[TowerTalk] Are All Low-Pass Filters Alike? -

[Michael Tope]

----- Original Message ----- 
From: "Don Havlicek" <n8de at thepoint.net>

> Correct me if I am wrong, but aren't "harmonics" exact multiples of a
> given frequency?

Yes.

> As such, they should be differentiated from the spurious frequencies
> generated by an amplifier which usually are NOT an exact multiple of the
> driving frequency.

Amplifiers generally don't produce non-harmonic spurious.
They produce harmonic distortion and intermodulation
distortion. Both harmonic distortion and intermodulation
distortion products have clear integer relationships to the
input signal(s).

> Those spurious frequencies are, correctly as stated, not filtered by a
> LP between the Xcvr and amplifier.

Correct. Distortion produced in an amplifier (harmonic or
otherwise) won't be affected by a filter placed before the
amplifier.

> The original question I raised was concerning HARMONICS, not spurious
> frequencies generated by the amplifier.

Yes, but among the "spurious frequencies" generated by an
amplifier are HARMONICS of the input signal frequency. If
you put a clean 7 MHz CW sine wave with absolutely no
HARMONIC content into a typical class AB HF power
amplifier, you will find that the amplifiers produce some
measurable level of 2nd, 3rd, 4th,.... HARMONICS. I think
the FCC rules state that these "Spurious" signals need to be
down something like -60dBc (those in the know, correct me
if I am wrong) to pass type acceptance. Generally, a PI-network
with a reasonable Q (~10) will attenuate these harmonics
sufficiently to pass FCC criteria. A PI-L will provide additional
harmonic attenuation. Neverthless, there will still be some
harmonic energy present at the output of a FCC type accepted
linear. In those cases where these fairly low-level signals still cause
a problem (pre-amplified TV front-end connected to an off-air
antenna), you can add a low-pass filter to the output of your
amplifier to get additional attenuation of those harmonics.

> Perhaps operating an amplifier ONLY within the linear portion of its
> curve would cure most of the spurious frequencies and limit the need for
> a LP filter post-amp.

If you ran class A, you could lower the harmonic content of the
amplifier output significantly, but the efficiency of the amplifier
would be dismal (< 25%). In most cases, if the amplifier is
properly designed and meets FCC type acceptance (good PI or
PI-L output tank) you won't need an external LPF.

> As for the statement that 'amps do not amplify harmonics' that was
> posted by another submitter .. I defer to the "experts" as to why
> harmonics ARE amplified by a well-designed amplifier .. a FACT that  can
> be easily measured.

Tune your tube amplifier up on 40 meters, and then switch the
exciter to 20 meters. Measure how much output power you get
from the amplifier when it is tuned up on 40 meters and driven by
a 20 meter exciter. It will be very small.

73 de Mike, W4EF.....................

> Don
> N8DE
>
> Jim Lux wrote:
>> At 01:10 PM 8/16/2005, Don Havlicek wrote:
>>
>>>Why don't hams place the LP filter between the Xcvr and the Amp?  That
>>>way, the amount of harmonics reaching the Amp will be minimized,
>>>rendering their output levels relatively the same, if not lower, than if
>>>the LP filter was AFTER the Amp.
>>>BTW:  this is what I have done since the 50's.  Never blew up a LP
>>>filter yet!
>>>Don
>>>N8DE
>>
>>
>> probably because the harmonics people are concerned about come from the
>> power stage, not the exciter.
>>
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>
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