The story is always funnier than expected.
I have done some measurements right now.
As HugeMan/lygra suggested, I feed the ch-A input (without any probe) directly with the signal generator. BTW: the instrumentation is still exactly the same.
I checked the signal either on the Quad display and with the LeCroy scope on U5 pin13.
The amplitude of the input was constant at 2Vpp.
Sine wave test.
A manual sweep from 1 to 15MHz (step by 1MHz) clearly shown that the band is NOT constant (ref U5 pin 13).
Assuming 100% at 1MHz, the amp will decrease briefly to 50% (-6dB) at about 3MHz. Still remain at 50% until you reach about 7-8MHz, then raises to about 75% around 10MHz. Hereinafter will raise even more: at 15MHz (the max I can test) is about 200%.
So, at 10MHz the attenuation is about 75% (i.e. -3dB)…that’s the way HugeMan says the bandwidth is 10MHz…but FOR ME the bandwidth has to be taken as the VERY FIRST POINT where the attenuation falls to -3dB…once again the point is around 2MHz.
Square wave test.
Just a simpler test than before: always scoping the pin 13 U5.
At 1MHz the wave has the rising edge clearly rounded. The displayed wave on Quad looks pretty the same as the LeCroy’s one.
Going higher with the freq, 3MHz, the square is almost a triangle/sine, with a well-visible spike just before the falling edge.
Above 4-5MHz the wave on the LeCroy scope looks as sine.
The spike is clearly due to the unexpected amplitude above 10MHz.
My conclusion:
the analog section has a bandwidth of 2-3MHz;
the “supposed” bandpass is very far to be flat (as should be);
the analog switching strategy is faulty: if the signal cannot be shown as it is, the Quad should constrain the user’s selection;
as the current analog section hardware, this Quad is good for signals under 100-200KHz.
I will leave my Quad in the lab, if someone of you is asking for any additional test.
Cheers
PS: suggestion to HugeMan.
I would think about an external box, embedding a good analog section, that can feed a good bandpass and a reliable way to display signals.
I am not so happy about this toy.