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Author Topic: noise from DDS VFO  (Read 10766 times)
YL3GND
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Posts: 55




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« Reply #30 on: October 08, 2017, 02:45:07 AM »

Yes, G3RZP, exactly this is the way how MC1350 works.

My guess about this transistor circuit: May be input and/or output impedance changed too much with gain adjustment?

Thanks, Dave! They had published these schematics only 2 month ago. I was unaware of this. Do You know something more about Si5351 usage in KX2 ? It would be interesting to know a bit more.

Egmont
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G4AON
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Posts: 1020




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« Reply #31 on: October 09, 2017, 12:22:28 AM »

Thanks, Dave! They had published these schematics only 2 month ago. I was unaware of this. Do You know something more about Si5351 usage in KX2 ? It would be interesting to know a bit more.

Egmont
Sorry Egmont, all I know is what is shown on the KX2 diagrams on the Elecraft site. I have a KX3 and a K3 with the updated synths, but don’t have a KX2.

73 Dave
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ZENKI
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Posts: 1421




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« Reply #32 on: October 09, 2017, 01:40:07 AM »

Dont look at the early K3 design. The design has terrible SPI bus noise. Elecraft did produce a mod for this noise that works OK with the old synthesizer. Upgrading to the new synthesizer is no guarantee that you can get rid of the bad layout in the K3's SPI bus lines.

 I have 3 old K3's  and upgrading all the  synthesizers to the new synthesizer including  the sub receiver made matters worst.  I have tried 8 of the new synthesizers already! I know of at least 10 people at the club with old K3's who have this SPI bus noise problem with  the new synthesizer. My advice is not to buy a old K3 and expect to upgrade to the new synthesizer you might get unlucky and end up with horrible noise as you spin the VFO.

I will dump these old K3's once something more viable with knobs comes along, maybe the K4.  DDS noise is evident on most designs if you know how to look for it.


Very interesting, Dave! About KX2 using Si5351, have not heard this before. Also I am interested in K3S LO circuitry (contemporary Elecraft is hiding their circuitry) - does it have PLL loop with Si-570, or it goes straight out of the chip?

Egmont
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G3RZP
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Posts: 8123




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« Reply #33 on: October 10, 2017, 07:35:13 AM »

The MC1350 is very old now. The device was meant for TV receivers: the same chip in the professional version in a metal can was the MC1590. Plessey produced the SL6140 which was basically the same thing but with much greater bandwidth.

As far as my discrete transistor IF was concerned, variation of input impedance of each stage didn't matter except for the input stage, which was low a noise fixed gain buffer so its impedances were defined.
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