Back to the SDR-2018: Examine Low-Pass Filters

To give credit where it's due, I completely stole this filter topology from Icom and adapted it to my circuit and junkbox inventory - it's NOT my original idea, I just think it's cool.

Here's the 40m LPF from my SDR-2018, as modeled in LT-Spice:

 When I first saw this circuit, it had me scratching my head for a moment.  In essence, it looks similar to pretty much every other LPF, but wait, what's with C4 and C5?  Then it hit me: They work in conjunction with the coils to create traps at the 2nd and 3rd harmonics of the 40m band: L2/C4 are resonant at 21 MHz and L1/C5 at 14 MHz.  Nice!

Here's a sweep of the circuit without C4-C5:

  And here 'tis with them:

Here's what makes it cool: Nobody likes winding toroids, but with the standard Chebychev or Butterworth topologies, you'd likely need at least one, maybe even two more stages to ensure the 2nd harmonic is down >40dB.  That's one or two more toroids to wind, not to mention the extra caps and board space.  Here, we get massive harmonic attenuation with only two additional caps. I don't know about you, but I find that appealing.

It's easy to apply to an existing design as well.

Pete, N6QW, recently posted on his blog (n6qw.blogspot.com) about the driver/power-amp circuit he's developed for his IC transceiver, that includes a low-pass filter that, in his comments, he mentions may not be adequate.  So, let's look at it:

Would it work?  Probably, but with a single-ended output, I'd be a little concerned about the 2nd harmonic. 

Now, we calculate the values of capacitors that will resonate with the coils on 20 and 15 m, select the nearest standard values and plug 'em in:

And Fannie's your aunt.  Look at that rejection, just look at it!

Note that I did have to tweak L2 to achieve resonance with the 47pf cap, one could probably accomplish this by simply spreading out the windings a bit, or, worst case, removing one - no big deal.

My next post will be the final installment of the Echophone restoration, and includes a tale of woe. Never lacquer over enamel.  Just don't do it.