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Reg wrote
They who attempt to grasp support by stating the manufacturer's type number of the instruments used are most in need of the self-confidence it falsly generates. Hi Reg. What exactly are you talking about? I had a few minutes in between Hurricane Ivans wrath to get the Emergency generator cranked up and had a chance to read this. Lucky you don't have these things in the UK. 73 Gary N4AST ============================ Gary, we have heard the news over here about the devastating Hurricane Ivan. We get them here at about 1/2 strength of yours only once every very few years. And even then it's only over a relatively small area. If you can't understand what I am wittering about then its due either to the storm stress you are under or because you are one of those suffering from delusions of accuracy. In your case I prefer the former excuse. I hope your generator started up OK and that you and your family suffer the bare minimum of danger and damage. At least communications between us are still intact. My best wishes. --- Reg, G4FGQ |
"JGBOYLES" wrote in message ... All radio people suffer from delusions of measuring accuracy. RF power measurements are the most inaccurate of all. The accuracy of measurements are a function of the instrument user. They who attempt to grasp support by stating the manufacturer's type number of the instruments used are most in need of the self-confidence it falsly generates. Hi Reg. What exactly are you talking about? I had a few minutes in between Hurricane Ivans wrath to get the Emergency generator cranked up and had a chance to read this. Lucky you don't have these things in the UK. 73 Gary N4AST Gary, I saw an interesting curve at a resistor manufacturer's web site. It plotted resistor error as a function of F(MHz) x R(Meg) for 1/4 W carbon resistors. To make a long story short, the resistor error will be about 20% where the Megahertz x MegaOhms = 1. That means the resistor value will be 1/ Frequency. So, at 30 MHz, the resistor will be in error by 20% if it is bigger than 1/30 =.033 Meg, or 33K. That, I believe ignores capacitive effects. Personally, I have never tried to put RF through a resistor bigger than a few hundred Ohms. It occurs to me that you can ignore capacitive effects if you make all resistors identical. For instance, if you want a 3:1 divider make the series resistor 10K, and the shunt resistor two 10K resistors in parallel. Of course, you will need a high impedance load on it. Let's see if anybody shoots this down. Tam |
On Thu, 16 Sep 2004 21:26:48 -0400, "Tam/WB2TT"
wrote: To make a long story short, the resistor error will be about 20% where the Megahertz x MegaOhms = 1. And not so curiously Trc = 1 MOhm · 1 pF = 10^-6 F = 1 / T F = 1 MHz perhaps the product rule should be: Megahertz x MegaOhms x picoFarads = 1 The 20% error is, of course, simply the rolloff response at the RC inflection point described by 1/Trc. Let's see if anybody shoots this down. Hi All, I think chipping at the clay feet of saints is more appropriate metaphor. What is the saint? The RF response of the resistor. It should be suspect right out the gate. Being suspect, you employ the conventional techniques already evidenced even by the cheapest Power Meter builder (MJF) by swamping the stray capacitance with series capacitors (paralleling the resistors). One capacitor is either variable, or further paralleled with a trimmer. The saint is also the unspecified requirement: is this divider BEFORE OR AFTER the detector? If before, and thus subject to RF, the simple RC compensated divider has served for eons. If after, and thus subject to only DC - who cares? The one clay foot of the discussion. The other clay foot of the discussion is that for placement before OR after the detector, ALL ratios are post-hoc determinations (in other words, design with variable components fully expecting you WILL be wrong). Further, ALL descriptions to this point have been of normalized levels. With the RC compensated divider, you are throwing the knee if rolloff into lower frequencies so that ALL frequencies of interest reside on the same slope. Hence the common "calibration" procedure has you adjust the resistors for the low frequency readout, and the capacitors at the high frequency readout. This "calibration" is simply distributing the error so that it doesn't accumulate outrageously. The greater challenge is how do you know how much power you are setting your meter to read? Compounding errors are common in RF. 73's Richard Clark, KB7QHC |
"Richard Clark" wrote in message ... On Thu, 16 Sep 2004 21:26:48 -0400, "Tam/WB2TT" wrote: To make a long story short, the resistor error will be about 20% where the Megahertz x MegaOhms = 1. And not so curiously Trc = 1 MOhm · 1 pF = 10^-6 I think the curve ignores C, and is based on skin effect only. There is no explanation for the data. F = 1 / T F = 1 MHz perhaps the product rule should be: Megahertz x MegaOhms x picoFarads = 1 Go to http://www.xicon-passive.com/resistor.html and click on CC. There is also info on resistor performance vs frequency in the W6SAI book. He shows curves for 5 different carbon resistors vs frequency without identifying the resistor values. As a gross average, they show about 50% error at 15 MHz. The 20% error is, of course, simply the rolloff response at the RC inflection point described by 1/Trc. The curve goes from 0 - 100. I arbitrarily picked 20 % as being a point where there is apreciable error. Tam/WB2TT Let's see if anybody shoots this down. Hi All, I think chipping at the clay feet of saints is more appropriate metaphor. What is the saint? The RF response of the resistor. It should be suspect right out the gate. Being suspect, you employ the conventional techniques already evidenced even by the cheapest Power Meter builder (MJF) by swamping the stray capacitance with series capacitors (paralleling the resistors). One capacitor is either variable, or further paralleled with a trimmer. The saint is also the unspecified requirement: is this divider BEFORE OR AFTER the detector? If before, and thus subject to RF, the simple RC compensated divider has served for eons. If after, and thus subject to only DC - who cares? The one clay foot of the discussion. The other clay foot of the discussion is that for placement before OR after the detector, ALL ratios are post-hoc determinations (in other words, design with variable components fully expecting you WILL be wrong). So true. I notice the series C in the Kenwood meter is variable. Further, ALL descriptions to this point have been of normalized levels. With the RC compensated divider, you are throwing the knee if rolloff into lower frequencies so that ALL frequencies of interest reside on the same slope. Hence the common "calibration" procedure has you adjust the resistors for the low frequency readout, and the capacitors at the high frequency readout. This "calibration" is simply distributing the error so that it doesn't accumulate outrageously. The greater challenge is how do you know how much power you are setting your meter to read? Compounding errors are common in RF. 73's Richard Clark, KB7QHC |
Oh, yea! I forgot the bit about Peak...silly me.
73, Steve K9DCI "JGBOYLES" wrote in message ... E = Root (P*R) Scaled V is V / 273 but you'll have to go further to stay in the dynamic range of the multiplier. V^2 should be around 10V or whatever the mult can output. Thanks for checking my calcs. Steve. I had to do what your spreadsheet did by hand. I should note that since I have to convert the voltage to the multiplier to DC, at 1500 watts we are working with 273*SQRT 2 or 386 volts. I size the divider so that 386 this gives 10.0 volts to the multiplier. With a dual polarity 15VDC supply, the multiplier has enough dynamic range. 73 Gary N4AST |
On Fri, 17 Sep 2004 10:35:25 -0400, "Tam/WB2TT"
wrote: I think the curve ignores C, and is based on skin effect only. There is no explanation for the data. Skin effect would tend to increase resistance which contradicts the trend. As for explanation: From "Electronic Components and Measurements," Wedlock and Roberge, 1969: "At high frequencies the performance of a resistor will depart from Ohm's law because of stray capacitance and lead inductance." [pg. 77] There is an identical curve to your reference shown in Figure 7.4, same page: "Change in resistance of a ½ Watt carbon-composition resistor as a function of frequency. Frequency in MHz times resistance in Megohms" In Chapter 18 "RF Impedance Measurements": "Such behavior is often termed stray capacitance or stray inductance. Because these effects are usually undesirable and serve to limit the high frequency performance of components, they are also called parasitic effects." [pg. 276] However, my expression of this being rolloff was too simplistic as the curve does not follow the typical 10dB/Decade characteristic. Rather, it shows a 6dB/Decade+. Some of this may be accounted for in lead reactance, but at the Megohm scale this is inconsequential for conventional leads. 73's Richard Clark, KB7QHC |
The other clay foot of the discussion is that for placement before OR
after the detector, ALL ratios are post-hoc determinations (in other words, design with variable components fully expecting you WILL be wrong). Further, ALL descriptions to this point have been of normalized levels. Hi Richard, I haven't been able to keep up with this like I wished because of that pesky Hurricane. If you put the detector circuit before the voltage divider, then the resistors see DC which they are a lot happier with. The detector diode will have to be 700VDC PRV rating, and the filter cap. will have to be sized properly. I guess the diode will have some frequency dependent properties, but as long as it still acts like a diode, and the forward bias drop is around .6V it ought to work. This looks like good alternative to frequency dependent resistors. What say you? 73 Gary N4AST |
FYI
I have used either 470 or 510 ohm carbon comp resistors way back when as 20dB probes for a sampling scope and Spectrum analyzer work. If I recall, they were quite comparable to the scope's regular probes for pretty fast digital signals. I would, however, get nervous with the high values stated / needed here. At first I was thinking I'd series-up 500 or 1K resistors if I had to do this, but then strays start to become significant as has been well discussed. If you have the equipment, I like the frequency response / sweep idea. This way you may even be able to adjust the compensating caps for best high freq (10M) response. You could also use the calibrator on most good scopes to start out.... 73, -- Steve N, K,9;d, c. i My email has no u's. "Richard Clark" wrote in message ... On Fri, 17 Sep 2004 10:35:25 -0400, "Tam/WB2TT" wrote: I think the curve ignores C, and is based on skin effect only. There is no explanation for the data. Skin effect would tend to increase resistance which contradicts the trend. As for explanation: From "Electronic Components and Measurements," Wedlock and Roberge, 1969: "At high frequencies the performance of a resistor will depart from Ohm's law because of stray capacitance and lead inductance." [pg. 77] There is an identical curve to your reference shown in Figure 7.4, same page: "Change in resistance of a ½ Watt carbon-composition resistor as a function of frequency. Frequency in MHz times resistance in Megohms" In Chapter 18 "RF Impedance Measurements": "Such behavior is often termed stray capacitance or stray inductance. Because these effects are usually undesirable and serve to limit the high frequency performance of components, they are also called parasitic effects." [pg. 276] However, my expression of this being rolloff was too simplistic as the curve does not follow the typical 10dB/Decade characteristic. Rather, it shows a 6dB/Decade+. Some of this may be accounted for in lead reactance, but at the Megohm scale this is inconsequential for conventional leads. 73's Richard Clark, KB7QHC |
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On Fri, 17 Sep 2004 16:25:56 -0500, "Steve Nosko"
wrote: At first I was thinking I'd series-up 500 or 1K resistors if I had to do this, but then strays start to become significant as has been well discussed. Hi Steve, This would probably improve the parasitic capacitance while increasing the parasitic inductance. Off hand, I think the inductance would probably be tolerable in the HF. 73's Richard Clark, KB7QHC |
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