[Elecraft] K3 receiver desensing on CW during contest

[N8LP]

When I first received my K3, I too was disappointed with the BDR until I
understood what was happening. 

In an effort to quantify the BDR of signals between the xtal filter bandpass
and the DSP bandpass, I set up the following scenario...

Two generators feeding the K3 through a hybrid combiner.
One set to about -85dBm (S7 on my K3), and the other variable.
I tuned in the -85dBm signal, with the xtal filter set to 500Hz, and DSP to
50Hz.

With the "interfering" signal set to 200Hz offset (within the xtal filter
passband) I started to see desense of the weaker signal at -34dBm output
level for the stronger signal. With the interfering signal set to 2kHz
offset, I started to see desense at +15dBm. The difference  of 49dB shows
the importance of the xtal filter to this design. Of course, different AGC
settings would affect this test. The promised variable roofing filters would
reduce the problem quite a bit, but that opens up other issues.

I think the days of receivers with xtal filters are numbered. High speed
ADCs capable of 140dB dynamic range without xtal filtering are on the
horizon. A 20-bit ADC with enough processing gain would do it. 

73,
Larry N8LP 















Bill W4ZV wrote:
> 
> 
> 
> K2MK wrote:
>> 
>> I thought it was interesting that recovery was noticeably slow on the DX 
>> station's frequency when this was happening. However, under normal 
>> conditions when US stations were calling directly on top of the DX
>> station, 
>> recovery was fast. Hence the ability to copy the DX station through the
>> dits 
>> when the QRM was on frequency.
>> 
> 
> Mike this definitely sounds like desense but I'm surprised you experienced
> it with only S8-9 signals.  I'm not sure what you mean about differing
> conditions on and off the center frequency.  Desense will affect
> everything inside the roofing filter.
> 
> What speed AGC were you using (F or S) and what were your AGC-F and AGC-S
> CONFIG settings?  I NEVER use AGC-S on either CW or SSB and like at least
> 120 dB/sec for AGC-F.  Also, help your AGC do its job by not running RF
> GAIN fully clockwise (as mentioned in my previous message).
> 
> 73,  Bill
> 
> P.S.  As I said before my K3 worked extremely well coping weak signals in
> the midst of massive ones on 160 this weekend.  I smiled when I saw the
> following comment from OL0W(OK1DSZ):
> 
> "Only for fun, when the band was already closing, I decided to try what I
> will
> be able to work with QRP. Using longer callsign OK1DSZ and the power
> reduced to
> 5W I spend about 10 minutes calling several east coast big guns. The award
> for
> the best ears goes to W4ZV, who was worked as easily as with the kilowatt
> earlier in the night. K8PO and W2FU were worked with several repeats. The
> others just kept CQing without any reaction with exception of K1TTT, who
> apparently heard something, but not enough for the QSO."
> 
> 

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