Re: BITX40 LPF mod

Jerry Gaffke
 

Yes, I've been following what you are doing with interest.
A significant contribution if knocking down two harmonics can be done with just a couple extra caps.

The original intent on the uBitx was to get by with fewer filters by assuming the push-pull final
would have a negligible second harmonic.  It seemed Farhan got that to work,
but it had to be adjusted very carefully. 
I'd expect the second harmonic to not be much of an issue.

On the Bitx40, the second harmonic is quite severe due to how unbalanced the final is.
With only 100ma of quiescent current, the IRF510 is not providing much ooomph on low going peaks.

I'm seriously considering putting together a signal analyzer along the lines of your earlier post.
    Step_Attenuator -- 50mhz_LPF -- diode mixer -- MMIC_Amp -- PX1002_SAW -- ad8307 -- Nano_ADC

In parallel with the AD8307, add an SA612A plus audio amp so I can hear it.
There's lots of information in that audio if you know what to expect.

Jerry, KE7ER
 


On Tue, Aug 7, 2018 at 09:19 AM, Arv Evans wrote:
This morning I posted more information for using an LCR series trap on the output to suck
out 2nd harmonic energy.  This too seems to work, at least in the LTSpice simulator.  Will
probably be trying this in the real circuit sometime in the next week or so. 

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