[Elecraft] Oscilloscope help part 2 ;-)

[Ron D'Eau Claire]

Keep in mind, Jean-François, that if you use that 'scope to search for
distortion in a waveform, you probably shouldn't try to investigate
waveforms above 1 or 2 MHz with a "14 MHz" oscilloscope. It will work well
at 14 MHz and above for simply "peaking" a signal for maximum amplitude as
Don says, but not for looking for spikes, noise and other artifacts in a
waveform. 

That's because noise, spikes, etc., appear at frequencies much higher than
the fundamental signal. For example, if you had access to a variable
frequency square wave generator and applied it to the scope input terminal,
you'd see nice square waves on the display until you raised the frequency
above 1 or 2 MHz. Then they'd start looking less square and more rounded. As
you continue to raise the frequency by the time you raised it to 14 MHz the
waveform on your scope would look more like a sine wave than a square wave. 

That happens because the leading and trailing edges of a square wave are
really at a much higher frequency than the fundamental  frequency of the
square wave. If you look at the plot of a square wave and carefully traced
over it a sine wave whose rise and fall matched that of the leading and
trailing edges of the square wave, you'd see that many cycles of sine wave
would  fit in the space of one cycle of square wave. Unless your 'scope can
handle that higher frequency, it can't reproduce the steep shape of the
leading and trailing edges and they will look rounded. 

So you might have a really distorted signal at 10 or 15 MHz showing noise or
parasitic oscillations, etc., but on your 14 MHz scope the signal might look
like a very clean sine wave. 

In practice, the usual "rule of thumb" is that the vertical amplifier in a
oscilloscope should have about 10 times the bandwidth of the
highest-frequency signal that you want to view to see the "shape" of the
signal and to look for artifacts. That's why most 'scopes used for HF up to
30 MHz have vertical amplifier bandwidths of several hundred Megahertz. 

Let me reiterate, that is a concern only if you are looking at the shape of
the waveform. As long as you can see some vertical deflection, it is FB for
"peaking" a stage for maximum output. 

Ron AC7AC

-----Original Message-----

My oscilloscope have only an input amplifier bandwidth of 14 Mhz 
maximum... will it be enough to adjust all bandpass on all bands ???


Le 04-06-20, à 18:29, Don Wilhelm a écrit :

> When adjusting the bandpass filters for transmit, I do use a 'scope. 
> The 'scope is doing nothing more that acting as a sensitive analog RF 
> voltmeter, so nothing complex is required on setting the 'scope time 
> base.
>
> Just connect the 'scope probe across the dummy load (I use a 10x
> probe),
> press TUNE on the K2 and you should see an RF envelope display on the 
> 'scope
> face - if not adjust the vertical amp and the trigger level until 
> something
> is displayed - then set the time base so you are seeubg the CW envelope
> rather than a few cycles of the RF.
>
> As you adjust the bandpass filter, you should see the vertical
> deflection on
> the 'scope face shrink and grow in response to the tuning - adjust for 
> the
> largest display height.
>
> 73,
> Don W3FPR
>
>