Very often hum is present on the signals under test. This can be easily determined from the screen because the hum is related to the line frequency. If a signal shows a kind of unexpected amplitude modulation, switching back the time-base setting to about 5 to 10 or 20 ms/div, and switching over the trigger source selector to MAINS (or LINE), will generally result in a stable picture in the event of hum.
For example, imagine a 250-kHz square-wave signal whose amplitude is rather vague. Switching back the TIME/orv to 5 ms/div and triggering from MAINS results in a stable display of a 20-ms envelope. This means that a 50-Hz hum is present on the square-wave signal.
Another example is a photograph (double-exposed) of a case where the power supply of a pulse generator is defective. The lower trace shows the varying amplitudes of the pulses. Switching the oscilloscope to 5 ms/ div and MAINS triggering showed the cause of the trouble immediately, a 50-Hz dip in the output which can only be caused by the power supply of the pulse generator (upper trace).
AC-DC INPUT COUPLING WITH X-Y MEASUREMENTS
A pitfall sometimes met in X-Y measurements is different coupling or the input channels, for example, X via channel A is dc-coupled and Y via channel B is ac-coupled.
The specification of a PM 3240 oscilloscope for ac-coupled input is stated by giving an “input RC time” of about 22 ms. With 1- MO input resistance this means a coupling capacitor of 22 nF. From this, it follows chat for 7.23 Hz a phase shift of 45° occurs (l /21rfC = 106 0) compared to a dc-coupled input. This must be borne in mind and avoided when operating the oscilloscope in the X- Y mode.
Now picture an X-Y display of the same sine-wave signal of 20 Hz. One channel is dc-coupled; the other ac-coupled. The fraction cut off the Y-axis is arc sin <p, with <p being the phase shift between the X and Y signals. From the illustration can be read arc sin <p = i; so that <p = 22°.
This result can be checked by calculation. For 20 H z, the X e of a 22-nF capacitor is 0. 36 Mil. Together with the I -Mil input resistance of the oscilloscope, this means that tan cp = 0.36 (current through a capacitor is 90° ahead of the voltage across it) and arc tan 0.36 = 22°. The readout from the screen is thus a fair approximation. Already it will be clear that at I kHz this effect is no longer noticeable.
FLOATING MEASUREMENTS ON TV RECEIVERS
Common-mode signals can be rejected by operating the oscilloscope in the A – B mode. The A – B mode must never be used for rejecting the line supply voltage.
This application of the A – B mode could be considered when signals have their zero level directly coupled to the line supply voltage. This is found in a great number of TV receivers. The illustration shows a simplified power supply of a typical TV receiver, equipped with both tubes and transistors. Node A of the Graetz diode bridge D is directly connected to the common (v) or chassis.
But in the oscilloscope, the common (v) and the ground (-. b) are very often connected. For this reason, the common connection of the oscilloscope cannot be connected to the chassis of the TV receiver. This would mean that the 220-V terminals are connected to ground (.!.) via the diodes of bridge D, causing a short circuit. At the very least the fuses VL would be blown, but very often the diodes D have to be replaced as well. Also, depending on the local 220-V supply, the filaments of half of the tubes may temporarily be connected to double the voltage.
Now, one may initially think that since both oscilloscope inputs have I-MO input impedance, if the A – B mode is used without ground connections, everything will be alright. But if the operator accidentally touches both instruments at the same time, a lethal electrical shock may be received. Thus, for safety, this procedure must never be permitted.
The only solution to this when making measurements on TV receivers is to use an isolation transformer, with separate primary and secondary windings. This not only applies when connecting an oscilloscope to a TV receiver but also for other equipment, such as TV pattern generators and digital voltmeters.
By means of the separation transformer, the TV receiver is made to float with respect to sound. Nevertheless, care should always be taken when working on TV receivers.