Re: a simple spur fix

Jerry Gaffke
 

How did you arrive at 47pF and 5uH for 12mhz?
freq = 1/(2*pi*sqrt(L*C)) = 1/(2*pi*sqrt(47e-12 * 5e-6)) = 10382123 Hz
Though that 30mhz LPF is a fairly complicated network to hang an LC trap on,
there may be more going on here. 

The 12mhz trap my not be needed if you reduce the loading on the si5351 clocks
since this would reduce crosstalk between clocks within the si5351.
 
Table 3, "DC Characteristics", in the si5351 datasheet says the output impedance is 50 ohms.
My best guess is that this means there is effectively a series 50 ohm resistor between the 
CMOS driver and the output pin.  If you then put a 50 ohm external load to ground on it and 
the driver is trying to drive high to 3v, then the expected current would be 3v/(50+50ohms) = 30ma.
But the output driver is limited to 8ma, so that output will be current starved.
And with three clocks all working hard as they possibly can, the crosstalk is pretty bad on the si5351.
So we want an external load that's more than 50 ohms on the si5351 clock pin,
but we want the mixer to see a 50 ohm source impedance to avoid reflections.
A pi network resistive attenuator is not a good choice.

I suggest each si5351 clock pin drive a series DC blocking cap and a series 100 ohm resistor
into the mixer port.  Assuming the si5351 has not hit its 8ma current limit, the mixer will
see a source impedance of 50 ohms (internal to the si5351) plus that 100 ohm resistor, so 150 ohms.
Add a 75 ohm resistor from mixer port to ground, that in parallel with 150 ohms makes
it a 50 ohm source impedance for the mixer.

Jerry, KE7ER

it 


On Fri, Nov 9, 2018 at 05:53 PM, Ashhar Farhan wrote:
allison and others,
 
the traps are capable of adding a notch of -20 db. so even if you don't hit the bottom precisely, a 10 db reduction is all that is necessary to clean it up to the required level.
 
these are meant to be fixes that work with existing boards : from v1 onwards. i tried to choose a method that was easy to do.
we will need a different post mix if amp design going forward
yes the traps are for 90 mhz and 12 mhz.
 
the 10 mhz insensitivity is something i will look into today, I havent experienced it. perhaps we should move the trap to tx filters for the 21 mhz to 28 mhz band? the 12 mhz is filtered for the lower bands in any case.
 
the other place to apply the 12 mhz trap would be the local oscillator output?
 
another simple mod to be applied is to change R26 and R46 from 470 ohms to 220. the darlington pair was having insufficient current adding to distortion woes.
 

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