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Author Topic: new "standard" for measuring RF power  (Read 30914 times)
TANAKASAN
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« Reply #15 on: September 15, 2013, 12:50:12 AM »

I've always used a scope and a dummy load.

Tanakasan
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W6RMK
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« Reply #16 on: September 15, 2013, 06:29:14 AM »

3% of reading or 3% of full scale on that range? It really is accurate to better than 1/8th of a dB over the full frequency range?

That's far better than any of the ISO standards ....What is the measurement uncertainty for an accuracy of 3%?

The measurement uncertainty of most modern power meters is based on how they do the measurement.  If they use something like a monolithic power measurement device like the AD8032, it is "linear" (in a volts/dB sense) to within 0.5 dB, and that's out of the box, without applying any cal curve, which is fairly easy these days with a microcontroller.

If they do it by using a diode and measuring the voltage, then you're looking at the stability of the voltmeter (a 16 bit ADC, for instance) and the diode characteristics.  The diode characteristics are somewhat temperature dependent, so you can factor that into your calibration.

These days, it's fairly easy to duplicate the performance of an Agilent power meter, at least from a electrical design standpoint.  What you get with the Agilent box (or the $600 USB pod from MiniCircuits) is a different user interface, mostly.  The underlying design (some sort of solid state power sensor, temperature sensor, and software to convert the sensor readings to power) is the same for everyone.   

The challenge is in building something that is "calibrateable", in the sense that the sensor returns the same value every time for the same power input, so that you can build that table of "sensor output to RF power".

The other aspect of good power measurement is "flat over frequency"  if you're measuring reasonably narrow band signals of known frequency, then you can use a frequency dependent calibration, but if you need to take wideband signals, then you need to design for flatness.  The monolithic chips are pretty good, but...

The real challenge is in the coupler between the RF line and the power sensor, and making it flat across the decades of bandwidth.  It doesn't take much ripple in the coupling, along with a bit of impedance mismatch between sensor and coupler, and that becomes the dominant uncertainty source.  People who care keep the coupler+measurement head as an assembly, and calibrate it as a unit.

However, returning to the original question about "3% full scale or 3% of reading".. most of the error sources (frequency dependence, temperature dependence, coupling ratios, mismatch) are "ratio errors", so the uncertainty is a constant "fraction" of the reading, not of full scale.  I would say that the "voltage measurement" part of modern power meters is not a significant contributor to the measurement uncertainty, unlike the venerable Bird, where the analog meter is a major contributor to measurement uncertainty, particularly at less than half scale.
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W6RMK
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« Reply #17 on: September 15, 2013, 06:36:17 AM »

I've always used a scope and a dummy load.

Tanakasan

Oscilloscopes don't necessarily have very good measurement uncertainty for voltage.  Tek claims 3% vertical accuracy on all scales for their low-end scopes. That's basically 3% of full scale, and if you get your signal to be nearly full scale, then 3% voltage uncertainty works out to 6-7% power uncertainty. So you're in the Bird 43 category of accuracy, with the advantage that you can just turn a knob to adjust the scale, as opposed to swapping slugs.

I would imagine that if you used the modern scope's calculation capability, it's in the same ballpark.  What you're really looking at is the gain accuracy and flatness of the vertical amplifier and the ADC. These days, fast, wide ADCs are easy to come by, so I doubt that's the limiting factor (3%, after all, is only 6 bits)
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KD8MJR
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« Reply #18 on: September 15, 2013, 12:54:27 PM »

The LP-100A is both Temperature and Frequency compensated.  You can even see the values on the LCD if you wish to. 
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G3RZP
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« Reply #19 on: September 15, 2013, 02:16:26 PM »

Is the calibration NIST traceable to the claimed accuracy?
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KH2G
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« Reply #20 on: September 15, 2013, 04:17:36 PM »

I submit that any measurement system is only as accurate as the standard with which it is calibrated. You must also factor in all external errors such as connector losses etc.
regards,
Dick KH2G
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W5JO
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« Reply #21 on: September 15, 2013, 08:27:33 PM »

G3RZP go to the N8LP page for the meter.  The specs are there   http://www.telepostinc.com/lp100.html

I think the specs and capabilities of the unit is very good and traceable.
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KD8MJR
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« Reply #22 on: September 16, 2013, 08:10:05 AM »

I submit that any measurement system is only as accurate as the standard with which it is calibrated. You must also factor in all external errors such as connector losses etc.
regards,
Dick KH2G

Yes Larry uses very expensive NIST calibrated equipment and I think he keeps the calibration up to date.
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K9COX
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« Reply #23 on: September 21, 2013, 02:09:51 AM »

Using watts is really rather meaningless when you consider that doubling your power means 3 db increase. dBm is the only standard that makes any sense.
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RFRY
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« Reply #24 on: September 21, 2013, 04:47:01 AM »

Using watts is really rather meaningless when you consider that doubling your power means 3 db increase. dBm is the only standard that makes any sense.

However doubling the power stated in dBm also produces a 3 dB increase.

 1 mW = 0 dBm
 2 mW = 3 dBm (essentially)

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W9PMZ
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« Reply #25 on: September 21, 2013, 07:20:32 AM »

"Note that the Bird 43 and the Coaxial Dynamics 83000-A accuracy (measurement uncertainty) is 5% of FULL SCALE and not 5% of measured power."

Accuracy and uncertainty are different items in the measurement.

Uncertainty is dependent upon the return loss's of the measurement device and the unit under test.
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K6AER
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« Reply #26 on: September 21, 2013, 08:32:32 AM »

In wattmeter readings,  Absolute Accuracy is a bit of fleeting target. You have meter accuracy and for most applications you only need a relative indication and a meter that increases in scale when the power is increased.

Other factors come into play when taking accurate readings. Is the coax the right impedance along with the load? What is the loss of the test lash up?  Is the source 50 ohms out. Any reading within a couple dB is very accurate. Do you need maximum accuracy or do you need accuracy through a whole range of power levels from millawatts to Kilowatts. What is the frequency coverage of the meter? At frequency limits the accuracy falls off. Many fine digital and analog wattmeter’s are available for a whole range of prices. The LP100A, Telewave, Bird, Array Solutions, Wave node and my personal HF favorite the Alpha 4510 series for HF and the Telewave for VHF and UHF. I have two of those.  Some of these metes are made for CW applications and other will handle complex modulation for peak wattage readings. For very accurate readings I use a spectrum analyzer but even with $40,000 in test gear the readings will only be accurate to 1 dB.

Peak reading watt meters must have electronic circuitry to hold the meter indication to a maximum level.  The circuitry will sample a number of peak readings to give an average peak reading. I the case of the Alphas they sample 200-300 peak readings. This is done in milliseconds and a microprocessor does the housekeeping chores. You can add an analog peak reading module to a Bird 43 but at best the accuracy will be 10-20%. A good peak reading wattmeter will cost more than $300 not including the sensors.  The longer the samples rate the more accurate the peak reading on analog peak reading modules.

Many hams get all would up in a few watts of discrepancy on wattage output from their amplifiers. Nobody will see 1 dB difference between 600 watts and 480 watts.

Last note, many overseas watt meters advertise their meters as peak reading units when they only just adjust the meter gain to read high. There is no free lunch. You must have active circuitry to have a peak reading watt meter
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AD5X
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« Reply #27 on: September 21, 2013, 09:18:28 AM »

I made a bunch of measurements on several bands using a Bird 43 with 100W slug compared to a Nist-traceable Minicircuits PWR-6GHS+ power sensor and precision attenuator.  The Bird 43 had the Array Solutions AS-43A digital read-out.  Here are the results:

Band   MC   Bird 43   MC Bird 43    MC Bird 43    MC Bird 43
80M   100     97         50     48         10     10          5      6
40M   100     99         50     49         10     10          5      6
20M   100    100        50     50         10     10          5      6
15M   100    100        50     50         10     11          5      6
10M   96       95         50     49         10     10          5      6

As you can see, the Bird tracks pretty well over a broad power range.

Phil - AD5X
« Last Edit: September 21, 2013, 09:21:58 AM by AD5X » Logged
W9PMZ
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« Reply #28 on: September 21, 2013, 06:51:20 PM »

Well...
I work as a manufacturing test engineer and I design test systems and write test software for cellular amplifiers.

It is possible to measure power to less than 0.2dB. Do it all the time with repeatable results. But to do this it is necessary to pay attention to all factors associated with the measurement, absolute accuracy, uncertainty and modulation scheme. Signals with a high peak to average ratio will require sensors designed for such measurements.

Also, spectrum analyzers aren't that great for absolute accuracy but really shine when relative measurements are made, in other words power level 1 compared to power level 2.

Do amateurs need this?  Not really.  But some engineers endlessly worry over accuracy and uncertainty.

73,

Carl - W9PMZ
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KC4MOP
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« Reply #29 on: September 23, 2013, 03:38:33 AM »

ya buy what you can afford. The high accuracy meters might be for broadcasters. But rarely has there been an FCC bust on a licensed ham station for extreme high power, unless it was being operated by a LID and causing tremendous interference.
Or a CBer transmitting 5KW and tearing up the bands with harmonics and splatter.
Fred
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