09-04-2018, 01:32 PM
The first varicap tuner I encountered was in late 1969 - a special VHF tuner covering 45 - 230MHz - using the BB105 varicap diode and designed/manufactured by AB Electronics.
This was before the TAA550 was developed (I'm not sure when varicap tuners started to appear in domestic TVs).
In this case, the problem of producing a stable varicap tuning voltage was to use a zener string with an overall temperature coefficient designed to match that of the varicap diodes. It consisted of three 6.8V zeners in series with three 3.3V zeners. As can be seen from Keith's graph, the temperature coefficient of these two values straddles the zero line, resulting in a small overall positive temperature coefficient.
This was before the TAA550 was developed (I'm not sure when varicap tuners started to appear in domestic TVs).
In this case, the problem of producing a stable varicap tuning voltage was to use a zener string with an overall temperature coefficient designed to match that of the varicap diodes. It consisted of three 6.8V zeners in series with three 3.3V zeners. As can be seen from Keith's graph, the temperature coefficient of these two values straddles the zero line, resulting in a small overall positive temperature coefficient.
