Part 1 - Front Ends
by Larry GadallahBack in the early 1980s, I decided that I would like to build my own state of the art receiver for DXing and SWLing. At that time I began collecting all sorts of engineering information about various circuits and devices to use as references for my receiver design. Today, almost a decade later I have revived my project. I thought that perhaps other DXers might be interested in what I have found. So I offer the first installment of my receiver design:
Front End Design:
There are two roles performed by a receiver's front end: The front end provides either gain or attenuation as needed, and it also often provides some amount of RF selectivity. This is what the "Preselector" or "Antenna Tune" control does on some sets. By themselves, these two roles are not too difficult to accomplish, but when you include the requirements for very high dynamic range and ultra-linear operation, they become more challenging. Having a high dynamic range, in simple terms, means that your receiver is able to receive a weak signal on one frequency without being affected by other much stronger signals nearby. In modern reception environments, often a receiver is faced with adjacent signals having more than 100 dB difference in strength, the front-end of a receiver bears the brunt of this onslaught of signals. Poor dynamic range in a receiver is almost always accompanied by a front-end design containing devices operating in their non-linear ranges which leads to cross-modulation, intermodulation distortion (IMD) and related undesired effects.
RF Selectivity
In the olden days, most receivers had the aforementioned "Preselector" or "Antenna Tune" controls, but they are generally avoided today because they make it impossible to change frequencies quickly. In general they have been abandoned in favour of banks of fixed bandpass filters that are switched in and out depending upon what frequency range you are listening to. Often this switching is done automatically, but on some sets, like the Drake R-7 the user was forced to switch the front-end filters manually. Some more exotic schemes used a true tuned preselector with electronic tuning done by the microprocessor controlling the receiver, but the use of non-linear devices such as varactor diodes in the front-end is begging for trouble with dynamic range and cross modulation. For the homebrew receiver, the best solution is the fixed bandpass filters. These are relatively simple to construct, they can be manually or automatically switched, and they cause minimal loss and non-linearity. In older receiver designs, image rejection was a problem that was typically solved with the use of selectivity in the front-end. In older designs, the first intermediate frequency (IF) was almost always below the receiving frequency. This caused problems in that the image frequency was relatively near the receiving frequency and had to be filtered out. Today most receivers use an up-conversion technique where the first IF is much higher than the receiving frequency and thus the image frequency is higher yet. This has eliminated the need for complex tracking preselectors, but it also makes it a good idea to include a good low pass filter in the front-end to reject both the IF and image frequencies while passing all receiving frequencies.
Attenuation/Gain
Older receivers, particularly those built after the introduction of the bipolar transistor often had an "Attenuator" control. This was a remedy for the very poor dynamic range that these receivers often suffered from. Tube-type receivers had a greater dynamic range and did not suffer as much from overload. Thanks to newer technology in transistors, this is not nearly the problem it used to be. However, the radio spectrum today is even more crowded. Now, more than ever before higher power broadcasts are sent via thousands of transmitters. So on many relatively recent receivers you can often find an "RF Attenuator" control. This is useful in some situations, but it requires manual control. The state of the art solution is to provide a PIN diode attenuator. This allows the same function and range as a manual attenuator, but it may be controlled by the receiver's AGC circuit or by a microprocessor.
Gain in the front-end is another design concept used in older receiver designs. In general, this lead to a number of problems, mostly related to the reduction of dynamic range by the RF amplifier stage used to provide gain. Several modern receivers still provide gain in the front-end, albeit in the form of a preamp which may be used or bypassed at the operator's convenience (i.e. Icom R-71). For many applications, especially in the lower HF bands, for MW and LW use, there is no real need for gain in the front-end. The IF and audio stages of the receiver will provide the necessary gain and the signal to noise ratio is determined by atmospheric and man-made noise. The use of a preamplifier stage in the front- end becomes useful at the higher HF bands (20 MHz and above) where it can improve the signal to noise ratio of the received signal if designed correctly.
Older preamplifier stages suffered from poor dynamic range and non-linear operation which led to cross modulation and other problems. These stages were often controlled by the receiver's AGC which lead to more problems with dynamic range and AGC "pumping" effects. Modern transistors, such as power FETs (field effect transistors) and bipolars, designed for ultralinear applications such as cable TV distribution amplifiers, can provide excellent dynamic range and linearity. Such devices can be used for front-end preamplifers, but such a preamplifier should be an optional item in the front end. At high frequencies, in some cases, the preamplifier can help enormously but in general the use of a preamplifier just shifts the burden of providing a good dynamic range and linearity to the first mixer stage so the preamplifier stage should be able to be switched in or out at will.
The next installment will be about the first mixer and VFO stages.