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KSYN3A Synthesizer problem?

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Roger D Johnson

KSYN3A Synthesizer problem?

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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Ross Primrose

Re: KSYN3A Synthesizer problem?

It sounds like you've only put one new board in a 2RX K3. I don't
believe that's a supported configuration...

73, Ross N4RP

On 4/6/2015 7:01 PM, Roger D Johnson 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
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

--
FCC Section 97.313(a) “At all times, an amateur station must use the minimum transmitter power necessary to carry out the desired communications.”

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Jim Brown-10

Re: KSYN3A Synthesizer problem?

In reply to this post by Roger D Johnson

On Mon,4/6/2015 4:01 PM, Roger D Johnson wrote:
> 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?

Is this with or without an antenna connected? What's the nature of the
noise on your spectrum analyzer? Broadband or with humps every 10-20
kHz? Do the peaks drift? Remember that our world is full of switch-mode
power supplies spewing out trash on harmonics of their square waves, and
the lower we go in frequency, the stronger they are.

73, Jim K9YC
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wayne burdick

Re: KSYN3A Synthesizer problem?

Administrator

In reply to this post by Roger D Johnson

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 <[hidden email]> 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
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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wayne burdick

Re: KSYN3A Synthesizer problem?

Administrator

In reply to this post by Roger D Johnson

Roger,

More on this...

Since KSYN3A buyers may not always see the FAQ, we'll be adding something like this to page 1 of the KSYN3A installation manual:

* * *

Note on K3 low-frequency (100-400 kHz) receive coverage with the KSYN3A installed:

With a KSYN3A installed, the K3 VFO can be tuned as low as 100 kHz. Sensitivity will be very good in the 600-meter band (470 kHz) if a KBPF3 filter module is installed, and the antenna is connected to either the RX ANT IN or XVTR IN jack on the KXV3 module. However, below 300 kHz, sensitivity starts to decrease quickly, because the K3's receiver was optimized for use on higher bands. Typical sensitivity at 300 kHz will be about -100 dBm. The preamp should be turned off at very low frequencies because it is not useful here; it will only increase noise level.

* * *

Let me know if you think further clarification would also be helpful.

Thanks,

Wayne
N6KR

On Apr 6, 2015, at 4:01 PM, Roger D Johnson <[hidden email]> wrote: