Practical rules to minimize spurs when using the SI5351

After more testing some rules:

Target output is: The output required to have minimum spurs
All dB levels in the rules refer to the target output level as 0dB

- Muting an output and stopping the clock will remove all spurs caused by that output/clock
- Any use of fractional PLL will increase the number of spurs on ALL outputs (at < -70dB)
- Any use of fractional PLL on output next to the target output will cause substantial spurs (> - 60dB)
- Avoid any frequency close to but not exactly a multiple of 5MHz
- The spurs in multiples of 5MHz can be above -55dB

In practice:
With an IF2 at 10.7MHz, set IF1 to xx.7Mhz i.s.o. xxMHz so LO2 can be set to xx-10 MHz and make sure xx is a multiple of 5MHz

Example:
IF1 is 40MHz, LO1 is 45MHz, IF2 is 10.7MHz, LO2 is 29.3MHz
The tested SI5351A uses a 25MHz XCO as reference, hence the multiples of 5MHz, will be different if you use a 27MHz XCO
The examples use output 0 for LO1 and output 2 for LO2 and keep output 1 muted low


Changing the IF1 to 40.7MHz
So IF1 is 40.7MHz, LO1 is 45MHz, IF2 is 10.7MHz, LO2 is 30MHz



Of course when changing LO1  to tune differently this will cause spurs in LO1 so there is never a generic good solution for the tuning signal but all other frequencies should be multiples of 5MHz to avoid a accumulation of spurs

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Using multiple SI5351 outputs

My home build VNA uses a single SI5351 to generate both the test signal and the LO for the downmix to the audio IF where the I/Q demodulation is done.
The VNA has some problems in isolation and one possible cause is leakage from one output of the SI5351 to the other output so we should measure how good/bad this is.
One output of the SI5351 is connected to the SA with 30dB attenuation. The other output of the SI5351 is not connected. The third output is always mute and not frequency has been setup
The SW steering the SI5351 uses for each output a separate VCO to have fully independent frequency control.
Different attenuation levels where used to verify the seen signals where not created due to IMD in the SA.
First with 10MHz on one outputs and the others muted. Seems OK, all spurs are at least 70dB down.
And yes, there is a small mistake in the auto marker algorithm of the SA




Then with a second output active at 15MHz. The second output is visible at -55dB, but also a strong spur at 5 MHz (-62dB). This already explains the isolation problems in the VNA




Will it get worse at higher frequencies? First with two outputs both set to 50MHz, a nice clean signal



And now with the connected output set to 50MHz and the open output to 52MHz. The level of the spurs is only 42dB down. When you send this as LO to a mixer with a LO-IF isolation of 30dB the not connected output will appear at -72dB in the IF output of the mixer which fully explains the (lack of) isolation performance of the VNA using this SI5351 



But is this a result of the output stages doing some mixing or is it earlier? Lets test with the connected output again set to 10MHz and and the open output to 66MHz



Measurement at other frequencies have confirmed that the direct output leakage increases with increasing frequency but the other spurs (like seen above) are not directly related to the selected output frequency. The modulo  difference between the frequencies of the two outputs determines the level and position of the spurs and these follow a pattern that repeats a number of times over the total frequency range. It may be possible that the coupling/leakage is through the PLL loop in the SI5351. More to investigate.