[Elecraft] K3s Notch Filter

[Richard Lamont]

On 12/05/2019 23:22, Richard wrote:

> Is there any way to make the manual notch filter really narrow to keep morons running legal limit on FT8 from wiping out my waterfall and making my ears bleed?
> 
> Don’t like what AGC does.

Probably not, but both the notch filter and AGC are the wrong tools for
the job.

The WSJT-X decoder has huge dynamic range (16-bit 48000 samples/sec
input decimated to 12000 samples/sec, so over 90 dB), so it's best to
use the RX as a dumb converter and let the decoder deal with it. It's
the right tool for the job.

This means keeping the bandwidth of the RX at least as wide as the band
displayed on the waterfall and avoiding the use of anything in the K3
that might improve the signal for aural CW/SSB reception such as notch
filtering, equalisation, noise blanking and noise reduction.

AGC should be avoided because the level of every FT8 signal in the
passband will be modulated by fading on the strongest signal, which
makes the decoder's job harder. The AGC recovery time may also cause the
loss of more symbols than necessary when there's impulsive interference
such as lightning static.

The trick is to set the gain manually through the chain, from the RF
gain knob on the RX right through to the waterfall settings, so that
levels are matched and no one stage in the chain is either clipping
strong signals or pushing weak ones below the noise floor.

If you ears are still bleeding then turn the AF gain down. If the WSJT-X
'thermometer' is in the red turn the RF gain down. If the waterfall
isn't showing both the weakest and strongest signals properly, adjust
the waterfall zero and gain controls accordingly.

The "morons" are not running legal limit to cause you trouble. We're
doing it (for example) because the loss on the 160m path between G and
JA is so high that we need legal limit to get even a faintly detectable
signal at the other end.

73,
Richard G4DYA