Low pass filter advice #vfo #40m #80m


Peter GM0EUL
 

Hi All
Can I request a bit of a reality check before I take my next steps.  I've made the vfo kit to add to my one-valve transmitter (this is the one that's the same as Hans's first transmitter, so arguably an ancestor of the QCX!).  The vfo has the 40m and 80m low pass filters and I had a look at their characteristics because I wasn't getting as much output as I expected.  

I've measured the peak to peak voltage output from the 40m filter on a scan from about 4 MHz to 11 MHz (have a look at the 1 min video here https://youtu.be/Lozyyh2OO9w  )  Ignore the actual numbers but look at the amplitude of the wave against the frequency (bottom left readout) The amplitude doesn't vary much from about 4 to about 9 MHz then it shoots up and peaks at 9.2 MHz then rapidly falls off and is almost fully suppressed by about 10 to 11 MHz. 

The 80m filter has similar characteristics and the peak output is at 5.3 MHz and its suppressed by just over 6 MHz 

I think I need to tweak the filters so that the peak output is at the bottom of the bands of interest (i.e pull it down by about 2 MHz by fiddling with the coils) is that right or am I interpreting the data wrong or misunderstanding what its telling me?

Any thoughts or definitive wisdom appreciated.

73
Peter GM0EUL

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