Receivers Over the Ages


Very early receivers were simple crystal sets which relied mostly on aerial size and sensitive high impedance headphones to resolve low level signals. These were followed by active (valve) detectors using reaction or super-regenerative techniques and some stages of amplification to increase their sensitivity. As the first amplifying valves were triodes, their use in tuned applications was limited because of the risk of instability unless neutralisation was employed. Later pentode valves significantly improved the instability issues.

TRF (tuned radio frequency) receivers were limited in what could be achieved in terms of sensitivity and selectivity so a different technology was required.

The superhet receiver concept was patented in 1901 by R A Fessenden but it took until 1917 for the first working example to appear. The superhet works by using a local oscillator and a frequency mixer to convert the incoming wanted signal to a fixed intermediate frequency (IF) where it can be filtered and amplified to the level required by the detector circuit (basically a crystal set). Early superhets used an IF around 465KHz which was fine for Medium and Long Wave broadcast reception but unsuitable for the higher HF bands due to poor image attenuation and poor oscillator stability.

If you need further information on superhet principles, look in the RSGB or ARRL Handbooks for the basic theory. In addition a number of typical receiver block diagrams are shown here and some thoughts on progressive receiver designs are shown here on this site.

In the lead up to and during WWII receiver development moved on considerably with the appearance of the HRO, AR88 and CR100 series, all well known to amateur radio operators for many years post WWII. The AR8516L, which was developed by RCA Marine to address the stability and image rejection issues of the AR88, used a combination of single and double conversions on the lower frequency bands and a tunable IF with a front end crystal controlled converter for the higher bands.

Later designs used an up conversion technique to a first IF above the maximum receiver frequency e.g. 37.3MHz, 40MHz and higher, so that the image problem was significantly reduced and digital techniques in the backend of the receiver for final amplification and mode detectors.

Current commercial designs (in the 2020s) are starting to use digital processing for the entire receiver where the incoming signals are RF filtered, digitised, processed, filtered and then converted back to an analogue form prior to application to the listener’s loudspeaker, headphones or recording system.

It has been long established that for an HF amateur bands only receiver, a frequency conversion to a fixed IF around 8-9MHz using a high performance mixer immediately followed by a diplexer, optional roofing filter(s), high dynamic range low noise preamplifier and the mode filters will produce excellent results - for example the Yaesu FTDx10 or the Kenwood TS-890S. This IF may be followed by a further conversion to a lower frequency IF like 455KHz for analog FM detection or a much lower IF like 24-36KHz for multimode digital signal processing and detection.

You can see a table of measured receiver performance figures by Sherwood Engineering Inc here.

Page under construction…..