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Varicap diode tuning characterics

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Varicap diode tuning characterics

Post by Terrykc » Sun Aug 14, 2016 9:00 pm

This is prompted by this thread. Apart from the multi-turn pot itself, it may not be generally known that the pot follows a special law.

When tuning the UHF bands, where the effects are more pronounced because of the large number of channels, it requires a much smaller rotation of a linear pot to tune through one channel than at the top end of the band. This can make tuning tricky if a linear pot (of any type) is substituted for one with the correct law.

I did some work on this many years ago to see if it was possible to make a linear pot track the varicap law inversely. At that time I had a very neat graph - from Egen, I think - which showed the response so I calculated the best solution to match it - with considerable success!

The result, from memory, was a 47k linear pot with a 15k resistor between the slider and earth. This gave a surprisingly accurate result but note that the resistance of the pot drops to less than 12k at maximum rotation, so it is important to ensure that the varicap supply can maintain its stabilisation with the increased load.

Unfortunately I can't find a copy of the Egen graph to repeat the experiment to prove the accuracy of my memory - this was done over 40 years ago!

 
Posts: 3781
Joined: Sun Feb 12, 2012 7:43 pm
Location: North Hykeham, Lincolnshire and Ilford, Essex (but not for much longer ...!)

Re: Varicap diode tuning characterics

Post by Terrykc » Sun Aug 14, 2016 9:38 pm

I note that I've mentioned this before in the context of altering linear pots to log:

http://www.forum.radios-tv.co.uk/viewto ... 86#p106868

 
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Re: Varicap diode tuning characterics

Post by Niall » Sun Aug 14, 2016 10:45 pm

Years ago I had a B&W telly whose push button unit packed up, I substituted a standard linear pot mounted through one of the former push button holes. It was quite manageable for the Blackhill / Kirk O Shotts channel lineup between channels 40 and 50. As far as I remember the calibration (marked on a bit of paper stuck under the knob) appeared to be linear.

Sounds as if I was lucky to have a midband channel lineup, and I suppose it might not have been manageable with a colour set where the tuning is more critical. Being LOS with the TX masts probably helped too.

Always meant to make up a set of switched presets, but never got round to it before the set was replaced by an ex rental G8.


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