On 23-May-10 6:15 PM, Joe Subich, W4TV wrote:
>
> > NO NO NO NO. You only need to know the RATIO between the output
> > and input power. That's what a dB is... A RATIO .. That's what
> > GAIN> is.... A RATIO.... Even an uncalibrated watt meter will
> > give you that information as long as it is linear.
>
> NO! You have to know what the two values are in order to know
> what the ratio is, Even if you're using a two channel scope,
> it is necessary to measure the specific RF voltages. Then, IF
> you know the impedances accurately (or your can guarantee they
> are identical) you can calculate the radio.
Oh now I understand. You are assuming that everything possible is going
to be worst case. Broken amplifier, broken load, broken exciter and
broken operator. Given that assumption, I have to agree.
Silly me, I assumed that if something was not right, it would get fixed
before trying to measure it.... But then given the posts that appear on
this list I guess yours is the more correct assumption.
>
> > What are you talking about? How does measured power applied to
> > the input jack have anything at all to do with what's going on
> > in the exciter?
>
> Many amplifiers do not have properly designed inputs and the
> solid state exciters operate at significant SWR. Add to that
> the changes in tuning based on power level which will change
> the input SWR, and simple wattmeters with insufficient
> directionality will mis-state the true input power.
Again, it boggles my mind that any intelligent person would try to do
anything under those conditions. Again, apparently a wrong assumption.
But given those conditions, you could not do a two tone test or anything
else that made sense.
>
> Of course, you could use a 6 dB pad between the exciter and
> the amplifier to isolate the exciter from the amplifier tuning
> - but then most users would not know anything about that either.
I agree.
However, I still maintain that given a reasonably technically competent
operator with equipment that is in reasonably good condition, the 1 dB
PO compression test will quickly establish the best linear operating
conditions for any given amplifier within acceptable ham radio
tolerances. It works for QRP, QRO, and mega-QRO; BJT's, FET's, triodes,
tetrodes, and even TWT's, anything that is supposed to be linear. If you
look across the industry P1dB is a standard, only in ham radio is it
ignored.
73, Larry
>
> 73,
>
> ... Joe, W4TV
>
> On 5/23/2010 8:28 PM, Larry wrote:
>>
>> On 23-May-10 4:06 PM, Joe Subich, W4TV wrote:
>>>
>>> > The beauty of 1 dB compression power is that unlike two tone
>>> testing,
>>> > absolute accuracy is not required to achieve proper operation of
>>> the
>>> > amp.
>>>
>>> Sure, if you end up reducing the drive by 3 dB because the user
>>> can't tell the difference between 1 dB compression in the amplifier
>>> and 1 dB of foldback in the drive due increased input SWR.
>>
>> What are you talking about? How does measured power applied to the input
>> jack have anything at all to do with what's going on in the exciter? I
>> don't understand your point. 100 Watt carrier doesn't care if the
>> exciter is in compression or folded back or standing on it's head. 100
>> watt carrier is 100 watt carrier period.
>>>
>>> > You DO NOT need to know what the actual power output is, you
>>> > only need to know at what point you should be adjusting the
>>> > drive to when running a linear mode.
>>>
>>> You do need to know both the drive and output accurately. That
>>> is the only possible way to accurately determine the gain of the
>>> amplifier and thus determine when the gain has dropped by 1 dB.
>>
>> NO NO NO NO. You only need to know the RATIO between the output and
>> input power. That's what a dB is... A RATIO .. That's what GAIN is....
>> A RATIO.... Even an uncalibrated watt meter will give you that
>> information as long as it is linear. Why do want to make this harder
>> than it really is?
>>
>> Larry
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>>
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