Progressive Receiver Design
Where do we start? This is the age old question. If the design is too simple then constructors may get bored or lose interest, if it is too complex it may appear too difficult and again never get finished. I have always divided projects into manageable sub-projects, each one with a well defined set of performance requirements. Make them all, one by one and then put them together to form the final product. This way minimises the risk of failure while encouraging the constructor by providing an increasing number of modules that have been made to work.
If you are unfamiliar with some of the terms used in this page then it is suggested that you obtain an introductory book on receiver construction. Both the RSGB in the UK and the ARRL in USA have such publications available. The G-QRP Club also specialise in easy to construct circuits and related information.
So, where do we start in designing and making a receiver? The simplest receiver is the classic crystal set which may be used to receive Amplitude Modulated (AM) signals from mainly local broadcast stations and a limited number of amateur radio stations. With a sufficiently large aerial you should be able to receive more distant stations. You will need a pair of sensitive high impedance headphones, or an audio amplifier as the signal levels from a crystal set are very low. Please note that low impedance headphones are not suitable for this application as they lack sufficient sensitivity will prevent the detector circuit from operating correctly.
If you do not have any high impedance headphones then some form of audio amplifier will be required to amplify the output of your project sufficiently to drive low impedance headphones or a loudspeaker. I therefore suggest that we should start at the power supply and audio amplifier and then work back towards the front end, one stage at a time, learning how each one works and gradually adding extra functions and gain to increase the sensitivity.
Crystal sets cannot be used for SSB reception without an additional oscillator to replace the carrier of the incoming signal. FM reception may be possible by tuning off to one side on the incoming signal and using slope detection.
For improved performance you can add a tuned radio frequency (RF) amplifier which will increase both the signal levels and selectivity of your receiver. The RF amplifier may be valve or transistor based. You can also change the diode detector to an active device.
Active detector
An active detector may take the form of a unity gain type such as the infinite impedance detector or a regenerative type such as the reaction detector or super-regenerative type both of which have variable amounts of gain or a direct conversion type as discussed below.
If significantly better performance is required then some form of superhet based receiver is required as crystal sets, super-regen, reaction detectors and TRF receivers, while educational and interesting to try out, are fairly limited in performance. If your main interest is in listening to the HF (short wave) bands then AM (Amplitude Modulation), CW (Morse) and SSB (Single Sideband) facilities will be required. If VHF operation on FM (Frequency Modulation), SSB or CW is desired then some form of superhet receiver is mandatory.
In the following example, the direct conversion receiver may be used in its own right as a simple receiver for CW and SSB signals and then used as product detector in a more complex receiver. It must be followed by an audio amplifier to provide sufficient gain and output power to drive a pair of headphones or a small loudspeaker.
When that has been made and tested, an RF amplifier can be placed in front to improve the sensitivity and later converted to an IF amplifier. An AM detector can be added which will provide another mode of operation and also a source of Automatic Gain Control (AGC) voltage for the IF amplifier. AGC is used to control the overall gain of the receiver so that it will work correctly with both weak and strong signals without deafening the user.
The next step is to add a mixer and local oscillator and we have the beginnings of a reasonable performance receiver but put together from a number of relatively simple modules. Adding IF filters for different modes of operation and an RF amplifier for improved sensitivity will significantly increase the overall performance - and the user’s construction and operating skills!
The frequency coverage may be increased by switching in additional tuned circuits in the front end or by adding an additional mixer and oscillator - both have their advantages and disadvantages.
Economics will determine the design of each module. For example, a multimode detector module could be made to cater for AM and SSB/CW but the individual sections would need to be separated/screened to ensure that they did not interact. Similarly an FM detector could be added but that may then make the module uneconomic for the majority of users.
We can also consider the application of Digital Signal Processing at a relatively low frequency by using the sound card in our PC with the appropriate software as a multimode detector/generator. This would then require the provision of the band frequency conversion hardware in an external assembly.