PURPOSE: To see beats on an oscilloscope between two
independant sources.
DESCRIPTION: Two independant oscillators are used to create sine
waves. The sine waves are displayed on the oscilloscope channel 1 and channel 2. By
slightly adjusting the frequency of one sine wave while keeping the other fixed, a small
difference in frequency is created. The beat pattern can be seen using the math function
SUM on the oscilloscope as the frequency of one of the sine waves is varied. One can
produce a slow change in amplitude with a small difference in frequency, or one can
produce the wave packet effect by making a larger difference in frequency.
EQUIPMENT: Two Function Generators, oscilloscope.
SETUP NOTES:
Start with the two function generators at 100 hz.
Time base 10 msec.
100 hz channel 1 on top trace. Synch the trigger to Ch 1.
100 hz channel 2 on the bottom trace.
Use Math to show the Sum (or Difference) on the middle trace between channel 1 and channel two. Make sure that all three traces are set to 1 volt. The amplitude of the sum should appear to be twice as large as the other two channels when the phases of the two channels are precisely the same.
From the doc called beats.doc, 2/10/99
Beats on the tektronics digital scope.
1)Start with the two function generators at 100 hz.
Time base 10 msec.
100 hz channel 1 on top trace. Synch the trigger to Ch 1.
100 hz channel 2 on the bottom trace.
Use Math to show the Sum (or Difference) on the middle trace between channel 1 and channel two. Make sure that all three traces are set to 1 volt. The amplitude of the sum should appear to be twice as large as the other two channels when the phases of the two channels are precisely the same.
Note: There is no phase feature on the Pasco function generators. To adjust the phase difference between the generators use the frequency control to create a beat. To do this, set the scope trigger on channel 1 (100.00 hz) and slowly adjust the frequency of channel 2 to be about 1/10th or 1/100th difference (100.10 or 100.01 Hz) to create a very slow beat between the channels. This will cause a slow change in phase between the two channels. Watch the sum change with the beat. At the moment when the sum reaches the maximum double amplitude quickly reset channel 2 back to exactly match the frequency of channel 1. This will eliminate the beat and maintain the two channels in the correct phase.
2) Adjust the ch 2 function generator to 105.00 hz while the other remains fixed at 100.00 hz
After the phase differences are synchronized properly at 100.00 hz for both generators, change the ch 2 generator to 105.00 hz while keeping ch 1 constant at 100.00 hz and the scope trigger synched on ch1. Note the beautiful beat in the middle trace math function, the sum (or difference) of ch1 + ch 2. Increase the time base from 10 msec to 50 msec to view the entire beat envelope on the oscilloscope screen trace.
