Hi John, thanks for confirming the command.
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This is a friend's u4b, that should have been working.
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Anyway after some further investigation:
The SW_SYNTH line is asserted correctly. The TCXO does start (the leakage is just much less than when it's calibrating using CLK2) and it does attempt to send CW.
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But the signal on RF1 and RF2 is not the requested 14 MHz etc, instead it is this 40 kHz waveform:??
1) both in phase. 2) RF_2 is stronger than RF_1 as it doesn't have the 100 n capacitor. 3) both appear AC coupled but the scope, probe etc is all DC coupled. 4) No other RF tone apart from harmonics of this one.
The frequency changes between 30 kHz and 50 kHz when I change requested frequencies, but with no obvious connection.
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I was about to say the Si5351 on this board is probably damaged by ESD, and this strange signal is what's leaking out of its damaged output circuits, but look at this, when I run Hardware Test, I get a nice 25 MHz signal on both ports (both in phase but still, it's working!)
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One idea now is that this is a bad or subtly damaged Si5351. In other experiments I find that it starts misbehaving at multipliers and clock frequencies that the datasheet say should work.
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If anyone has working BASIC code that controls the Si5351 with a series of I2CW statements, please let me know.
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Thomas VE7TOA