[Top] [All Lists]

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

Subject: Re: [TowerTalk] Are All Low-Pass Filters Alike? -
From: Jim Lux <>
Date: Tue, 16 Aug 2005 20:49:07 -0700
List-post: <>
At 06:23 PM 8/16/2005, Don Havlicek wrote:
>Correct me if I am wrong, but aren't "harmonics" exact multiples of a 
>given frequency?
>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.
>Those spurious frequencies are, correctly as stated, not filtered by a LP 
>between the Xcvr and amplifier.
>The original question I raised was concerning HARMONICS, not spurious 
>frequencies generated by the amplifier.
>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.

Virtually every power amplifier you're likely to encounter (unless you've 
got $100K plus to invest in a nice Amplifier Research unit) generates some 
significant (as in readily measureable) harmonic content.  That is, you put 
20 MHz in, and you get output at 40MHz, 60MHz, 80MHz, etc.

This is aside from intermodulation effects (i.e. put 20.001 and 20.003 MHz 
in, and get that, plus some 20.004, some 20.006, some 20.002, etc.), which 
would usually be termed spurious.

Sure, you can operate an amplifier in a more linear part of the curve.

This would be, for example, Class A, and is not particularly 
efficient.  The aforementioned AR boxes have very good harmonic distortion 
and spurious performance, are wideband, but are pretty hefty units (a full 
rack for a kilowatt of output power, and drawing  tens of kW of three phase 
power, not to mention the HVAC you'll need).  If you're running an EMC test 
lab and need such a thing, you just bite the bullet and pay the 
bucks.  Compared to the multimillion dollar investment in the rest of the 
lab (TEM cells and screen rooms), it's not that big a deal.

However, the vast majority of inexpensive amplifiers operate Class B or 
Class AB, with the pass device (tube OR transistor) only "on" for part of 
the cycle. There's crossover distortion (as one device stops conducting and 
the other starts) for one thing. And the transfer curve isn't particularly 
linear close to zero anyway.  (In op amps, you get around this by using 
gobs of negative feedback, which is easy when you've got 60 dB of open loop 
gain.  Not so easy when you've only got 10-15 dB of gain in a stage.)

So, what you do is use inexpensive devices, running in a reasonably 
efficient biasing mode, and filter the output (since ham applications are 
narrow band, this works fairly well).


See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>