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Author Topic: Low Pass Filters in HF Amps  (Read 3176 times)
AI4NS
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« on: July 08, 2007, 06:54:19 PM »

In looking at schematics for various amps, I don't see anything in the way of LPF circuits. Would that not be good engineering practice to have them? Or do amp manufacturers rely on the spectral purity of the exciter?

Mike
AI4NS
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KB9CRY
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« Reply #1 on: July 08, 2007, 08:11:58 PM »

Yes
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K6AER
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« Reply #2 on: July 08, 2007, 08:15:03 PM »

Most amplifiers have a fairly high Q tank circuit. As a result very little generation of the second and third harmonic is possible. For brut force VHF harmonics many will install a after market low pass filter after the amplifier.

Today’s transceivers are broad banded in final operation and as a result have switchable low pass filters for the harmonic outputs. If the transceiver output is clean then typically the amplifier output will also be clean.

Some specialty amplifiers such as the Commander VHF 1200 for 6 meters have a final band pass filter after the tank circuit before the output connector. This is to reduce TVI amplifier phase noise.
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WB2WIK
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« Reply #3 on: July 09, 2007, 08:26:54 AM »

It depends on the amplifier.

Solid state amplifiers that actually meet the emissions requirement (limits) of Part 97 have either bandswitched low pass filters (internally) or automatic antenna tuners which perform the same function.  I don't know of any that don't.

Tube type amplifiers use the typical pi-L output network as a low-pass filter, because it is one, and a very effective one.

WB2WIK/6
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KE3WD
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« Reply #4 on: July 09, 2007, 09:08:04 AM »

Solid State amplifiers that are not FCC type accepted for use on the Amateur bands do not have output filters in them.  These are the infamous "CB" solid state amplifiers that sell under names like "Texas Star", "Palomar"(NOT the original Palomar but clones using the name) etc.  While they can be used on 10 meters ONLY, a ham shouldn't try that unless they have the service and test equipment to prove that the things will not radiate harmonics or spurs.  Overdriving these amps is easy and will lead to a mess on the bands, too.  

Whether using tube or solid state amp purpose designed for the Amateur Bands, it is still a very good idea to invest a few dollars in a Low Pass filter able to handle the power level, place it between the amp output and antenna tuner using the shortest possible coax jumpers.  Good insurance IMO.  

Overdrive splatter is by far the worst culprit on the Amateur Bands in my experience looking at sigs on the spectrum scopes, those amateurs who are adament about not hooking up the ALC properly may not realize when they are exceeding amp drive limits.  Splatter-splatter.  


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AI4NS
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« Reply #5 on: July 10, 2007, 04:03:41 AM »

Thanks everyone for the replies. Makes sense. I looked at some solid state amps, and they have filter assemblies.

Mike
AI4NS
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