Shielding and induced signals

Disconnect all signal inputs and outputs from the breadboard frame. Connect a shielded cable to the scope input, but leave the other end of the cable unconnected. Observe the display and record the amplitude of any open circuit signal present. Connect the cable to the breadboard frame and again measure the signal amplitude. Now insert a job board and again measure the signal amplitude. Also determine the signal frequency. The signal can be further increased by connecting a wire to a job board contact to the scope connector and touching the other end of the wire. The amplitude of the signal is also affected by whether you are touching the frame (or other ground) or not.

What is the source of the signal observed and why is it affected by the amount of unshielded conductor exposed?

Connect the FG signal output to a frame connector. Connect the FG output to the Ch1 scope input through the contacts on a job board. Set the FG for a 1 kHz, 10 V sine wave. Connect the Ch2 scope input to another frame connector, but do not make any connections to the corresponding job board contact. Record the magnitude of the signal observed on Ch2 at the frequencies in the table below. Keep the Ch1 amplitude constant at 10 V. Now connect a $10$k$\Omega$ resistor between the Ch2 input contact on the job board and common. Repeat for a resistor of $1$k$\Omega$.

Frequency	Ch2 amplitude, V
	no load	$10$k$\Omega$	$1$k$\Omega$
10 Hz
100 Hz
1000 Hz
10 kHz
100 kHz
1 MHz

NB: What you have observed is called cross-talk. It occurs when a signal in one conductor can induce a signal by capacitive, inductive or electromagnetic coupling in another conductor. The small currents induced in the ``receiving'' conductor produce voltages across the impedance between the conductor and common. The larger the impedance, the larger the voltage as your data shows. When high impedance signal lines must be used, they should be shielded. Examples of shielded lines used in high-impedance voltage measurements are BNC cables and oscilloscope probes.^1.2