Permeability tuner's last stand

Created: Sep. 2020

The seminal work on permeability tuning were by Polydoroff in 1933 [1]. He intended it as a solution to the variable capacitor's changing selectivity over frequency. As a sign of the work's importance to that era, it was subsequently summarized in no less than "Nature" - a prestigious science journal which didn’t normally feature engineering works [2].

Old car radios used permeability tuning instead of a variable capacitor because the latter is more susceptible to vibration. Another reason behind its popularity in car radios is the permeability tuner is smaller than the air-spaced variable capacitor that pre-dated the polyvaricon (subminiature variable capacitors using polyethylene film as dielectric). 

The 1990s was permeability tuning's last decade, and after that it was supplanted by the cheaper & more reliable varicap / varactor tuning. Among the last members of the extinct species is Blaupunkt Bristol CC21, incidentally manufactured in my hometown [3]. Its permeability tuner is assembled mostly from die-cast parts. The reduction gear consists of a screw drive which takes 9 turns to tune one band. To vary the inductances, the screw drive moves a set of ferrite rods in & out of the coils. The coils' construction is similar to the FM IFT, consisting of ferrite pot over bobbin, & moveable rod. If the coils had not been enclosed by ferrite pots, their Q would have been been severely lowered by the diecast housing. The ferrite rods are attached to trimming screws for adjustment of RF-LO tracking.

Like its variable capacitor-tuned cousin, the permeability tuner is also susceptible to frequency drift. The coils’ ferrite material has a positive temperature coefficient. In the FM section, automatic frequency control (AFC) corrects the drift via a varicap / varactor diode that is lightly coupled to the oscillator coil. The AFC derives its control voltage from the FM discriminator.  

References

[1] https://ieeexplore.ieee.org/document/1685372

[2] https://www.nature.com/articles/133956a0

[3] https://youtu.be/7F-Zgwo5H34

In the FM section, the permeability tuner simultaneously varies L15, L25 and L45. Like its variable capacitor-tuned cousin, the permeability tuner is also susceptible to frequency drift. In the FM section, automatic frequency control (AFC) corrects the drift via a BB505 varicap / varactor diode that is lightly coupled to the oscillator coil. The AFC derives its control voltage from the FM discriminator