Re: Receiver front-end filter #rcvr


Roelof Bakker
 

Hello Ton,

Varicaps can be used, but show a much lower Q than a normal capacitor.
In the previous century I have measured the Q of the BB112, a 20 - 500 pF varicap.
It varied from 50 (400 pF) to 70 (30 pF).

I build a pre-selector using a Wes Hayward, W7ZOI double Peak Low Pass Filter topology.
The beauty of this topology is that it offers very little loss variation over a 1 to 4
frequency range, typical 2 dB.

Using BB-112 varicaps to tune a 10 - 30 MHz pre-selector, the loss was:

MHz loss dB
=============
28 3 (3)
25 3 (4)
21 3 (5)
18 4 (6)
14 5 (8)
10 8

The figures between brackets apply to using two BB-112's back to back instead of a single
BB-112.

73,
Roelof, pa0rdt

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