Re: Can you use multiple outputs of SI5351 in a high performance VNA?
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Add to that -- termination at the operating impedance level will mean (if it's 50 ohm out) a 50 ohm resistor across the driven and return lines of each output, which naturally shunts all output to ground.
If you just need to pull the unused outputs down, a 1K resistor is what's usually specified by the? manufacturer, but you should check the datasheet first.
I certainly would NOT just tie an output to ground, because that gives a zero-ohm, extremely low impedance path for any stray energy to get out.? Kinda like (from my previous "water" analogy) punching a hole in the side if the toilet tank because the flapper valve is leaking.
There's a good possibility that SiLabs may have missed it on this one, since we do things that they may not do.? So opening a dialog with their tech support may be a useful idea.? And as it happens, they're in the same town I am, and I'm a tech writer in need of a job right now ... maybe I should ping them too.
I'd like to thank all who've shown interest in this topic.
73 Jim N6OTQ
On Fri, Jan 18, 2019 at 10:37 AM Jerry Gaffke via Groups.Io <jgaffke= [email protected]> wrote: Programming the chip to drive an unused output low, and then externally shorting that pin to ground,
will effectively give that chip a second ground.? A very good thing, since the si5351 only has one ground pin.
Could make a difference when we're talking about crosstalk 1000+ times weaker than the desired signal.
Perhaps my career as a mostly digital guy was showing there.
However, I would suggest you verify that the pin is at ground potential after the firmware has loaded it
before shorting it to ground with a wire.? On the si5351 that unused output should be clk1, between clk0 and clk2.
Jerry, KE7ER
On Fri, Jan 18, 2019 at 06:35 AM, Jim Strohm wrote:
If practical, why don't you try terminating the unused outputs as close to the chip as possible, just to see what happens??
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