3-6. Use As FM Signal Generator
3-6-1. Procedure
A. Set up equipment as described for function generator operation. Use the
frequency and amplitude controls to set the carrier to the desired frequency and
amplitude.
B. Connect an AC modulating signal with no DC component to the VCF IN BNC on
the front panel of generator.
C. Adjust amplitude of the AC modulating signal for the desired frequency deviation.
3-6-2. Considerations
A. The approximate frequency deviation for a given VCF IN signal can be
determined as follows,
The 0.1 V change at the VCF IN BNC produces a frequency change of 1% of the
highest frequency obtainable on a given range. For example, the highest frequency
obtainable on the 100 K range is 500 KHz. One percent of 500 KHz equals 5 KHz.
Therefore, 0.1 V change at the VCF IN BNC will deviate the output frequency 5
KHz on the 100K range. Following table summarizes the frequency deviation
versus VCF IN voltage for all ranges.
HIGHEST FREQ.
FREQ. DEVIATION FOR EACH
RANGE
OBTAINABLE(Hz)
0.1 VOLT VCF IN CHANGE(Hz)
1
5
0.05
10
50
0.5
100
500
5
1K
5K
50
10K
50K
500
100K 500K
5K
1M
5M
50K
Frequency deviation versus VCF IN voltage.
B. For an example, it is assumed that we wish to generate a 455 KHz signal with
FM deviation of
±
15 KHz (30 KHz swing). 1M range will be used to obtain the 455
KHz carrier with the FREQ. dial set to 0.455. The highest frequency obtainable on
the 1M range is 5 MHz. One percent of 5 MHz is 50 KHz. Our requirement of 30
KHz deviation is 0.6 times less than 50 KHz deviation produced by a 0.1 volt VCF
IN swing, thus we will use 0.6 times as much peak-to-peak voltage swing, or
0.06 V.
STATED ANOTHER WAY: SUBSTITUTING THIS
EXAMPLE:
desired deviation 30 KHz
1% deviation 50 KHz
x0.1 V = required VCF IN signal
x 0.1 = 0.06 x 0.1V = 0.06V
