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Low Power Output for Hendricks BitX20A - Watt meters and Scope readings do not agree....
#bitx20
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I solved my stability problem for the Hendricks Bitx20A ?kit by applying the?Martiens mod. ? ? Now I need to resolve a low power output problem. ? I had been just calculating the power output from my peak to peak O-Scope readings with a 10x probe. ? However I notice that while I had 70 VPP on the scope with the 10x probe ( ~ 12.2W = (70^2)/400 ); my wattmeters both read a much lower 4 to 6 watts. ? My wattmeters are a NorCal QRP Club watt meter and a Heathkit HM-9 QRP Wattmeter. ? I am driving the microphone input with a 600 Hz phase shift sine wave oscillator.
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Very weird, your math looks right to me.
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Could be a bad scope probe, have you tried more than one probe? Could be a bunch of bad scope probes. You have 4 measurements saying around 5 Watts and one saying 12 Watts. My bet is that it is around 5 Watts. And that since the 10x probe works, the scope is doing fine. If your scope is one that attempts to read the switch and scale the 1x vs 10x readings for you, could be that the?scaling feature is hosed. Not much going on in a scope probe: ? ?? In 1x mode, my cheapo scope probes show about 100 ohms from probe tip to BNC center pin, that 100 ohms might be there to prevent massive currents when I do something really stupid. In 10x mode, it's 8.9 meg. In both cases, it is almost certainly driving a 1 meg load inside the scope, just like the link above says. ? Would be interesting to replace your 1x probe with a short piece of coax. Or maybe just a twisted pair of hookup wire. Keep it below a meter in length to avoid trouble with reflections at 14mhz. Then try adding a 9meg series resistor, see if it behaves properly as a 10x probe. Jerry On Thu, May 17, 2018 at 09:13 pm, ab2ts wrote:
What am I missing? |
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Correction, 10x does not work, 1x does.
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10x would read double if that 9meg resistor inside the probe was more like 4meg. And that could easily be measured with an ohmmeter from probe tip to bnc center pin. On Thu, May 17, 2018 at 09:58 pm, Jerry Gaffke wrote:
And that since the 10x probe works, the scope is doing fine. |
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The check would be you must be putting st least 2A into (24W to the finals, ~50% efficiency)
the finals to be in the 10-12W region. I don't ever believe scope readings as accurate. For power out I use a dummy load with a diode detector or a 30-40 db power attenuator as I have a calibrated unit 30db and 10db that into the spectrum analyzer gives good reading for power.? If all else fails if my 10W dummy gets hot its likely more than 10W.? The latter with a thermometer can be calibrated for power equal temperature using a variable power supply (DC power=RMS power). At some point you need an instrument you can believe. Allison |
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Ok, ?some improvement. ?Readings are a bit more uniform.
I checked the frequency compensation of my probes and the 10x was out of adjustment, 1x was OK. ? Adjusted the frequency compensation capacitor in the 10x position and both 10x and 1x looked good. ? (In retrospect this makes sense because the compensation capacitor is part of the 10x voltage divider network.) Now I read 54 Vpp??(7.3 W) with the 10x probe and 44.8 Vpp (5.0 W) with the 1x probe which is in nice agreement with my -20 db coupler 46.8 Vpp (5.5 W). ? The 10x and 1x still differ but are in much better agreement than before. ?My Watt meters read 5 to 6 Watts. ? I can't say why the 10x probe reads high. ?I expect the 1x probe to be low since the probe is only rated to 6 MHz in the 1x position. ? I have no feel for the frequency variation of my -20 dB coupler. I can get 7.2 Watts out if I crank up the drive, however the output is no longer stable; even with the?Martiens mod. ? So I have turned it down below 6W. ?? Did anyone really ever get a stable 12 Watts out of the Hendricks Bitx20A? |
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I also have the Hendrix Bix 20. ?I think I measured 70vPP on the scope, but my QRP kits dummy load read about 7 watts (with the dC voltmeter method.
I also measure about 7-8 watts output on my antenna tuner. ? I used to get more output (as measured by current when tuning up so may have fried something) but people say the signal sounds good and Im getting out ?(1500 miles), so havnt gone after the? problem yet. Andy ?KM4TRT |
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Mark McNabb
Hi ab2ts,
Old post, but how are you reading the PP off your scope?? If you just take the number readout from it, you may have an erroneous reading due to noise etc.? Always verify scope readings by checking against the display graticules.? You can also get more reliabel readings by using trace averaging, and/or the cursors to set the readings to the peaks. 73, Mark. |
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K4MRT,
I sincerely doubt you getting 24W (70V-PP).? I can believe 7W. Into? a 50ohm load I'd expect to see maybe 35VPP. for around 6-7W. Most people put a scope probe across? the output and and cause serious measurement error.? ? You can check most scopes with a DC power supply and a multimeter if the two agree great.? Then you need to use a properly calibrated probe or better yet review EMRFD (measurements chapter) for correct procedure. Even then I consider a scope the least accurate. A dummy load with diode detector can be calibrated with DC supply. Plenty of site on how its done and its dirt simple.? Just remember? mode diode measurements are PEAK voltage and you convert that to RMS to calculate power. Allison |
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Timothy Fidler
all due respect Allison Bitx20A is a dual IRF510/A? class B? design with substantial drive frm small signal drivers . I is suppose to produce 12W clean if well set up. Basically the parameters are similar to a uBitX but the output drive transformer is a bit different from most other designs.? It has the indications of a lot of? dev work done on it and it is supposed to be clean on OP? if built and biased as per directions.
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you may be assuming it is? class A amp as are many BIT designs. It is not.? of course they may have gutted it to cheapen it and repackaged it but the old version from Hendricks was as above. ? Timothy E. Fidler : Engineer BE Mech(1) Auckland , NDT specialist AINDT UT /RT3 , MT2 CB #2885,?
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Timothy,
>>all due respect Allison Bitx20A is a dual IRF510/A? class B? design with substantial drive frm small signal drivers . What is a bitx20A??? Also I made no assumption any Bitx has a Class A amp most builds I've seen would either oscillate or melt at 1-2 A no signal drain current.? ? Most people run them starved at 50ma to lower the gain of the IRF510 and that near class B. I built the bitx20 and it was a single device. Maybe? your talking about the hendricks bitx20 could as it had BS170 push pull driving IRF510 push pull? However, its was unspecified so I covered the known case. For the Hendricks they could do much more if they didn't break into oscillation first.? Most I got to see were better power oscillators than transmitters.? Minor changes tamed them and usual was 12-15W. Both cases 70V PP would be 24W output across 50ohms , you would only get that with the PA at more than 18V.? or its an incorrect measurement. Allison |
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Here's a video of a Hendrix BITX20a looking at the modulation, carrier balance and harmonics rejection. I'm not an expert but I believe my way of testing is relatively accurate.
I start out with 16 watts power out. The 20a has push-pull output so gives more power and good second harmonic rejection. This was made 10 years ago or so. The 20a is available from It is considerably more expensive though. Arv had a big hand in the design and testing of the kit. Leonard kc0wox |
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Leonard Very good video.? For those who were not around several years ago, Leonard KC0WOX played a large part i popularizing the BITX and in helping others build and fix their own BITX transceivers.? His videos are as relevant now as they were back then.? During the past few years the QRPKits company of Doug KI6DS has been transfered to Pacific Antenna, and the BITXxxA series seems to have been dropped from production.? The BITXxxA assembly manuals are still available from Pacific Antenna/QRPKits Arv _._ On Tue, Aug 14, 2018 at 3:42 PM Leonard <dredger@...> wrote: Here's a video of a Hendrix BITX20a looking at the modulation, carrier balance and harmonics rejection. I'm not an expert but I believe my way of testing is relatively accurate. |
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Yes.
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70 volts peak-to-peak is? 70/2 = 35 volts peak,? is 35v/sqrt(2) =24.75 volts rms. Into a 50 ohm dummy load, that's volts*amps = 24.75vrms * (24.75vrms / 50ohms) = 12.25 Watts. Though when you said? ? "Given the formula for Watts vs P-P" that formula takes volts peak, not volts peak-to-peak. On Sat, Aug 18, 2018 at 08:40 PM, KM4TRT wrote:
Given the formula for Watts vs P-P ?(Meca electronics) |
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Power output is notoriously difficult to measure.? To measure power out at one frequency, using an SA is probably the only way as you suggest.
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But then, the SA has a Spec for level, typically +-1dB say, for the HP8560A I have. And unless it's just been calibrated, even that is of course,? suspect.? Calibration in my experience simply means that the instrument being tested falls with it's specification, not that it's absolutely accurate. Then there is the problem of measuring the attenuators required for higher level power measurements.? An attenuator "checked" using an uncalibrated SA is also suspect. Take a measurement of a? 10W? signal (+40dBm) It could be +39dBm = 9.9W or +41dbM = 12.6W On Fri, May 18, 2018 at 10:23 PM, ajparent1/KB1GMX wrote: The check would be you must be putting st least 2A into (24W to the finals, ~50% efficiency) |
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typo in my previous post, for +39dBm
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should be Take a measurement of a? 10W? signal (+40dBm) It could be +39dBm = 7.9W or +41dbM = 12.6W On Sun, Aug 19, 2018 at 03:41 PM, Glenn wrote: Take a measurement of a? 10W? signal (+40dBm) |
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Glenn,
1db error cannot be heard at the other end. However most do better than 1DB more like less than .1db. However you then need to know the loads and attenuators accurately. If you using random cables you have to measure them especially at upper HF and above.? for example 1M of RG402 at 1.5ghz can eat many db.? I have measured mine to better than .1db and frequency and keep a record.? both for the precision cables and attenuators.? If it changes as in broken and two its not constant over dc to UHF and higher.? So a record is then good for building a calibration tree. However the solution is a calibration source to assure that what ever device being used is indeed indicating accurately. Its not hard to build and once conformed you write it on the box and use that as a reference.? I do that even with a recent calibration as that assures me I didn't grab a bad cable or fry something.? Then if suspect or just need to know the answer is use the calibration source and verify. Same for loads, a peak reading load for more than a few milliwatts can be verified with a power supply and a known accurate voltmeter. The last items is sanity check the numbers.? If it seems inconsistent it might be. For example a case of an insistence it can hear to -140dbm simple radio. Problem was his RF source leaked like a sieve and the radio heard well to about -118dbm!? Once he got near that level all the attenuators in the world were overridden by leakage. Allison |
Leonard
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