[Elecraft] Filters and Configuration Questions

[Paul Christensen]

> Only Elecraft can answer this question.  In early 2008, I tried using a 1
> kHz filter for CW TX (telling the configuration it was a 2.7) for CW TX 
> and
> got some very bad signal quality reports.

If the CW Tx frequency is not reasonably centered in the filter passband, 
poor keying quality reports will result, especially if placed on, or near 
the upper or lower skirt of a crystal filter.  This asymmetrical group delay 
issue plagued the Omni VI+ as the CW carrier frequency was placed right on 
the skirt of the 2.4 kHz @ 9 MHz filter.  The placement of the Tx BFO 
frequency near the filter skirt was done in order to better accommodate 
variable CW Tx offset in the Omni 6+ design.  In the Omni V, the CW BFO was 
fixed and located about 400 Hz above where Ten Tec placed it in the Omni 6+. 
That shift of a few hundred Hz made all the difference in the world.  Among 
several other contributions to poor keying in the Omni 6+, including a poor 
ALC design that mutated from the Omni 5 and 6 (non plus), the primary 
culprit was the BFO placement near the skirt of the filter.

The overall solution included filter replacement with a custom 2.8 kHz 
filter from INRAD and re-working the ALC with a handful of components.  I 
ended up bypassing the filter with a hard wire in CW Tx.  Passing the BFO 
through the filter simply isn't required when the rig is capable of 
generating a clean waveform.  Plenty of old homebrew boatanchor rigs were 
capable of producing a high-quality RF envelope and didn't rely upon ALC nor 
a filter to pass the CW waveform.  True, just as many homebrew rigs produced 
horrendous signals on the air.

Since keying bandwidth is a function of the RF envelope shape, I'm not sure 
why anyone would want to narrow the K3 filter on CW transmit.   As long as 
the ALC is stabilized and the slope is reasonably smooth and absent sharp 
discontinuities, all a narrower filter will accomplish is to produce 
envelope distortion if not placed squarely in the center of the filter 
passband.

Paul, W9AC