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Low Power Output for Hendricks BitX20A - Watt meters and Scope readings do not agree.... #bitx20

ab2ts
 

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.


Interestingly, when I switch to 1X on the probe my reading drops to 42 VPP which calculates out to 4.4 Watts and is more inline with the external wattmeters.   My scope is a new Siglent 200 MHz dual trace digital unit, with stock 10x/1x probes.

I have been reading up on scope probes and it seems that my 200 MHz probes are good to 200 MHz only in the 10x position.  In 1X the spec says 6 MHz, well below the 14 MHz carrier.    Based on this alone I would tend to accept the 70 VPP reading as accurate and accept that I am getting 12W out.    However given the lower readings from the external wattmeters I am not sure what to believe.

I have a homemade -20 db coupler that when terminated to 50 ohms at the tap and fed to the scope with RG-58 also agrees with the lower power output.

My older 20 MHz analog scope agrees with the lower 42 VPP / 4.4W readings independent of the 10x/1x setting.  (Calibration of this scope is reasonable but not traceable. )

All readings into a 50 ohm dummy load.

What am I missing?


Jerry Gaffke
 

Very weird, your math looks right to me.
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:
    http://www.radio-electronics.com/info/t_and_m/oscilloscope/oscilloscope-probes.php
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?

Jerry Gaffke
 

Correction, 10x does not work, 1x does.
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.

ajparent1/kb1gmx
 

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

ab2ts
 

I am definitely not running anywhere near 2A, so clearly 4.5 W is my output.   

Driver bias at 20 mA.  Finals biased to 50 mA each.  I have plenty of drive and can reach the point where more drive provides no more power.

So what do I try to get the output power to double?

ab2ts
 

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?

KM4TRT
 

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

 

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.

 

reliable   :-p

ajparent1/kb1gmx
 

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

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.

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, 
Telephone Whangarei   022  691 8405
e: Engstr@...



----- Original Message -----
From:
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To:
<BITX20@groups.io>
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Sent:
Sun, 12 Aug 2018 18:56:13 -0700
Subject:
Re: [BITX20] Low Power Output for Hendricks BitX20A - Watt meters and Scope readings do not agree.... #bitx20


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

ajparent1/kb1gmx
 

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

Leonard
 

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.

https://www.youtube.com/watch?v=uPzC8wUUNcI

This was made 10 years ago or so. The 20a is available from http://www.qrpkits.com/bitx20a.html It is considerably more expensive though. Arv had a big hand in the design and testing of the kit.
Leonard
kc0wox

Arv Evans
 

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. 

https://www.qrpkits.com/

https://www.qrpkits.com/bitx20a.html


The BITXxxA assembly manuals are still available from Pacific Antenna/QRPKits

https://www.qrpkits.com/files/BITX20_Assembly_Manual.pdf

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.
I start out with 16 watts power out. The 20a has push-pull output so gives more power and good second harmonic rejection.

https://www.youtube.com/watch?v=uPzC8wUUNcI

This was made 10 years ago or so. The 20a is available from http://www.qrpkits.com/bitx20a.html It is considerably more expensive though. Arv had a big hand in the design and testing of the kit.
Leonard
kc0wox

KM4TRT
 

Hi Allison,

Given the formula for Watts vs P-P  (Meca electronics)
one would use  the peak voltage(35Vp) and that would
crank out to about 12.25 watt, which I believe I had at one time,
but now at about 7 watts
Power -to - volts voltage equation forumla - RF Cafe

Feel free to correct me if I'm off here
Andy,  KM4trt

Jerry Gaffke
 

Yes.
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)
one would use  the peak voltage(35Vp) and that would
crank out to about 12.25 watt, which I believe I had at one time,
but now at about 7 watts
Power -to - volts voltage equation forumla - RF Cafe

Feel free to correct me if I'm off here

Glenn
 

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.

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)
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

Glenn
 

typo in my previous post, for +39dBm

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)
It could be +39dBm = 9.9W
or +41dbM = 12.6W

ajparent1/kb1gmx
 

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

ajparent1/kb1gmx
 

Jerry,


using Andy's    V^2 pk /100=P

There is an assumption of sinewave power and 50 ohms.
If measured at the drain of the mosfet the latter is likely not true.

Then there is O'Scope error.  Scopes are generally better at measuring
an events timing than accurate voltages.

Allison