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Erik,
You originally put the attenuator at the very front end.? Your latest schematic shows it after
the low pass filter.
Which do you reccommend?
Also, I see you redesigned the filter for 350MHz?
73,
Gary
WB6OGD

It should be at the front.
The 350MHz filter has not been tested, its an elliptical filter and maybe too critical in component values.
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HBTE Files section:?/g/HBTE/files
Erik, PD0EK

I have built a 9th order Bessel 300MHz lowpass, according to the parameters from?
I set the minimum capacity to 2.2pF and minimum inductivity to 10nH:

I have built this filter with SMD parts in "ugly construction":

and measured the S21 with a NanoVNA with these results:

The -3dB cutoff frequency is a bit lower, most probably due to parasitic capacities. Moreover the stopband attenuation is MUCH worse than calculated, which is most probably due to the construction technique: no shielding, potential issues with undefined currents on the ground plane.
To validate, that this is not a measurement deficiency of the NanoVNA, I replaced the filter with a 60dB (20dB+40dB) attenuator and got a pretty clean result:

The peak at ~300MHz is an artifact of the NanoVNA, that is switching output power of the SI5351 at 300MHz.

I will further investigate, how to improve the stopband attenuation of the 300MHz lowpass using a different construction.
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Joerg, ex-DB2OO

Joerg,

You have to ensure the inductors can not "see" each other otherwise the magnetic fields will destroy the stopband. At least put consecutive inductors on 90 degrees angle?
The SMD capacitors interact with the inductor windings when mounted on top of each other. Difficult to get a bit distance....
--
HBTE Files section:?/g/HBTE/files
Erik, PD0EK