Re: Narrow filter for CW #ubitxcw

Jerry Gaffke

The 12/11mhz crystal filter is after all IF amps on receive.
The IF amps are operating at very low signal levels, most of the gain is in the audio amps.
It's the audio pre-amp (a 2n3904 just before the volume control) that overloads first,
by a very wide margin:
That thread is about the Bitx40, but the uBitx will have the same issue.

So, if you have a very strong CW station 500 hz away from the weak signal you are trying to hear,
the strong signal will cause audio distortion in that audio pre-amp and any audio filter
that follows the pre-amp won't be able to clean it up.

The solution is to have a choice of wide crystal filter for SSB, and narrow for CW.
But that's not a big deal for most of us, easier to implement the audio filter for CW.

Jerry, KE7ER
Jerry, KE7ER

On Wed, Aug 14, 2019 at 04:56 PM, Tom, wb6b wrote:
It would seem the reason for this is, the crystal filter may filter out the strong unwanted signal while the signals are still in the first stages of the IF chain. The signals are still relatively weak at this point and the undesired signal is removed before it can get amplified to the point that overloading starts to occur. 

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