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.

thank you
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.

thank you

1852 2008-07-31 04:14:05 kerrypwr Re: FFT

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
have done.

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