Re: Measuring output watts with an oscilloscope

ajparent1/KB1GMX

Vpp is not equal to Vpp*2.

I presume you meant Vpp= 2*Vp.

I'd forgotten that as well, as I use a diode detector (rather than scope) for
power measurement so it is Vp= DC+diode drop [Vp+.06 for silicon).
Then /1.414 for RMS  (if waveform is sine.).

The problem with scopes is often the attenuator or probes are not
compensated correctly and the result is higher or lower voltage than actual.

Allison
-------------------------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Re: Measuring output watts with an oscilloscope

Kārlis Goba

50W into 50 ohms is indeed 50V rms, but that would give you 141 V peak-to-peak.

Just mind that the peak-to-peak voltage (Vpp) is not the same as peak voltage (aka amplitude or maximum swing from zero) (Vp), and for a sine wave is double the peak voltage.

Vpp = 2*Vpp
Vrms = Vp / 1.414 = Vpp / 2.828

--
Karlis YL3JG

Re: Measuring output watts with an oscilloscope

ajparent1/KB1GMX

Applying the scope across the dummy load is safe as its measuring voltage across
the resistors and that is what scopes do.

Make note that you are then measuring Peak to Peak voltage (very top of
waveform to very bottom).  IF you divide it by 1.414 you get RMS voltage
and that is used to calculate power.

so for 50W into 50 ohms that should be 50V (RMS) or 70.7V P-P.

Power is than V(rms) squared then divided by the resistance of your dummy load (usually 50 ohms).

P=V^2/R    where    R=50 ohms

As an auxiliary project look into making a Dummy load with a power meter.  THat adds
a diode to render DC and then a meter to display the power.  Look around the net for a
design you like.

Another project is a SWR (tadnam match ) that reads directly in watts.  Again consult
the net for a design or kit that you like.

Allison
-------------------------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Measuring output watts with an oscilloscope

Jon Reck W8REA

I would like to accurately measure the wattage my QCX radio is putting out, with and without my brand new 50W amplifier but I do not have a suitable watt meter.
However, I do have a brand new digital siglant oscilloscope that I don't really know how to use yet. The scope does not have a built in 50 ohm impedance input, just the 1 meg ohm.

Best I can figure from the internet, I should place my 10x probe across my dummy load and read the RMS voltage, square it, and divide by 50 ohms. I've done that with my QCX 20 with varying results that I don't trust so I bet I'm doing something wrong. Also, I am concerned that I might damage the scope if I do that with the output of the 50 watt amplifier.

Any pointers?
Thanks,
Jon Reck W8REA

Re: Measure the value of the inductance

Giulio Paris

Thank you

Re: Measure the value of the inductance

Arv Evans

IZ0FVD Giulio

Measuring inductance with a signal generator and oscilloscope or DVM with detector probe is
relatively easy.  Make a parallel circuit with your inductor and a known value capacitor.  Then
couple your signal generator to that circuit with a 2.7 to 3.2 pf capacitor.  Connect the oscilloscope
or DVM diode detector to the parallel circuit using a 2.7 to 3.2 pf capacitor.  Now tune the signal
generator across the frequency range to find where the scope or detector indicator peaks (it may
be quite sharp).  Once you have the resonant frequency (from your signal generator) and the
value of your parallel capacitor you can calculate the value of the inductance.

Sig-Gen----->>--||-------|---------|-------||-------Oscilloscope or detector probe
2.7pf      |         |      2.7pf
L1        Cx
|         |
GND -------------------|---------|----------------GND

If you have a "calibrated variable capacitor that can be used in place of Cx and varied instead
of varying the frequency of the signal generator.  This lets you set the signal generator at the
desired frequency and just tune the capacitor, read the cap value, and calculate the inductance.

Arv  K7HKL
_._

On Mon, Jan 20, 2020 at 10:14 AM Giulio Paris <giulio.paris@...> wrote:
Good evening
I would like to know if there is a way to measure the inductance value (i.e L1) before soldering it to the PCB. I have a signal generator, an oscilloscope and a VNA (rigexpert zero)
Thank you

--
IZ0FVD Giulio

Re: T1 Winding

Dave Edwards

Is it soldered in yet?

If not, just unwind, and start over.

20 meter T1 should not be tight. Lower frequencys get a bit crowded!

....Dave

On 1/20/20 8:59 AM, John Brock wrote:
I have a question about forming the windings. On the large winding I have a couple turns in the middle of it where I over lapped o turn. I'm building the 20 meter version.
I think I recall someone telling me in the past that if you overlap then those turns don't count. Would I have to add a couple turns?

Thanks
John
WA9US

Measure the value of the inductance

Giulio Paris

Good evening
I would like to know if there is a way to measure the inductance value (i.e L1) before soldering it to the PCB. I have a signal generator, an oscilloscope and a VNA (rigexpert zero)
Thank you

--
IZ0FVD Giulio

Re: 50W Amp X2 Success

Curt M.

Hi Ron, didn't think of that at the time that I was posting this but yes, I can see how that might be misleading.

Curt M.

Re: 50W Amp X2 Success

Ron Carr

This topic " 50W Amp X2 " made me think you were driving two amps, out of phase with a combiner on the output, and getting 100 watts out. ;-)

Re: T1 Winding

Alan G4ZFQ

Would I have to add a couple turns?
John

No, it will make no noticeable difference. Sometimes it is necessary to overlap if there is not enough room.

73 Alan G4ZFQ

T1 Winding

John Brock

I have a question about forming the windings. On the large winding I have a couple turns in the middle of it where I over lapped o turn. I'm building the 20 meter version.
I think I recall someone telling me in the past that if you overlap then those turns don't count. Would I have to add a couple turns?

Thanks
John
WA9US

Re: 50W Amp X2 Success

Curt M.

I ordered a small bench power supply that I can use for other things so I think I have it covered. We’ll see how it works out.

Curt M.

Re: 50W Amp X2 Success

Erik Kaashoek

--
Erik, PD0EK

Re: How to zero beat QCX?

Dan Pflugrath

Derek,
Your method is similar to what I did to calibrate my QCX only I have set my QCX offset to be 600 Hz.

I have a FT450D which is also set to a 600 Hz CW offset.  It has a feature that by holding down the ATTN button the radio produces a 600 Hz tone in the speaker.  The idea is if the tone of the CW note is the same as the tone when holding down teh ATTN button then you are zero beat with the other CW station.

I hook up a dummy load to the QCX, set the VFO for the same frequency that the FT450D is set to and then adjust the QCX calibration factor so when the QCX is key down the tone produced in the FT450D is exactly the same as when I hold down the ATTN button on the FT450.

Not a prefect process but the RBN indicates I am right on and the music pitch on shortwave stations sounds normal.  I have found that with the uBITX and the QCX the calibration  is very critical to getting the correct pitch for music on shortwave stations.

Dan KA7GPP

Re: Polyphase RX kit: No output if USB or LSB selected

Ryan Flowers

I examined the receiver and the polyphase filter, and just can't find the flaw. Any suggestions?

--
Ryan Flowers - W7RLF
The BITX40 FAQ

50W Amp X2 Success

Curt M.

Hi guys, this evening I finished building up 2 of the 50W amps kits. Sort of an off and on all weekend project while trying to do other things. I’m here to report that they both work great. The 40m one puts out about 52w with 19v in and the 20m one puts out about 37w with 19v in.

I modified both of my QCXs to output the 5v keying signal via a 3.5mm female jack so that I can use a standard audio cable for keying. I’m sure most people have already done this but it seemed like the appropriate thing to do.

I tried using a 19v laptop power supply which just can’t supply a quick enough burst of power for the amps. You can see a little lag in the power going up and down as you key and unkey. Has anyone found anything that’s reasonable in size that puts out 20v at the recommended current?  I’d rather not use a big bench type supply and stay as compact as I can.

Curt M.

@CurtisM

Been a while but I am remembering that the peak is not sharp on my 40m qcx.

The fun is the null, takes a while to get through all the levels. Note as you reach the null,  the display rescales, so don't be startled.

73 curt

Daniel Conklin

Thanks Ron! I couldn't tell based on the photo in the assembly manual because it looks very different from this.
--
73, Dan - W2DLC