Need help understanding Oscilloscope behavior!!

I doubt the terminations are fine...
Usually this is caused due to reflections by wrong termination.

It should look like:
                                     Scope(1Meg)
                                        |
FGen(50R terminated)---[BNC-cable]-----[T]---[BNC-cable]---Circuit(50R terminated)

I guess it looks like:
                                     Scope(50R terminated)
                                        |
FGen(50R terminated)---[BNC-cable]-----[T]---[BNC-cable]---Circuit(Hi-Imp)

The set up is as follows:

Scope Ch.1 (50R)
|
FGen(50R terminated)---[BNC-cable]-----[T]---[BNC-cable]---Circuit (Scope Channel 2 [1Meg])**

**So because I want to check the amplitude levels at the circuit end of the T-connector, I am connecting that end to Channel 2 of the Scope instead of the actual circuit, in order to simply observe if my circuit is being fed the correct input. But turns out that the level in Ch. 1 is going down by 0.1-0.5 V and Ch. 2 is stepping up.

Tapping the 50 ohm impedance matched connection with a BNC-T and a high impedance oscilloscope input is only a halfway correct termination scheme, but it should give acceptable signal quality for medium fast signals. Correct wideband matching would use a -6dB resistive power splitter and 50 ohm oscilloscope termination.

sohamsurve should report the actual setup.



As sketched by rho-bot, the end nodes need to be 50 ohm terminated and the device at the T-connector high impedance.

It means the unterminated coax and load reactance plus coax capacitance has a resonant effect with a gain up to 3. If you sweep the f , then you will find even higher voltages at the self resonant frequency.

This is why we don't use this method to drive high impedance loads on the end of a long cable without tuned cable lengths or careful consideration to the "impedance transformer" effects of long mismatched cable.

After the parallel resonant frequency, you may find a series shunt resonant frequency at the 1/4 wavelength with no signal using an RF sweep.

yes I too was wondering HOW LONG that cable was with the high impedance voltmeter trying to measure it? The line IS a transmission line, and will behave like a transmission line. With an OC load, there is a forward wave, and a equal reverse wave at the end. That can look like 2X the expected voltage

A matched transmission line will double when termination is disconnected but not 3x. There must be an inductive element raising the Q at the end with partial resonance with cable capacitance.

Since impedance rises when this occurs with parallel resonance, there is no power gain.