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Passband Tuning – SSB Receiver Innovation

passband tuning

At the beginning of the sideband era, engineers found interesting ways to move signals around to get different filtering effects – called passband tuning. 

Early single sideband receivers introduced passband tuning. With PBT, you could move the signal around within the final filter’s passband to remove interference. You adjust the signal pitch without changing the audio frequency by tuning two oscillators in opposite but equal directions.

Passband tuning was first introduced in the 1955 Collins 75A4 receiver, shown left above. In this old radio, tuning was mechanical using shafts and gears. To create PBT, the VFO frequency had to be moved in one direction, while the BFO moved in the other. And all this while the main tuning knob remained constant. Collins used a metal belt to link the PBT knob to the VFO shaft mount and BFO tuning coil. Adjustment was very precise over ± 3 kHz. This method works fine for SSB/CW which rely on a BFO.

A second method for passband tuning works with any mode as long as the receiver is a multiple conversion superhet with two IF filters at different frequencies. In this case, the second and third local oscillators are moved in different directions. As this is done, signals are pushed across the virtual passband formed by two different filters. Drake introduced this approach in their 1-A receiver in 1957, shown above right, and continued thereafter.

This type of PBT has been used on most SWL and Ham receivers over the years. Alternately, manufacturers call it passband tuning or IF shift. The techniques vary, but you get flexibility in adjusting response and width of the receiver’s final passband. This works best when both IF filters have similar characteristics in terms of skirt response.

Watch this demonstration of both PBT and IF Shift on an ICOM radio.

Passband Tuning in Software Defined Radios

Passband tuning and variable width filters are relatively easy to implement in SDR. It’s all math, really. Most SDR software contains a graphic interface to let you drag the sides of filter response to change width, and often to move signals around within a passband.

Every time you make a change, the software re-calculates the filter parameters (math) and then applies the new filtering scheme to the next batch of I/Q samples.

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