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Author Topic: LPF with HPF  (Read 7490 times)
K9FV
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Posts: 489




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« on: May 20, 2012, 07:47:06 AM »

Hello all, Please take a look at this link: http://wb6dhw.com/KWDiplexer.html for the 1.5KW LPF with a HPF.  The stated purpose of the HPF is to divert the higher harmonics into the 50 ohm dummy load so they are not reflected back to amplifier causing increased heat in the finals AND to reduce IMD of the amp.

Just how much increase in heat is involved in a typical AB biased amp?  How much will will this HPF reduce IMD in the amp?

With all things considered, is it really worth the extra expense to include the HPF in a LPF for a solid state amp?

Thanks to all for comments and guidance,

73 de Ken H>
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M0HCN
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Posts: 530




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« Reply #1 on: May 20, 2012, 11:21:28 AM »

Well third harmonic is usually about 13dB or so down on the fundamental, so that would imply 5% power reflected by a simple LPF (At a KW. the reduced heat in the filter may be a bigger win then the reduced heat in the PA).

IMD is pretty much a suck it and see issue here as it could be better, could be worse depends on how things combine, and IMHO so many amps are so poor here anyway that there are much easier ways to make big improvements before worrying about reflected power.
Once you have the amp itself doing better then maybe -45dB ref PEP IMD3 then this sort of thing becomes worth worrying about, but with many SS amps not even managing to hit -30.....

Iff you have done everything else to build as clean a solid state amp as state of the art permits, then it may be worth it, but for something like an EB104 or such, I have my doubts.

73, Dan.
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TANAKASAN
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« Reply #2 on: May 20, 2012, 12:36:24 PM »

A few years ago William Sabin published a series of articles in QEX where he detailed a power amplifier, power supply and a full set of diplexer filters. In his words using an additional HPF "cuts spurious emissions, while avoiding problems caused by out-of-band mismatch: reduced output, instability, poor linearity or efficiency".

Not quite the word of God but he probably helped the Almighty design his shack  Smiley

Tanakasan
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N8CBX
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Posts: 476




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« Reply #3 on: May 21, 2012, 09:15:54 AM »

I have designed my own diplexer LPF boards for my 600W EB-104 amp and I strickly used the Sabin design & component values for my boards. Here's a picture:
http://janswerks.com/n8cbx/jans_filters_2.jpg

I am using only three bands at a time, because I'm on the lower HF bands (160/80/40), but if I wanted to go higher, I designed it so the LPF just plugs into a motherboard.

Here's a picture of a LPF & motherboard mock-up:
http://janswerks.com/n8cbx/jans_filters_1.jpg

The motherboard has a general utility purpose that will serve the three LPFs, 2ea SWR/power bridges, band relay switching, 10db exciter attenuator and RX/TX relays.

I looked at WB6DHW LPF design and he is using a higher order filter, which may or may not be necessary, I don't know. Also, he is using some larger toroid cores than what I'm using. I used T130 cores which I think will be okay for legal power levels.
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
K9FV
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Posts: 489




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« Reply #4 on: May 22, 2012, 08:05:16 PM »

Thanks to all for the input - it sounds much like I was thinking..... a solution in search of a problem.  OR perhaps I need to learn more about this aspect of RF - that is most likely.

@Jan - that is a nice setup you've got there. I understand that's with HPF in front of LPF?  What was your idea of using the diplexer rather than a simple LPF?  Can you tell any difference in IMD - if you've measured with and without HPF.

Nice setup - did you etch those boards?  They look professional!  I wish my etched boards where that nice.

73 de Ken H>
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N8CBX
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Posts: 476




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« Reply #5 on: May 23, 2012, 04:43:46 AM »

My bare LPF PCB were made by a vendor (Pad-2-Pad) and you use their free CAD software to make the artwork.
I found & read the Sabin article and that's what started my interest. I haven't fired up the amp yet, I'm still building it.
Jan
« Last Edit: May 23, 2012, 04:47:56 AM by N8CBX » Logged

Dayton Ohio - The Birthplace of Aviation
ZS6BIM
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Posts: 67




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« Reply #6 on: May 26, 2012, 03:07:39 AM »

I believe the problem of low efficiency and bad IMD at some frequencies when using only low pass filters at the output of a broadband HF amplifier is due to a specific set of impedances “seen” by the harmonics in the LP filter stop band.

These “bad” impedances seen by the harmonics cause early saturation resulting in degraded IMD and poor efficiency.

The effect is normally observed over a couple of MHz in the pass-band of a specific LP filter.

For amplifiers that are only going to be used over the relatively narrow amateur bands LP filters can be designed not to exhibit these undesirable stop band impedances or the length of transmission line between the amplifier output and LP filter input can be adjusted to rotate the stop band impedance to a more benign region.

However when an amplifier is required to operate say 1.6MHz to 30MHz these undesirables are often unavoidable and the HP diplexer is then put to good use.

73
mike
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K9FV
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Posts: 489




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« Reply #7 on: May 26, 2012, 07:32:49 AM »

Thank you Mike - that helps understand why the use of a diplexer is good rather than just a LPF. 

73 de Ken H>
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