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KPA1500 Linearity

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K9MA

KPA1500 Linearity

I just repeated the linearity measurements, on both 40 and 15 meters. I
used the KPA1500 meter to measure output, and the K3 power setting for
input. Results on the two bands were similar, though 15 required much
more drive.

On 40, the maximum gain occurred at about 800 W out, 17.2 dB. At 1 kW,
it was 16.9 dB, and at 1.5 kW it was 15.7 dB. However, at 1.45 kW the
gain was 16.2 dB, so there was a lot of compression in that last 55 W.
The gain compression at 1.5 kW vs 1 kW is 1.2 dB, but only 0.7 dB at
1.45 kW. Overall, I'm told this compression is typical of the KPA1500,
and probably doesn't indicate excessive IMD, at least compared to other
solid-state amplifiers. It's probably a good idea to be very careful not
to overdrive it on SSB, though.

The gain drops off at low power, to 16.0 dB at 200 W. This is
consistent with the BLF188 data sheet. However, the data sheet shows the
gain almost perfectly flat to over 1 kW per device. They show maximum
gain, in fact, at about 1 kW per device, which means gain ought to be a
bit HIGHER at 1.5 kW than 1 kW for two devices. Since there are two of
them in the KPA1500, the compression at 1.5 kW is a bit surprising.
(That's about what one would expect from ONE device at 1.5 kW.) Yes, the
data sheet curves are for a specific test circuit at 108 MHz, but it's
still surprising. On the other hand, anyone who has designed electronics
knows to take the published data sheets skeptically. Elecraft has, no
doubt, tested lots of them, and knows their REAL characteristics.

73,

Scott K9MA

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Scott K9MA

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John Oppenheimer

Re: KPA1500 Linearity

Hi Scott,

I wonder how the linearity of the two power meters was evaluated?

Some years ago I checked the K3 and KX3 power meter against two other
meters. They were just within 10%
http://www.kn5l.net/Elecraft/Power.html

John KN5L

On 06/19/2018 04:56 PM, K9MA wrote:
> I just repeated the linearity measurements, on both 40 and 15 meters. I
> used the KPA1500 meter to measure output, and the K3 power setting for
> input.
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Don Wilhelm

Re: KPA1500 Linearity

John,

You are exactly right for bring that factor to attention. Typically
wattmeters use diodes in their detectors, and the response will vary by
frequency and by the power level.

I would trust only something that has been calibrated to NIST traceable
standards. The Telepost LP-100 is one example (and those are used on
many Elecraft test benches).

While 10% is an OK deviation for amateur purposes, two wattmeters each
with 10% accuracy can lead to a 20% error in the final measurements.

It is too easy to jump to conclusions by not considering the potential
errors in measurement accuracy. If you want 5% accuracy in your
conclusions, your measurement tools should be accurate to 0.5% - a far
stretch for wattmeters as we know them.

73,
Don W3FPR

On 6/19/2018 6:25 PM, John Oppenheimer wrote:
> Hi Scott,
>
> I wonder how the linearity of the two power meters was evaluated?
>
> Some years ago I checked the K3 and KX3 power meter against two other
> meters. They were just within 10%
> http://www.kn5l.net/Elecraft/Power.html
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k6dgw

Re: KPA1500 Linearity

In reply to this post by John Oppenheimer

Most, if not all analog measurement devices are spec'd at % of full
scale which can lead to high absolute variances if the levels being
measured are significantly below full scale. A Bird with a 100W slug
accurate to 10% of full scale [+/- 5W] would be within spec if it
indicated a real 10 W signal at 5 W. Many folks would say, "My K2/10
will only produce half the power it should." Most digital measurements
involve processing circuitry/computation, again which can introduce
errors in the lower end of a range, and be dependent on frequency as well.

Mathematics can be hard. Arithmetic can be tricky. [:-)

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 6/19/2018 3:25 PM, John Oppenheimer wrote:

> Hi Scott,
>
> I wonder how the linearity of the two power meters was evaluated?
>
> Some years ago I checked the K3 and KX3 power meter against two other
> meters. They were just within 10%
> http://www.kn5l.net/Elecraft/Power.html
>
> John KN5L
>
> On 06/19/2018 04:56 PM, K9MA wrote:
>> I just repeated the linearity measurements, on both 40 and 15 meters. I
>> used the KPA1500 meter to measure output, and the K3 power setting for
>> input.
> ______________________________________________________________
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Bob McGraw - K4TAX

Re: KPA1500 Linearity

Yes and most power meters are voltage sensing instruments which are
calibrated to a specific value or load. Thus is the load being used may
not be the identical value to which it was calibrated, hence another
error enters the equation.

To wit, I have 3 "allegedly" 50 ohm dummy loads. Only one is actually
50 ohms but others are as low as 45 ohms and as high as 58 ohms, while
one actually measures 50.5 ohms. If one is trying to measure power
then both voltage and current should be measured with a known load and
then do the math. If you are using an antenna, it most likely is
somewhat reactive. This throws another curve into the ballpark in terms
of power measurement.

73

Bob, K4TAX

On 6/19/2018 7:15 PM, Fred Jensen wrote:

> Most, if not all analog measurement devices are spec'd at % of full
> scale which can lead to high absolute variances if the levels being
> measured are significantly below full scale. A Bird with a 100W slug
> accurate to 10% of full scale [+/- 5W] would be within spec if it
> indicated a real 10 W signal at 5 W. Many folks would say, "My K2/10
> will only produce half the power it should." Most digital
> measurements involve processing circuitry/computation, again which can
> introduce errors in the lower end of a range, and be dependent on
> frequency as well.
>
> Mathematics can be hard. Arithmetic can be tricky. [:-)
>
> 73,
>
> Fred ["Skip"] K6DGW
> Sparks NV DM09dn
> Washoe County

K9MA

Re: KPA1500 Linearity

In reply to this post by Don Wilhelm

Yes, the linearity of the two power meters is suspect. However, the
compression of interest is near the high end for that of both the
KPA1500 and the K3, so I wouldn't expect their nonlinearity there to
make much difference. Note that absolute accuracy isn't critical.

73,
Scott K9MA

On 6/19/2018 17:42, Don Wilhelm wrote:

> John,
>
> You are exactly right for bring that factor to attention. Typically
> wattmeters use diodes in their detectors, and the response will vary
> by frequency and by the power level.
>
> I would trust only something that has been calibrated to NIST
> traceable standards. The Telepost LP-100 is one example (and those
> are used on many Elecraft test benches).
>
> While 10% is an OK deviation for amateur purposes, two wattmeters each
> with 10% accuracy can lead to a 20% error in the final measurements.
>
> It is too easy to jump to conclusions by not considering the potential
> errors in measurement accuracy. If you want 5% accuracy in your
> conclusions, your measurement tools should be accurate to 0.5% - a far
> stretch for wattmeters as we know them.
>
> 73,
> Don W3FPR
>
> On 6/19/2018 6:25 PM, John Oppenheimer wrote:
>> Hi Scott,
>>
>> I wonder how the linearity of the two power meters was evaluated?
>>
>> Some years ago I checked the K3 and KX3 power meter against two other
>> meters. They were just within 10%
>> http://www.kn5l.net/Elecraft/Power.html
> ______________________________________________________________
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Scott K9MA

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Ignacy

Re: KPA1500 Linearity

The saturation can be due:
- transistor limit -not the case here
- power supply drop - not if well designed switching PS or a commercial
unit
- winding ratio in final transistor module

You can test the last one by measuring max power at saturation. Should be at
least 2 KW.
I have Expert 2k-fa that has 6 transistor modules rated at 300W. Saturated
power around 2.0 KW as measured by LP100; 2k's meter shows 2.4 K.

Ignacy, NO9E

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Wes Stewart-2

Re: KPA1500 Linearity

In reply to this post by Don Wilhelm

To make these kinds of measurements with amateur equipment and accuracy, I
suggest acquiring some directional couplers and/or high power attenuators. A
spare K3 or a decent SDR receiver then makes an excellent "power meter" with
significant dynamic range. Gain measurements are really ratio measurements and
seldom, if ever, require NIST traceability. The ancillary hardware can be
characterized by substitution.

One further point: Although it's possible that two 10% accuracy power meters
will have 20% uncertainty, it's highly improbable. In industry, all
uncertainties: mismatch errors due to reflection coefficients of sources and
power sensors, linearity errors, noise, temperature effects, etc are combined
into one uncertainty by RSSing the individual errors. You never add all the
worse cases to get the answer.

Wes N7WS

On 6/19/2018 3:42 PM, Don Wilhelm wrote:

> John,
>
> You are exactly right for bring that factor to attention. Typically wattmeters
> use diodes in their detectors, and the response will vary by frequency and by
> the power level.
>
> I would trust only something that has been calibrated to NIST traceable
> standards. The Telepost LP-100 is one example (and those are used on many
> Elecraft test benches).
>
> While 10% is an OK deviation for amateur purposes, two wattmeters each with
> 10% accuracy can lead to a 20% error in the final measurements.
>
> It is too easy to jump to conclusions by not considering the potential errors
> in measurement accuracy. If you want 5% accuracy in your conclusions, your
> measurement tools should be accurate to 0.5% - a far stretch for wattmeters as
> we know them.
>
> 73,
> Don W3FPR
>
> On 6/19/2018 6:25 PM, John Oppenheimer wrote:
>> Hi Scott,
>>
>> I wonder how the linearity of the two power meters was evaluated?
>>
>> Some years ago I checked the K3 and KX3 power meter against two other
>> meters. They were just within 10%
>> http://www.kn5l.net/Elecraft/Power.html
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]
>