10007	2014-05-12 09:22:29	Neil Martinsen-Bu...	Calibration thoughts for popcorn power meter
Message	Date	From	Subject
I am a beginning homebrewer building the test equipment to follow along with the projects in EMRFD 9and beyond!). I'm working on the simple transmitter from chapter 2 and I wanted a way to verify the power output of my oscillator and buffer amplifier. So I built Todd VE3BPO's popcorn power meter (http://www.qrp.pops.net/RF-workbench-5.asp) which is very similar to the AD8307-based one in chapter 7 of EMRFD and the June 2001 QST article. The power meter is working, but now I am calibrating it. Todd's design includes a trim pot that can be adjusted to give a 20 mV/dB slope for the output. In order to adjust this correctly, one needs two sources of known power. That same web page gives a design for a CMOS-based -10 dBm source which I have built and calibrated using a DVM as specified. My problem is with getting a second calibration level. Todd specifies "Don't connect an attenuator pad to the CMOS square wave calibrator— error in the AD8307 arises" but it isn't clear to me why. That page seems to suggest that I now need to build another calibrated source, this one with sine wave output which can be used with an attenuator to easily get the two levels necessary to calibrate the power meter. (The only reason I'm not excited about this is it means buying another crystal, a JFET, toroids, trim caps, etc.) I'm a bit confused why I can't just use a 10 dB pad with the square-wave source to get a second level. Are the higher harmonics not attenuated correctly with a simple resistive pi network? And in the big picture, I am doing this calibration of the slope so that I will only need one calibrated source when I go to use the power meter. (If I know the output voltage for -10 dBm, and the slope is stable at 20 mV/dB, then the math for any measurement is easy.) If I'm going to build a sine wave source with switchable -10 dBm and -20 dBm outputs, then what do I need to do the slope calibration for in the first place? Can't I just take output voltage readings at both levels and then plug them into a quick formula to get the true power for any other measurement? This is my first message to this group and I am most grateful for any advice or clarity you can offer. Thanks in advance. -Neil KD0UKC
10009	2014-05-12 10:47:15	John Marshall	Re: Calibration thoughts for popcorn power meter
Neil, Look in the EMRFD Files section, in the folder ""K3NHI_8307PM&Calibration for the file "AN AD8307 ADVISORY.doc". K3NHI explains why you shouldn't use an attenuator with the PM calibrator. What it comes down to is that the AD8307 isn't accurate for square waves below -20 dBm or so. You don't necessarily need to build a dedicated sine wave source to do your slope calibration. Any RF source you have that can be set to -10 dBm will do the job. Since you're just getting started, a more general purpose signal generator, along with an accurate step-attenuator might be more useful. John, KU4AF Pittsboro, NC
10010	2014-05-12 12:52:05	Neil Martinsen-Bu...	Re: Calibration thoughts for popcorn power meter
Thank you John, that explains the situation very well. Some further reading (thanks to ZS6RSH for documenting his process: http://zs6rsh.blogspot.com/search/label/AD8307%20Power%20Meter) and a little bit of sleep makes me see that I can calibrate the slope using ANY sine wave source and a 10 dB pad. The next question for me is whether or not the crystal oscillator with the buffer amplifier in the Chapter 2 simple transmitter is sufficiently sinusoidal for this purpose. I lack an oscilloscope or spectrum analyzer to examine the waveform directly. Is there any way to indirectly determine the harmonic content of my oscillator relative to the needs of the AD8307? Alternatively, since the buffered oscillator puts out +11 dBm (measured on a dummy load with a diode detector: http://www.k4eaa.com/dummy.html), I suppose I have enough headroom to build a low pass filter and use the filtered output as my signal source into an attenuator. Is there a particularly simple filter design that I could use for this relatively undemanding application? You don't necessarily need to build a dedicated sine wave source to do your slope calibration. Any RF source you have that can be set to -10 dBm will do the job. Since you're just getting started, a more general purpose signal generator, along with an accurate step-attenuator might be more useful. If I had a more general purpose signal generator, I most certainly would use that, but I haven't built or bought one of those yet either. I suppose that even my TenTec Argonaut 505 might be able to be turned down low enough to feed the AD8307 directly. (And it had better have sinusoidal output!) Thanks for your response. -Neil KD0UKC
10011	2014-05-12 15:40:52	John Marshall	Re: Calibration thoughts for popcorn power meter
Since you don't have a way of measuring harmonic content, adding a low pass filter to your oscillator-buffer combination is a good way to assure clean sine wave output. The one shown in Fig 1.37, with component values appropriate for your crystal frequency, should do the job. When you're ready for a signal generator, the one shown in Fig 7.27 is a good choice as is K3NHI's 50 MHz RF Generator in the Files section. You certainly could use your Argonaut as a signal source if you have or build attenuators that can handle its output. Keep in mind that a sensitive power meter and a strong signal source will teach you the importance of careful shielding! 73, John, KU4AF Pittsboro, NC
10012	2014-05-12 16:11:46	Todd F. Carney / ...	Re: Calibration thoughts for popcorn power meter
Some time ago, Joe Everhart N2CX and George Heron N2APB designed what they called the Signal Quality Meter, and it was available as a kit from the AmQRP and NJQRP clubs. It's basic function was to use a tunable filter to notch-out the desired frequency and then to measure the remaining RF energy--none of which you want (and neither does the FCC). They used a 10-step bar graph, but you could also use a meter, I suppose. Here's the link for the still-available documentation: http://www.njqrp.org/sqm/index.html It's a pretty-inexpensive device, and it's only a rough measurement, but it's something useful anyway. 73, Todd ------------------------------------------------------------------------------------ K7TFC / Medford, Oregon, USA / CN82ni / UTC- 7 (P D T) ------------------------------------------------------------------------------------
10013	2014-05-12 16:35:55	vasilyivanenko	Re: Calibration thoughts for popcorn power meter
"................and a little bit of sleep makes me see that I can calibrate the slope using ANY sine wave source and a 10 dB pad......." Good work Neil; exactly right. Also the website software applet "L. Calculate Power from the DC Output of an AD8307 Meter" will accept any 2 DVM rendered DC voltages 10 dB apart to calculate the slope. V.
10014	2014-05-13 02:43:33	Roelof Bakker	Re: Calibration thoughts for popcorn power meter
Hi, There are quite a few treasures to be found on the web and this is one of them: http://dk4sx.darc.de/ Click on "Projekte", scroll down the page and look for 2004: "Intermodulationsmessplatz für hohe Interzeptpunkte" The crystal oscillator has a very low harmonic content. I have build one and it is typical > - 40 dBc. In selected cases and low output I found > - 60 dBc. The transistor is not critical. I have used a 2N2222, but a 2N3904 will work fine as well. The beauty of this design is that the output can be easily adjusted by the potentiometer in the emitter lead. 73, Roelof Bakker, pa0rdt
10016	2014-05-13 08:29:14	bkopski	Re: Calibration thoughts for popcorn power meter
Hi Neil, Here are some reference articles related to your post. Collectively these references should answer some power measurement questions for you: 1. "An Advanced VHF Wattmeter", QEX for May/June 2002 2. "A Simple Enhancement for the Advanced VHF Wattmeter", QEX for Sept/Oct 2003 3. "A Simple RF Power Calibrator", QEX for Jan/Feb 2004 4. "Tech Notes" QEX for May/June 2010 These four references detail the design and the design evolution of my two basic HB RF test instruments - a power meter and a calibrator. The "evolution" is the result of (1) simple operational feature enhancement of the power meter and (2) changes in performance of the production 8307 with time. The latter was the most disturbing matter but has been accommodated per reference 4. As I understand your post, I think you would benefit from the four references taken in the order shown. Basically, despite the continuing description in the AD 8307 data sheets referenced in the articles, comtempory ICs respond well to SINE inputs and not so well to other wave shapes (crest factors) whereas early production ICs did. The calibrator was inspired and encouraged by this latter fact. As originally described (ref 3) the Calibrator worked very well with the 8307 over nearly the entire dynamic range. It was convenient to choose a Calibrator output level of -20 dBm and step attenuator changes in this beginning level resulted is good adherence to the IC's log transfer curve over most of the dynamic range - just as was the case with sine input signals. However "something" in more recent production 8307's transfer behavior changed such that the response to non-sinusoids deteriorated over the range. This occurred at just about -20 and lower levels - but not with higher levels. Since the Calibrator outputs a square wave this fact led to the Calibrator design change from -20 to -10 dBm. where IC performance continues fine. More detailed info can be found in the references. As described in the references, the power meters have several adjustments within the circuitry following the 8307 itself. One of these is for setting the response to a nice "volts per dB" response while another sets the response (calibrates) to a given input test power. The Calibrator provides an accurate known test level at -10 dBm but an attenuation of that level to say -20 puts the 8307 operating level at a questionable operating level as above. This is the only reason for not using a simple attenuator to set the "volts per dB" factor with this Calibrator. Given all this (whew!) I suggest that the Calibrator with it's known -10 output equivalent signal level is perfectly OK to use for "set point" purposes. Then one needs somclean sine wave signal to cal the "volts per dB" factor. Note that any power level (in the dynamic range) including unknown power levels can be used for this purpose. Let's say you have a sine source of -15 - for example - except your may not know the exact level. That's OK - for now all you need to know would be a change in that level - say with a pad or step attenuator. The known change then allows setting the associated trimmer to yield that "volts per dB" cal factor. I acknowledge that all this is a little bit awkward but is perfectly workable compared with "earlier" 8307 performance wherein the Calibrator easily allowed both adjustments with just the Calibrator. KOPSKI's LAW says that in the long term average of life everything changes for the worse. You have just experienced one example! Sorry this got so long - I just wanted to cover "everything" for you - and for other readers too since this subject matter comes up now and then. Good Luck with your pursuits, Bob, K3NHI
10017	2014-05-13 10:12:53	Guy Mengel	Re: Calibration thoughts for popcorn power meter
Roelof, Was für eine wunderbare Website und Station! Ulrich Graf hat ziemlich viel Talent in so Amateur-Radio-Geräte. Vielen Dank für dieses Posting. mit freundlichen Grüßen Guy (AI1g) ---------------- Guy Mengel (434-260-0439) -------------
10021	2014-05-13 11:22:50	David	Re: Calibration thoughts for popcorn power meter
I knew about the crest factor limitations but from the discussion here, I wondered if something else was going on. It is even stranger that this behavior changed over time. My solution for absolute power meter calibration has been to use a sampling voltmeter or sampling oscilloscope (not a DSO). On 13 May 2014 08:29:14 -0700, you wrote: > These four references detail the design and the design evolution of my two basic HB RF test instruments - a power meter and a calibrator. The "evolution" is the result of (1) simple operational feature enhancement of the power meter and (2) changes in performance of the production 8307 with time. The latter was the most disturbing matter but has been accommodated per reference 4. As I understand your post, I think you would benefit from the four references taken in the order shown. > > Basically, despite the continuing description in the AD 8307 data sheets referenced in the articles, comtempory ICs respond well to SINE inputs and not so well to other wave shapes (crest factors) whereas early production ICs did. The calibrator was inspired and encouraged by this latter fact. > > As originally described (ref 3) the Calibrator worked very well with the 8307 over nearly the entire dynamic range. It was convenient to choose a Calibrator output level of -20 dBm and step attenuator changes in this beginning level resulted is good adherence to the IC's log transfer curve over most of the dynamic range - just as was the case with sine input signals. However "something" in more recent production 8307's transfer behavior changed such that the response to non-sinusoids deteriorated over the range. This occurred at just about -20 and lower levels - but not with higher levels. Since the Calibrator outputs a square wave this fact led to the Calibrator design change from -20 to -10 dBm. where IC performance continues fine. More detailed info can be found in the references.