Re: BITX40 LPF mod

Howard Fidel

I tried a similar approach tonight in my simulator. Seems promising to me also. Once I fix my uBitx, the Arduino died, I will try it.

On 8/8/2018 10:50 PM, K5ESS wrote:

This is the filter approach taken by Wes Hayward W7ZOI for his “Updated Universal QRP Transmitter”  (QST April 2006) although Wes didn’t add in the series resistors to broaden the notch..  Wes has a table of component values for all the ham bands 80 through 10 meters (including the WARC bands)




From: [] On Behalf Of Arv Evans
Sent: Tuesday, August 7, 2018 10:46 AM
Subject: Re: [BITX20] BITX40 LPF mod


Re.  BITX-40 LPF Mod

When I added the parallel capacitance to notch out the 3rd harmonic the simulator showed

a very narrow notch.  Today I tried adding a resistance in series with that parallel capacitor.

The result is a much broader notch at approximately 21 MHz, but still with 63 db of attenuation.

LTspice IV -

The upper trace is frequency response and the lower trace shows phase shift.  Adding the resistor seems to have

tamed the phase shift so it does not show a dramatic transition at resonance of L7 and C3.

Next possibly illogical step is to notch out any 2nd harmonic energy at 14 MHz.  This was done with a series trap

(R3, C1, L4).  With R3 at 1.5 ohms this looks promising.

LTspice IV -

Upper trace shows frequency response and lower trace is phase shift.   
Results look like (1) no additional insertion loss, (2) Reasonably broad notch at 14 MHz, and

The notch at 21 MHz is still there, and (4) overall LPF roll off still starts at 10 MHz.

Will this actually work in a real-world circuit?  It will be interesting to find out.  If it does work it

could insure that 2nd and 3rd harmonics can be reduced to way below FCC requirement for the

BITX-40 transceiver.  The trap idea is interesting because more traps could possibly be added

to take care of any 12 MHz IF leak-through, or any other spurious signal that looks like it might

be a problem. 

Could this approach be applied to the uBITX filter problem...maybe, but I will leave that to people

who are much smarter than myself.



On Mon, Aug 6, 2018 at 10:22 PM Arv Evans <arvid.evans@...> wrote:

Glen VK3PE

That is something that can be tested.  It doesn't show in the simulator, but real-world

may be different.  When I get to actually wiring this it will be easy to test.

Thanks for the info.



On Mon, Aug 6, 2018 at 10:18 PM Glenn <glennp@...> wrote:

I read somewhere that its a good idea to drop the inductor value by 25% or so, (cant recall exact %) then calculate the parallel Cap value for the required notch.  Otherwise i think the RL is compromised.
Or my memory is suspect, quite possible........

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