EMRFD Message Archive 1846
Message Date From Subject 1846 2008-07-30 11:28:46 wb7dmx FFT I just bought a DS0-2250 usb oscilloscope that has the FFT spectruim
analyzer features in it, up to 100Mhz.
I want to look at the output od a bitx-20 V3 20 meter ssb xciver.
output is about 5 watts, the max input of the scope is 35 v.
I am thinking that all I need to do is use a 10 db attenuator with a
10 watt dummyload to reduce the power level to a safe level, and then
connect it to the scope input,
do I also need to have the 50 ohm resistor across the scope input or
I also have a 30 db rotary step db attenuator I can place in the line
to the scope.
the scope has a 100 Mhz probe, or can I connect the scope directly to
the step attunator.
I just do not want to do anything that could damage the scope input.
1847 2008-07-30 14:18:09 wimmie262000 Re: FFT 1848 2008-07-30 14:48:49 bob bailey Re: FFT yes, I already have all the power taps and pads as described in the EMRFD book and have been studying the chapters relating to this, and is why I asked the question. it was still a bit confusing to me the proper way to hook up with this set up.
I am not too concerned about accurate power measurement as I am of seeing all the harmonics and / or any spurs that might be in the signal.
I am just experimenting with this to learn the correct way to us all the features of the new scope.
1852 2008-07-31 04:14:05 kerrypwr Re: FFT Bob;
Using the formulas Power = RMS Voltage squared/Resistance and Peak =
1.4 X RMS, your 5 watts will provide 16 volts RMS/22 volts Peak/44
volts P-P at the 50-ohm load.
Use the 'scope probe on its X10 setting and you will have 2.2 volts
Peak at the 'scope input. I'm assuming that the 35 volts is
Peak-above-ground, not P-P but, in any case, you will be well below
the danger line.
A simple way is to use a BNC T-piece; one leg via co-ax to the
amplifier/transmitter, one leg directly on the dummy load (with UHF or
other adaptor if required) and the 'scope probe (which probably has a
BNC adaptor) in the third leg.
The X10 setting should be used always; I regard X1 probes as
close-to-useless. All my probes are fixed X10 types.
As a general rule it's virtually impossible to damage a 'scope or a
X10 probe with the kinds of voltages that we encounter in our
low-power solid-state gear; it pays, however, to think about it as you
Just for interest, if you measure using the 40 dB tap (which is not
necessary anyway) you would terminate the 'scope in 50 ohms I think,
as the tap is designed to feed a 50-ohm power meter iirc.
I looked-up your new 'scope; very nice.
You will have lots of fun with it!!
1853 2008-07-31 13:37:45 Kenneth Stringham Re: FFT You could use a 50ohm feedthru termination as well.
Ken - AE1X