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-
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-
1. 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 very limited number of amateur radio stations using that mode. This type of receiver has little selectivity and no power gain so requires a reasonably large aerial together with a pair of sensitive high impedance headphones or an audio amplifier to obtain adequate signal levels. 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.
The crystal set may be used in subsequent more complex designs.
Crystal sets cannot be used for SSB reception without an additional oscillator to provide the carrier required for correct detection of the incoming signal. FM reception may be possible by tuning off to one side on the incoming signal and using slope detection but this is unlikely to be very efficient, particularly at VHF.
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 and you can also change the diode detector to an active device to increase the received signal levels.
When you move on to more complex receivers the crystal set may be used as the AM detector and to provide the control voltage for the AGC system.
2. An active solid state or valve detector may take the form of:
3. Crystal sets, super-
If your main interest is in listening to the HF (short wave) bands then AM (Amplitude Modulation), CW (Morse) and SSB (Single Sideband) reception facilities will be required. If VHF operation is required then FM (Frequency Modulation) will be required in addition to AM, SSB and CW.
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.
4. 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 provide a reasonably constant output level with both weak and strong signals.
The following AF amplifier will provide gain and sufficient output capability to drive a small loudspeaker:
A simple audio filter will help in reducing unwanted noise although it is not as effective as an IF filter:
5. 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 -
The frequency coverage may be increased by switching in additional tuned circuits in the front end or by adding an additional mixer and oscillator -
The requirements and 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.
You 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 module.