Dual beam oscilloscope 

The dual trace oscilloscope cannot capture two fast transient events,
as it cannot switch quickly enough between traces. The dual beam oscilloscope
has two separate electron beams, and therefore two completely separate vertical
channels. The two channels may have a common time base system, or they may
have independent time base circuits. An independent time base allows different
sweep rates for the two channels but increases the size and weight of the
oscilloscope.

 Lissajous Figures:  
When sine-wave signals of different frequencies are input to the horizontal and
vertical amplifiers a stationary pattern is formed on the CRT when the ratio of the
two frequencies is an integral fraction such as 1/2, 2/3, 1/5, etc. These stationary
patterns are known as Lissajous figures and can be used for comparison
measurement of frequencies.
The ratio of the two frequencies is 

Measurement of Frequency

The frequency of a signal is measured using oscilloscope in two methods. They are, 
 1. Using calibrated oscilloscope          
  2. Using uncelebrated oscilloscope. 

1.) Measurement of Frequency using Calibrated Oscilloscope 

It is the indirect method of measurement of frequency. In this method, the frequency of unknown signal is measured by measuring its time period. 
Initially, the unknown frequency signal is applied to the vertical inputs of the CRO. Now the horizontal sweep is turned ON and the display appealing on the screen is adjusted by varying different control knobs provided on the front panel of CRO, till the signal is suitably displayed on the screen. After obtaining the display of good deflection, count the number of horizontal division for a complete cycle. From the counted horizontal divisions, the time period is computed as,  
                                                   
                                                               T=m*n
Where  
m=Number of division in one complete cycle
n=Setting of time base =Time/Division
From the measured time period of the signal, the unknown frequency is calculated as, 
    f =1/T

2.) Measurement of Frequency using Uncelebrated Oscilloscope 

It is the direct method of measurement of frequency. In this method, initially a signal whose frequency is known is applied to the horizontal input terminals whereas a signal whose frequency is to be known is applied to the vertical input terminals of CRO. Now different control knobs provided on the CRO are adjusted till a pattern of loops appear on the CRT screen. Here, the displayed pattern of loops cut by horizontal line and also vertical line. The number of loops that are cut by horizontal line gives the frequency of signal applied to vertical plate (fv), whereas, the number of loops cut by vertical line gives the frequency of signal applied to horizontal plates (fh). 

Therefore we have.
 
 f v / fh = Number of loops cut by horizontal line/Number of loops cut by vertical line 
If the frequency of the signal applied to the horizontal input is 100 Hz. For the 100 Hz frequency signal, say the number of loops cut by horizontal line is 1 and the numbers of loops cut by vertical line are 2.