[Elecraft] CW Crystal Filter Signal Loss

[Lemkes]

My K2 S/N 3202 exhibits what seems to be undue loss of sensitivity as the CW 
crystal filter is narrowed.  After calibrating and recalibrating with 
Spectrogram, and signal tracing through the receiver IF section, I purchased 
a new set of seven CW crystals.

I installed the crystals today and recalibrated with Spectrogram.  I get 
maybe 1 dBm of sensitivity improvement, probably due to my increased 
calibration proficiency.  I am using an Elecraft XG1, 1 microvolt, 7040 kHz, 
signal generator to measure sensitivity as minimum detectable signal, MDS.

I measure a sensitivity of -137 dBm with the 2.4 kHz SSB filter,
                                    -132 dBm with a 1.2 kHz CW filter,
                                    -129.5 dBm with a 800 Hz CW filter, and
                                    -126.5 dBm with a 400 Hz CW filter.

That's over 10 dB loss of sensitivity as I narrow the filters which can 
easily lose a weak signal as I try to isolate it from nearby QRM.  I can't 
believe that such loss of performance is normal.  Has anyone else measured 
sensitivity with an XG1 1 microvolt signal source?

The filter varactor bias is working evenly across all seven crystals.  If a 
varactor was shorted or installed backwards an abnormal bias level would 
show up.  Spectrogram shows the filter widths to be proper and also centered 
properly on the sidetone offset.  At each filter setting the XG1 signal is 
also loudest at the sidetone frequency, 590 Hz.  So, the calibration seems 
as good as it can get.

My next steps might be to remove the SSB option, and the Noise Blanker.  Any 
other suggestions?  Has anyone had a similar experience?

Thanks in advance for your help es 73, Dick, AD7AF


Dick and Marty Lemke
PO Box 1038
Ocean Park, WA 98640
Tel. 360-665-2438
www.lemkestudios.com