EMRFD Message Archive 8336

Message Date From Subject
8336 2013-03-14 18:22:49 Jerry Haigwood W7ZOI & W7PUA Wattmeter Calibration
Hi All,

I am building the W7ZOI/W7PUA wattmeter from QST June 2001. In that
article, figure 5 shows a DVM voltage versus generator power. When I
compare this chart with the calibration formula in the document titled
"Power meter updates.pdf I see that the chart and formula have differences.
Take the example where the DVM reads 1VDC. The chart of figure 5 shows that
1VDC is -68dbm. If I calculate the power level for 1VDC, I get:

Pdbm= -77.143 + (14.286 x 1V) which is equal to -63dbm. So, what am I
missing here? Perhaps the formula was based on a different meter that read
differently than the meter in the original article? Maybe someone can help
me out here.

Jerry W5JH

"building something without experimenting is just solder practice"





[Non-text portions of this message have been removed]
8337 2013-03-14 19:24:44 john lawson Re: W7ZOI & W7PUA Wattmeter Calibration
Jerry please make sure your signal you are using for calibration is a very clean sine wave....see Bob Kopski's post......
 
Files > K3NHI_8307PM&Calibration,  AN AD8307 ADVISORY. Doc 
 
as it can affect the results if not clean.......Hope this will helps........John K5IRK

8338 2013-03-14 20:05:20 Jerry Haigwood Re: W7ZOI & W7PUA Wattmeter Calibration
Hi John,

I plan to use my home made signal source with a good low pass filter after it. I wish I had something like an HP8640B but I have to use what I have. My original question was why there is a difference between the calculated equation and the chart on figure 5. My best guess is that two different power meters were used.

Jerry W5JH

"building something without experimenting is just solder practice"







8339 2013-03-15 08:57:37 Ashhar Farhan Re: W7ZOI & W7PUA Wattmeter Calibration
To get best filtering for caliberation, you will want the signal to be
pure. However, it doesn't have to have very high spectral purity. This
is because the fundamental will dominant the measurement. I would
guess that anything south of -30dbC is good enough. I used a double
tuned circuit following a crystal oscillator to overcome bob's issue.

- farhan

On 3/15/13, Jerry Haigwood <jerry@w5jh.net> wrote:
> Hi John,
>
> I plan to use my home made signal source with a good low pass filter
> after it. I wish I had something like an HP8640B but I have to use what I
> have. My original question was why there is a difference between the
> calculated equation and the chart on figure 5. My best guess is that two
> different power meters were used.
>
> Jerry W5JH
>
> "building something without experimenting is just solder practice"
>
>
>
>
>
>
>
>
8340 2013-03-15 16:08:18 boblarkin02 Re: W7ZOI & W7PUA Wattmeter Calibration
Jerry, it is great to see your meter project coming together. I just re-measured my power meter, the one
8341 2013-03-16 00:48:44 Kerry Re: W7ZOI & W7PUA Wattmeter Calibration
Some time ago I did some testing on my AD8307 meter to help a builder who had no calibrated RF source; this is the DC output from the AD8307;

http://i49.tinypic.com/zksxg4.jpg

I have two lab-standard generators, a Rohde & Schwarz SMS and a HP 8657B; neither are in current calibration but they agree almost exactly so far as frequency and output level are concerned so the output power accuracy is probably fine for amateur work.

My AD8307 has the matching network at the input (was it Wes or W7PUA who designed it?) and the usual 52.3 ohm termination.

For a builder without any calibration source at all, I think that adopting these figures would provide quite good calibration, certainly better than nothing.

Kerry VK2TIL.
8342 2013-03-16 02:56:23 ha5rxz Re: W7ZOI & W7PUA Wattmeter Calibration
I calibrated my power meter using a digital oscilloscope. 10 dBm is a 2 volts P-P sine wave and 4 dBm is 1 volt P-P, this should give you your two reference points.

Peter HA5RXZ

8343 2013-03-16 04:48:59 Ashhar Farhan Re: W7ZOI & W7PUA Wattmeter Calibration
I am not sure if this is correct, I hope someone can explain this to me...
A perfect square wave goes from 0 to full power instantly. Hence, the
peak power and rms values will be exactly the same.
So, if we use a clock like the 10 MHz TTL oscillator, we could factor
in the difference between a sinewave and a square wave's energy
envelops and caliberate instruments accordingly.
A pure sinewave is not as common as we believe it to be. Outputs of
power amps, mixers, even some passive networks can output
non-sinewaves. (Depending upon how they are being driven).
- farhan

8344 2013-03-16 10:45:13 Bob Re: W7ZOI & W7PUA Wattmeter Calibration
Hi Farhan et al -

You can find a detailed explanation to your query in my Jan/Feb 2004 QEX article "A Simple RF Power Calibrator" with an important related update article in the Tech Notes section of May/June 2010 QEX. These references present an easy HB way to calibrate not only an 8307 based power meter but a "real" power meter and a spectrum analyzere as well. Only a DVM is needed to calibrate the Calibrator which outputs a known square wave signal.

From the Tech Notes article the updated Calibrator outputs a -10 dBm
level to an 8307 based instrument. The same sq. wave "looks like"
different power levels to the other instruments mentioned above as
explained in the references.

As for the relationship between a quality square wave and the sine wave "equivalent", the relationship is given by:

V (sine) p-p = 4 X V (square) p-p / (n X pi)

where "n" is the selected harmonic within the sw. wave. The Calibrator utilizes a 10 MHz CMOS clock padded to output a 50 ohm sourced 100 mV p-p square wave. The sine fundamental therein is 200 mV p-p which is equivalent to -10 dBm FOR AN 8307 BASED INSTURMENT. (The equivalent signal has a power level of -13 dBm for a "real" power meter and a fundamental spectral equivalent of -14 dBm as seen by a spectrum analyzer.)

While all this may seem complicated, both the updated Calibrator circuit and it's application are really very simple and reliable. AS an incidental, the described "update" was brought about by a change in the performance between "early" and "later" versions of the 8307 with non-sinusoids as explained in the Tech Notes article.

Cordially,

Bob, K3NHI






8345 2013-03-16 11:03:45 Ashhar Farhan Re: W7ZOI & W7PUA Wattmeter Calibration
Bob,
I am amazed at the effort! This is something I must emulate. Small
things are indeed so illuminating once you apply yourself to the
problem. I hope to apply your methods in things I build, measure and
understand

On 3/16/13, Bob <kopskib@gmail.com> wrote:
> Hi Farhan et al -
>
> You can find a detailed explanation to your query in my Jan/Feb 2004 QEX
> article "A Simple RF Power Calibrator" with an important related update
> article in the Tech Notes section of May/June 2010 QEX. These references
> present an easy HB way to calibrate not only an 8307 based power meter but a
> "real" power meter and a spectrum analyzere as well. Only a DVM is needed
> to calibrate the Calibrator which outputs a known square wave signal.
>
> From the Tech Notes article the updated Calibrator outputs a -10 dBm
> level to an 8307 based instrument. The same sq. wave "looks like"
> different power levels to the other instruments mentioned above as
> explained in the references.
>
> As for the relationship between a quality square wave and the sine wave
> "equivalent", the relationship is given by:
>
> V (sine) p-p = 4 X V (square) p-p / (n X pi)
>
> where "n" is the selected harmonic within the sw. wave. The Calibrator
> utilizes a 10 MHz CMOS clock padded to output a 50 ohm sourced 100 mV p-p
> square wave. The sine fundamental therein is 200 mV p-p which is equivalent
> to -10 dBm FOR AN 8307 BASED INSTURMENT. (The equivalent signal has a power
> level of -13 dBm for a "real" power meter and a fundamental spectral
> equivalent of -14 dBm as seen by a spectrum analyzer.)
>
> While all this may seem complicated, both the updated Calibrator circuit and
> it's application are really very simple and reliable. AS an incidental, the
> described "update" was brought about by a change in the performance between
> "early" and "later" versions of the 8307 with non-sinusoids as explained in
> the Tech Notes article.
>
> Cordially,
>
> Bob, K3NHI
>
>
>
>
>
>
>
8347 2013-03-16 16:47:15 Kerry Re: W7ZOI & W7PUA Wattmeter Calibration
Jerry's initial question remains unanswered.

I've been puzzling over the anomaly and I agree with Jerry that there is conflict between Fig. 5 of the QST article and the equation derived by W7ZOI in the Power Meter Updates document.

It seems that two different meters were used, one in each case; the difference in calibration may arise from tolerances in the AD8307 and in the following circuitry, particularly in the gain-setting resistors of the op-amp.

The difference of 5dB is certainly substantial.

Errors such as these can be minimised by measuring the AD8307 DC output before it reaches later stages.

My measurements of this voltage agree closely with the measurements shown in Fig. 5 of the AD8307 data sheet, indicating that the AD8307 itself is fairly consistent and that, in the absence of any other calibration source, these voltages will provide a reasonable calibration until a better one can be done.

Kerry VK2TIL.
8353 2013-03-16 20:40:06 Paul Re: W7ZOI & W7PUA Wattmeter Calibration
CW RF better be a sine wave. If you are transmitting an RF square wave, you will also transmit harmonics from here to infinity.

de Paul, W8AEF