the atten­uation of any image frequency. Of course, selection of a suitable IF is governed to a large degree by commercial avail­ability of multi­pole crystal filters. Another cause of degraded receiver performance is non line­arity of the RF stages. The linearity of an RF amplifier is always best at low levels. This means that there are two con­flicting design goals; ie, to maximise amplifier gain for best sensitivity, and to minimise RF gain to ensure linearity. The solution is to distribute the gain of the receiver across several stages. It is better to reduce the frontend gain of the receiver by several dB, thereby improving the front-end overload immunity by 10dB or more, and make up for the reduction in gain after conversion. Another beneficial effect of reducing the RF gain of the receiver input stage is to minimise the affect of compression or “blocking”. Blocking occurs when a strong signal is present within the passband of the receiver front end, causing the first stage to become saturated and therefore unable to pass AMPLITUDE LOCAL OSCILLATOR OSCILLATOR NOISE FLOOR (a) FREQUENCY AMPLITUDE LOCAL OSCILLATOR NOISE SIDEBANDS (b) FREQUENCY Fig.2: this diagram shows the difference between a clean & a dirty local oscillator. The sideband noise can fall within the IF passband & therefore become audible. a weak signal. Blocking immunity is thus a measure of the ability of a receiver to detect the wanted signal without exceeding a pre­scribed level of degradation, caus­ed by the presence of an un­wanted signal. A typical blocking test for commercial receivers calls for 90dB of immunity to any interfering signal from 1-10MHz either side of the wanted signal. Intermodulation When two or more interfering signals combine in any non-linear semiconductor, the result is a set of intermodulation products. For example, if there are only two signals present, the primary result will be f1-f2 and f1 + f2. These are called second order products. The additional products of 2f1, 2f2, 3f1 and 3f2 are normally well outside the coverage of the receiver. However, odd order intermodulation products (ie; 3rd, 5th and 7th order harmonics) can be a problem. Using two input signals, f1 and f2, 3rd order products of 2f1 - f2 and 2f2 - f1 are generated, as are 5th order products 3f2 - 2f1 and 3f1 - 2f2, and 7th order products 4f2 - 3f1 and 4f1 - 3f2. Each pair of products is separated from its partner by a frequency equal to the difference frequency of the two originating signals. Fig.1 shows 3rd, 5th and 7th order products in a 144MHz receiver. A good receiver should be able to exhibit 60dB of intermodul­ ation immunity to two mathematically related simultaneous interfering signals within several hundred kilohertz of the wanted input frequency. When a combination of products is fed into a mixer stage having some degree of non linearity, a spurious response is generated. This is further complicated when one or both of the original signals is modulated. Careful allocation of gain is essential and the importance of linearity can also be seen. The commercial market demands receivers able to exhibit at least 70dB of spurious response immunity from 100kHz to 1000MHz, regardless of operating frequency. Equally important is the design of the local oscillator. An impure local oscillator can cause a significant problem in receiver perfor­mance, called “reciprocal mixing”. This problem is caused when the receiver local oscillator signal contains significant noise sidebands. Fig.2 shows the difference between a clean and a “dirty” local oscillator, caused by a poorly designed syn­thesis­er. The combina­tion of an off-channel input signal and the sideband noise of a dirty local oscillator produces a signal in the receiver IF passband, along with the on-channel signal, degrading the input signal due to noise masking. In general this problem is limited to synthesised designs (crystal oscillators are normally quite clean) and hence is a very important consideration. Most of the above characteristics relate to the internal effects of mixing products. However, it is just as important that no conducted spurious signals emanate from the receiver to the antenna system. Commercial specifications limit conducted spuri­ ous emis­­sions to an absolute level of -57dBm for mobile tran­sceivers and -47dBm for handheld transceivers. Careful considera­tion must therefore be given to effective antenna filtering which minimises spurious emissions, without adversely affecting receiver sensitivity. From these few points, it can be seen that there are a significant number of factors which affect the design of a receiver. Having an appreciation for these factors can result in a better selection for a given application. SC October 1993  69