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Add a "Q-Control" to your Preselector's Notch Circuit |
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L vs C for best Q
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Through recent experimentation, and practical usage, it has been determined that there is significant utility in an adjustable Q circuit. Based on the Type-III Preselector circuit, the following tests were conducted: |
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This is the standard Notch (null) series-tuned L/C configuration in a 50 Ohm circuit. The series resistor was adjusted for 0.0 Ohms and 50 Ohms. |
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Here is the same circuit with the Notch filter operating within a 200 Ohm loop. The higher impedance was created by using the standard 4:1 transformer, that are used to create the 12 Ohm environment for the Bandpass circuit. The series resistor was adjusted for 0.0 Ohms and 50 Ohms.
Note the availability of a deep 20+ dB notch down to a more tunable 8 dB notch.
This represents a refinement of the original circuit and is not a necessity. Obviously, tuning such a steep notch without the aid of a "Q-Spoiler" can be somewhat touchy, but with practice becomes (sort of) second nature.
Deeper notches can be realized by using an 8:1 (or higher) transformer, but bear in mind that the addition of each transformer causes some amount of signal attenuation.
I have built many preselectors without any transformers and, except for the most discriminating user, are just fine for serious DX and weak-signal use. |
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8 Mar 06 |
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