RECEIVER PRE-SELECTORS

BACKGROUND:

Long ago when our radios used Vacuum Tubes instead of transistors, most receivers had built-in tunable pre-selectors between the antenna and its pre-amplifier and down-stream circuitry. 

The built-in tunable pre-selector did not improve the ultimate selectivity of the receiver, it only reduced the signal strength of stations outside of the passband of the pre-selector, helping to prevent receiver front-end overload. 

When tuning across a band, we had to manually "peak" the pre-selector to assure maximum sensitivity on the received frequency.

Europe was plagued with dozens of high-power shortwave broadcast stations whose signals often reached 20 to 40 dB over 9 at night.  This was especially caused by stations broadcasting in the 41m and 49m shortwave broadcast bands.

Thus, the sum of the RF reaching the front end of our radios often overloaded the RX, causing severe Intermodulation Distortion.  Without an external pre-selector, the only way to use our receivers at night on the low bands (i.e., 160/80/40m) was to insert lots of attenuation; often more attenuation than the radio's own built-in attenuator.

Later, Transistorized receivers and transceivers used multiple Bandpass-Filters (BPF's) between the antenna and the RX circuitry.  These were in addition to the BPF's used between the transmitter and the antenna.  There was a separate RX BPF for each ham band.  These reduced the signal strength of all stations outside of the associated ham band. 

Although some hams began using external pre-selectors in the 1970, the didn't become popular until the the 1980's.  This accelerated when the transceiver manufacturers began to put General Coverage receivers into their radios. Instead of just the radio's receiver just covering the ham bands, GC receivers covered the entire Medium Range (AM) broadcast band up through the shortwave band (30 MHz).

Unfortunately, this new GC "feature" was a step backwards in receiver performance, causing a reduction in the receivers' 3rd order Dynamic Range (DR3).

SOFTWARE DEFINED RADIOS (SDR)

The recent migration of technology to SDR has resurrected the need for pre-selectors.

Like all technology, SDR comes with many advantages, but also some disadvantages. 

IMO, the biggest advantage is the transition to using DSP filters instead of Crystal filters for the primary selectivity of the receivers.  Whereas the use of multiple crystal filters was very expensive, with each filter costing $100 or more, good DSP circuitry costs less than one single crystal filter, and if implemented correctly, provides as good or better selectivity as using crystal filters, with the additional advantage of being variable in bandwidth.

At the same time, most SDR receivers are General Coverage and have very poor front-end bandpass filters.  Most have BPF's which are very broad, covering several MHz, and the low cost receivers often have no BPF's at all.  There are exceptions, of course, but these are only found in expensive receivers and transceivers.

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