Re: a simple spur fix

Jerry Gaffke
 

Farhan's 90mhz filter is clearly meant to take out the 2'd harmonic of the 45mhz IF signal.
It's perhaps 90mhz and not the calculated 91mhz due to some capacitance in the node it connects to.
May need a trimmer cap to tune it for exactly 90mhz unless low tolerance components are used.
The user could have Nano code that sets clk2 to 90mhz and unbalances the mixer with CW-KEY,
then adjusts the trimmer cap for a null while monitoring the 90mhz clk2 with a diode RF probe.

Perhaps no diode RF probe is needed for this if we also set clk1 to 90mhz+12mhz+500hz,
and we monitor audio in the receiver for a null in the resultant 500hz tone while adjusting the trimmer.
I'm assuming some of that 90mhz energy can sneak by the 45mhz filter somehow.

The  proposed 12mhz (10.4mhz?) trap in the 30mhz LPF might be more problematic.
Perhaps easier to add a high pass filter to clk2 instead, removes any 12mhz energy present on clk2
before it gets to the D1,D2 mixer.  No critical tuning required, and does not trash 12mhz (30m?) reception.
That in addition to monkeying around with the 6dB pads on clk0,1,2 to reduce crosstalk.

Jerry, KE7ER



On Fri, Nov 9, 2018 at 10:20 PM, iz oos wrote:
The 91mhz might make sense to notch out the harmonics of 45mhz if this is the cause of the spur.

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