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I tested transmitting from my new zBit into a tinySA with 50 dB of attenuators in between.
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First the good news, the 2nd and 3rd harmonics seem ok (43 dBc down or more).
But I also see spurs at carrier +-2 MHz, and a 2 MHz, that seem to be a higher level than 43 dB below carrier.
These 2 MHz sideband spurs were still visible when testing output on multiple other bands, from 40M to 12M.
I don't see such 2 MHz spurs when testing my other transceivers, so they don't seem to be from the tinySA.
Wierd measurement error? ?Or is my zBitx bad? ?Or could this be a hardware design or software issue.
Or inter-mod from one of the digital circuit clocks or IO or display, requiring more shielding.
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I also hear noise on the audio, even with no antenna connected, pulsing at about 3 or 4 Hz.
Ideas? ?Potential solutions?
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Thanks and 73, Ron, n6ywu |
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If you probe the Si5351a output? with a series resistor? of say 2k2 or more to attenuate the Oscillator level before going into the Analyser. Do you see the spurs.
The chips don't like heavy loading nor DC coupling so may introduce issues.?
I haven't got my Zbitx as I ordered it recently so can't compare.? |
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Ron, when I was working on one of the used sBitx V2's that I bought I had problems with spurs 4MHz or so off, and I believe I recall a 2MHz absolute spur once.? ?Of course the low pass filters won't stop most of these.? ? I went back through my records but couldn't find what I did to deal with those (it may be recorded in the posts I made however).? ?I suspect it might be adjustable by the comparator bias setting, but I'm not sure.? ? ?I'm sorry I don't have more remembrance.? ?But I've seen things like what you are setting?in the zBitx.? ? Perhaps others can provide more light. Gordon KX4Z On Mon, Apr 14, 2025 at 9:09?AM Ron, N6YWU via <ron.nicholson=gmail.com@groups.io> wrote:
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I found a note where I had seen 2MHz spurs.? ?You can see them in the screen of the spectrum analyzer. In that threat (of > 340 entries)? I finally apparently got everything fixed by adding a ton of capacitors and adjustments.? ?Some bypasses apparently made it WORSE.? ? In this post, I listed all the changes I made -- including moving all the Si5351 oscillators to 2 mA drive instead of 8mA Building radios is not for the faint of heart!!? ?I have a lot of appreciation for Ashhar and all he has built -- this is NOT easy and bringing a product to market almost by himself is quite an achievement. Gordon KX4Z |
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Hello Ron,
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I can confirm, that there are spurs +/- 2.4MHz around the carrier in the zBitx output signal.
This is the output spectrum of my zBitx with factory calibration operated @ 8.0V with sBitx V3.052 firmware in CW mode @ 7.074MHz, Drive=100:
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You see that there are spurs +/- 2.4MHz around the carrier, which are only 28dB (!) below the carrier and additional spurs (intermodulation?) at -46dBc.
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Measurement with a TinySA Ultra connected via 50dB (20dB + 30dB) attenuators.
Adding another 30dB of attenuation does not make a difference in the measurement result: The TinySA will then keep its internal attenuator at 0dB, but the spurs will still be -28dBc.
Reducing Drive to 50 will reduce the spurs to -31dBc.
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Doing the same measurement with Drive=100, mode CW @ 14.074MHz also shows a lot of spurs, that are not sufficiently suppressed:
The worst one is only -23dBc @ approx. 2.3MHz. So it seems, that something in the range of 2.3 to 2.4MHz is somehow injected into the output signal.
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And finally 15m 21.074MHz:
Again a strong spur -32dBc 2.1MHz? below the carrier.
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--
73, Joerg - DB2OO
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开云体育It looks like the things that I did way back then to my developer edition to fix this issue involved:Reducing SI 5351 current to 2 mA Adding additional bypass to various power supply lines, usually by simply paralleling on an existing bypass capacitor Increasing final mosfet idle current Gordon kx4z? On Apr 14, 2025, at 13:19, Joerg DB2OO via groups.io <db2oo.ju@...> wrote:
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If zbitx/si5351v2.c is the correct place to look, then in lines 243, I see the variable drive_current being set to 2ma. But then that variable is being ignored, and the Si5351 is being configured in line 249 :
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set_freq_fixeddiv(clk, pll, frequency, pll_div,?
? ? ? ? ? ? ? ? ? ? SI5351_CLK_DRIVE_STRENGTH_8MA); ?
Changing this to 2 mA might help reduce the spurs according to KX4Z (Thanks!).
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- Ron
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开云体育Cannot guarantee it, but might be worth a try. Gotta fix those little problems. All radios have issues.On Apr 14, 2025, at 17:40, Ron, N6YWU via groups.io <ron.nicholson@...> wrote:
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