[Elecraft] KSYN3A Synthesizer problem?

[Wayne Burdick]

Hi Roger,

The noise floor is coming up because the RX circuitry was optimized for use from 250 kHz and up. Multiple sources start contributing significant noise below 250 kHz.

Even though the new synth tunes down to 100 kHz, we tried to make it clear in the KSYN3A FAQ that the receiver isn't very sensitive in this range. It does very well in the 600-meter band (472 kHz), and has reasonable sensitivity in the 300-450 kHz range where there are many LF beacons.

To quote from the KSYN3A FAQ:

> Q: What is the K3's 100-490 kHz receive sensitivity?
> 
> A: With preamp OFF, we measured about -125 dBm at 475 kHz....

At 250 kHz the sensitivity is about 10-20 dB lower, and it will go down from there. Yours is better than our baseline measurement. 

The FAQ entry continues:

> We have not measured MDS below 250 kHz, but the receiver will gradually become less sensitive as you go lower in frequency due to band-pass filter roll off (in the KBPF3, which is required for operation below 160 meters). These tests were made with the signal routed through the RX ANT IN or XVTR IN jacks on the KXV3 option module.


It would be possible to redesign the KBPF3's lowest-frequency filter to improve sensitivity below 250 kHz. However, the receiver uses PIN diodes extensively, and the rolloff from this band-pass filter helps prevent intermod in the diodes.

I can see that we should clarify this topic further in the FAQ and installation manual.

73,
Wayne
N6KR


On Apr 6, 2015, at 4:01 PM, Roger D Johnson <n1rj at roadrunner.com> wrote:

> My new boards arrived today. I popped one into the K3 for some testing. I was especially interested
> in the low frequency performance as I like to DX the LF broadcasts from 153 to 279 kHz. What I found
> was that below 400 kHz, the noise level rises and at 100 kHz it's almost 20 over 9 on the S meter!
> I calibrated my NaP3 spectrum analyzer program and took some measurements of the noise level.
> 
> 500 kHz  -127 dbm
> 400         -127
> 300         -124
> 200         -113
> 100         -93
> 
> That's a 34 db increase in noise level!! Is this the way the new boards are supposed to work?
> I expected a decrease in sensitivity below 500 kHz but not this. Any ideas?
> 
> 73, Roger
> 
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