Wednesday, May 12, 2021

Low Pass Filter Construction



Background

Next step after building a functioning transmitter: clean up the signal. 

Moving Forward

To create a Low Pass Filter, I used information readily available at the G-QRP site. A good page is the A Short Guide to W3NQN Harmonic Filters for QRP Transmitter Output. - George Dobbs G3RJV. This is a good overview, but I found the older version that I think, well, would be an easier for the time being. A copy of George, G3RJV first paper can be found here: A Complete Do-It-Yourself Kit ....... with just a few simple calculations (George Dobbs G3RJV)

Design & Build

Here's the design:




And here's my parts:

  • 3 x T37-6 (for L2, 4, 6) 
  • 2 x 220 pF and 47 pF capacitors. These will be in parallel for a 267pF cap substituting for the 270 pF caps for C1 & C7
  • 2 x 470 pF and 220 pF capacitors. These will be in parallel for a 690pF cap substituting for the 680 pF caps for C3 & C5

Finished Product

Here's what the final product looks like:

Top Side

Underneath

Results/Performance

I listened for oscillation at 2x (14.260 MHz), 3x (21.369 MHz), and 4x (28.492 MHz). I never really heard anything on 14.260, but certainly did on 21.369 and less so on 28.492. The following video shows the test on my receiver and scope plus a quick voltage calculation on the dummy load.



Next Steps


  • Finalize capacitor value
  • Assemble into an enclosure. Add BNC antenna connector and 3.5mm jack for a straight key
  • Make some QSOs




6 comments:

  1. Anonymous13/5/21 11:15

    I think this one is better then the dual band I've made since it has more coils and capacitors. For sure I will try this version at some point as well and make it the same dual band as the one I made for the 40 and the 80m bands.

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  2. FB, Ciprian. I found the design on the GQRP pages (indicated above). There is a "newer" version out there with an additional capacitor across the center inductor. My scope indicated that the "original" version that I built did just fine....

    Waiting on a 2.1mm barrel connector for power and then I can put the entire rig into a nice enclosure.

    Calculated power 1.2W is where P(watts) = (V + 0.25)2 /25 (0.25 is the voltage drop over the dummy load diode)

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    Replies
    1. Anonymous13/5/21 16:40

      I will look into the "new version" as well but I'm pretty sure even this one does better than mine so it's good enough for me hehehe. I'm about to built a new dummy load now with a wat metter incorporated to make my life easier ))) So far the only tools I had were a multimeter and a capacitor meter... good enough for me, but it seems that it's nice having other stuff too... makes my life easier )) 73 and have an amazing weekend. I started working on the new version of the Sputnik receiver so I can post it on Sunday.

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    2. Ciprian, looking forward to seeing the latest version of your receiver....

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    3. Anonymous14/5/21 09:54

      Meanwhile I'll also wait for your finished transmitter )) 73 and have a nice weekend.

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    4. Ciprian: the transmitter is now "on the air:" https://jackhaefner.blogspot.com/2021/05/ten-minute-transmitter-on-air-with.html

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