Analogue oscilloscope preamplifier to compensate for reduced bandwidth.

[neazoi]

I have made a 60's combo Tek 564 scope with Tek 3A6 and Tek 3B4 plugins. Despite the 3B4 specs are 20MHz triggering, it can trigger ok on 30MHz.
The 3A6 can show a 30MHZ signal, but the level of the signal is reduced much of course, since this is a 10MHz vertical plugin.
In fact the flatness of the 3A6 level starts to degrade from 3-3.5MHz or so (the 3:1 bandwidth rule).

It won't be easy to compensate for the response loss curve of the vertical plugin, but I wonder, if I can built a small HF pre-amplifier to compensate for the loss and push the scope to the limits of 30MHz?
I am not familiar if this will cause any serious distortion issues (tested only with sinewave) apart from level enhancement.

 

[D.A.(Tony)Stewart]

The old tube & BJT plug-in could be replaced, but without analyzing the 3A6 schematics and deflection BW, there won't be a simple tweak.

How about a nice low-noise CMOS 100 MHz Op Amp.

 

[neazoi]

The old tube & BJT plug-in could be replaced, but without analyzing the 3A6 schematics and deflection BW, there won't be a simple tweak.

How about a nice low-noise CMOS 100 MHz Op Amp.

View attachment 188055

By experimentation, I came up with 2 useful points:

1. Even if I increase the level of my generator, the scope cannot cope with it on high frequencies. I think it has to do with input saturation of the vertical amplifier? So obviously a preamplifier is not the way to go.

2. But I thought, how about loading the input to the scope less as the frequency is increased? And THAT worked to an extent! I was able to read the same level signal as that of 3MHz, but at 15MHz now! So it worked like being a 45MHz scope now (1/3 BW)

You see, the generator has 50R output impedance, and I connect a 50R coaxial to the scope. Directly at the scope 1M input, there is a coaxial 50R resistor, so that to satisfy the load conditions and the measurements to have a meaning.
But if I increase this load resistor as the frequency goes up, I could read the same level signal, "fooling" the scope, in essence.

Yes, I know about reflections and impedance mismatch that can cause problems, but when talking about CONTINUOUS HF signals like a sinewave and not pulsed, this reflections do not play a big role I think.

So for continuous signals and 50R measurements (only when meet these criteria), increasing the input impedance of the scope as you go up on the frequency, seems to virtually increase the usable bandwidth of the scope.

Of course, some kind of pre-calibration is needed, to know what impedance (shunt resistance) corresponds to each frequency. Then it can be set manually or electronically using some kind of MCU and a variable resistance device.

I would like your comments on this, since my experiment seems to work. But there might be other issues on this approach that I am not aware of.

 

[D.A.(Tony)Stewart]

RC= 0.63/80%*0.35/fc =0.28/f-3dB so for 28MHz BW , RC= 10ns
If Cin at scope is 20 to 30 pF, then source , then the max is Rs = 330, for a 1:1 probe with a short ground ESL so that it doesn't resonate with the ~70 pF/m probe capacitance near f max.

You could easily make an emitter follower to buffer high impedance AC signals with Re= 300 Ohms and Zo= Rin/hFE || Re, Zin=hFE Re.
or a JFET buffer.