Re: a simple spur fix

Jerry Gaffke


So no second 45mhz filter, no surface mount inductors at L5,L7,  L1,L2,L3,L4 ?

TP13 is not shown in the v3 schematics, only in the v4 schematics.
I assume these mods work will work on the v3 boards too.
TP13 is the node at T2-pin3 in the first mixer.
I assume "across TP13" means that we add the cap and inductor in series from that node to ground.
Will be interesting to see if it matters where we physically place it, at T2-pin3 would be nice
because it could be added to the bottom of the board directly to the ground plane.

The second series cap and inductor get placed across C202, in the middle of the 30mhz low pass filter
on the far left side of the schematic.
So just leave C202 as it was, but add the cap and inductor from
the through hole pin for L2 (or L3) to the ground plane.

Will be interesting to see how picky we have to be about those inductors.
If we're lucky, they can be most any small surface mount inductor.

Jerry, KE7ER

On Fri, Nov 9, 2018 at 07:35 AM, Ashhar Farhan wrote:
so, there is one spur fix that is now consistently working for me.
among rhe solutions, i was looking for one that would be easy to apply.
first, the distortion in the 45 mhz amp is creating harmonic distortion. so a 0.3uh in series with a 10 pf is used across TP13 (output of the 45 MHz IF amp going to the front end mixer). 8 turns on T30-6 gives you 300nh.
second, there is a cross talk inside the Si5351 where the 12 mhz from the bfo leaks into the local oscillator. A 5uh in series with a 47 pf across C202 of the LPF will suppress it. 35 turns on T30-6 gives you 5uh.
- f

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