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Technical Topics => The RF Workbench => Topic started by: OZNRH on July 27, 2014, 1105 UTC

Title: Don't use a LP filter, use a Diplexer instead..
Post by: OZNRH on July 27, 2014, 1105 UTC

Hi here  :)

What happen with your harmonic in a normal LP filter? They are sending back to your PA transistor as 'mismatch and heat'. Therefore send them to a separate 'harmonic load' instead, this is what a Diplexer do.

Best 73 from Ole  :)..

Title: Re: Don't use a LP filter, use a Diplexer instead..
Post by: OZNRH on July 27, 2014, 1111 UTC

Here you can see the curves for a 49M Diplexer. There was no 'image space' in my first post.

Ole :).

Title: Re: Don't use a LP filter, use a Diplexer instead..
Post by: ff on July 27, 2014, 1435 UTC

An interesting concept Ole.  All of my previous builds have been about 40 watts carrier power or less so a diplexer is kinda overkill.  However, the Class D circuits I am now playing with would definitely be happier.  I generally calculate my LP cutoff for about 1/10 octave above the operating frequency.  What would you suggest for the LP and HP cutoff frequencies in a diplexer?

Title: Re: Don't use a LP filter, use a Diplexer instead..
Post by: OZNRH on July 27, 2014, 1738 UTC

@ ff  :)..

As a princip, I like 'overkill' in my equipment, both on my RF harmonics and the AF spectrum I send out (20 Hz to 6 kHz. with a soft clipper) and with a low distortion as possible. My listeners tells me, my AM sounds is near 'FM quality'.

Okay I know a Diplexer gives not a better harmonic spectrum, but it give my PA transistor in my driver more better work condition and that a good thing for both linearity and intermodulation in the stage.

The cutoff frequencies in a diplexer?

As you can see on my curve pic, its give me possibility to work down from 6 MHz and up to about 7 MHz. with reasonable data. Everything over 12 MHz. go to the load on the HP part.

Ole :).