> For the average ham, it's far better to measure PO1dB (1 dB output
> power compression level) and always operate linear modes below that
> point. That guarantees the best compromise between power out and
> linearity on any amplifier with minimal test equipment and
> experience.
The average ham has even less ability to accurately measure the
1 dB compression point on an amplifier than he has of accurately
measuring IMD. Most don't have one accurate wattmeters much less
two ... nor the ability to understand the issues of exciter foldback
and the changes in SWR between the exciter and amplifier with tuning.
73,
... Joe, W4TV
On 5/23/2010 3:49 PM, Larry wrote:
> Well Jeff,
>
> I both agree and disagree with your statements below. I agree with
> your statement about guys not measuring at all. But I strongly
> disagree with telling these same guys they can do "adequate"
> measurements with an incorrect setup.
>
> The guys you are targeting are not competent enough to discern the
> difference between sound card setup issues, exciter issues, and
> amplifier issues. The average guy needs to have an idea what is
> required to do real testing of an amplifier BEFORE you give him the
> easy way out. And you need to detail the "easy" setup including the
> tests at each stage.
>
> I strongly disagree with proliferating half ass measures which will
> invariably produce even more junk science than we are already dealing
> with.
>
> For the average ham, it's far better to measure PO1dB (1 dB output
> power compression level) and always operate linear modes below that
> point. That guarantees the best compromise between power out and
> linearity on any amplifier with minimal test equipment and
> experience.
>
> 73,
>
> Larry - W7IUV DN07dg - central WA http://w7iuv.com
>
>
> On 23-May-10 12:11 PM, Jeff Blaine AC0C wrote:
>> Larry,
>>
>> You are 100% right if an absolute accurate measurement is needed.
>> But if you simply want to know if you are in the 20's or the 30's,
>> then the method I describe is fully adequate to the task.
>>
>> I just completed protos of a crystal modified Wentzel design with
>> some tweaks added by Bill Carver that is for high DR3 range
>> receiver testing. We are talking about IMD levels for the entire
>> system of sub -110dbc. Right on all counts - LPFs needed - ferrites
>> needed - lots of added return loss via pads sprinkled. I'm fully on
>> board your claim here. But just not for Joe's basic ham shack
>> needs. For the basic "is my tx/amp combo generating crap" kind of
>> test, the 10dbc better rule of thumb for the sources is good enough
>> and the modern sound card without being overdriven is fully up to
>> the task.
>>
>> The problem as I view it is that guys are not measuring at all. Or
>> if they are it's with something like an SB610 or whatever. To my
>> thinking, better to have some minimal indication even if it's
>> "gross" in resolution than none at all. This solution provides that
>> - it's better than the SB610 at least. And given the number of
>> "I've never built anything before and don't really know what I'm
>> doing" guys comments we see on the boards, this has got to be a
>> step in the right direction. Even if it's filled with significant
>> shortcomings.
>>
>> 73/jeff/ac0c
>>
>>
>> -------------------------------------------------- From:
>> "Larry"<larry@w7iuv.com> Sent: Sunday, May 23, 2010 1:51 PM To:
>> "Jeff Blaine AC0C"<keepwalking188@yahoo.com>; "AMPS"
>> <amps@contesting.com> Subject: Re: [Amps] How to read the 3-500Z
>> spec sheet?
>>
>>>
>>> On 23-May-10 10:45 AM, Jeff Blaine AC0C wrote:
>>>>
>>>> Of course, a real IMD test requires a 2 -tone source and you
>>>> can download those applications on the web free of charge.
>>>
>>> Wrong answer for amplifier testing. While you can approximate
>>> proper "two-tone" testing with a sound card tone source when
>>> evaluating a transceiver, a LOT more is required for accurate and
>>> technically correct amplifier testing.
>>>
>>> To do accurate IMD (and more correctly OIP3) testing on an amp,
>>> you NEED two separate clean signal sources. This means two
>>> exciters that are known to be very clean and free from harmonics
>>> (much greater than 60 dB down) plus a high power combiner and
>>> usually a set of bandpass filters at each of the tone
>>> frequencies. Plus, just to keep everything happy, a bunch of
>>> attenuators sprinkled around the bench.
>>>
>>> Anything less and you are only testing the source(s) and not the
>>> amp. Some time back, W8JI detail the correct test setup for
>>> amplifier testing and I think it's on his web site someplace.
>>> Anything less and you are just fooling your self.
>>>
>>> 73,
>>>
>>> Larry - W7IUV DN07dg - central WA http://w7iuv.com
>>>
>>>
>>
>>
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