Specifications

1. Re-create the analog Voltage represented by the digital
word by turning on or off units of dc Voltage (or RF
Voltage) and holding it constant for one time interval.
2. Pass the reconstructed audio through a low pass filter to
remove the steps. This low pass filter may also be called
a reconstruction filter. It acts as a “smoothing filter,”
removing the steps to provide a high quality analog
signal.
STEP 1:
Re-create the analog Voltage represented by the digital word.
Each bit of the digital word represents some amount of Voltage.
For a five bit digital word and an analog Voltage range of zero to
eight Volts (used in Table 1), each bit represents a Voltage as
follows:
Bit 1, 4.00 Volt (MSB)
Bit 2, 2.00 Volt
Bit 3, 1.00 Volt
Bit 4, 0.50 Volt
Bit 5, 0.25 Volt (LSB)
Note that the Most Significant Bit (MSB) represents one-half of
the maximum analog Voltage, and each additional bit represents
one-half of the Voltage of the bit before it.
The analog Voltage can be reconstructed by providing a Voltage
source, either dc or RF, for each of the Voltages represented by
bits in the digital word, then using these Voltages as inputs to a
summing circuit with a switch to turn each Voltage OFF if the bit
is zero or ON if the bit is one. An example from Table 7-1 will
be used to illustrate this process. When the input to the A/D
converter is +3.914 Volts, the digital word constructed is 01111.
The D/A converter, then, sums (2.00 + 1.00 + 0.50 + 0.25) for a
total of 3.75 Volts. If the digital word is 00101, the output of the
D/A converter is (1.00 + 0.25) = 1.25 Volts.
The least significant bit in the digital word represents 0.25 Volts
in this example, so that the output of the D/A converter must
change in 0.25 Volt steps. Note that this is the same as the 0.25
Volt quantization error in the example of the Analog to Digital
conversionused in the previoussection. The analog input Voltage
to the Analog to Digital converter changes in continuous manner,
but the output of the Digital to Analog converterchanges in steps.
The re-created Voltage at the output of the D/A converter is an
approximation of the original analog input Voltage. The maxi-
mum round off or quantization error in the re-created analog
Voltage is the size of the steps. As the number of bits in the digital
word increases, the Voltage step represented by the least signifi-
cant bit becomes smaller, the steps in the re-created analog
Voltage from the D/A converter become smaller, and the re-cre-
ated Voltage more closely approximates the original analog
Voltage.
STEP 2:
Pass the reconstructed audio through a low pass filter to remove
the steps. Because the low pass filter smoothes the steps, it may
be called a “smoothing filter”;itisalsocalleda“reconstruction
filter” because it reconstructs a better approximation of the
original audio signal from the stepped output of the D/A con-
verter.
Sharp “corners,” steps, or transitions in a waveform are caused
by high frequency harmonics in the signal. The low pass filter
attenuates or removes these harmonic frequencies, and therefore
also smoothes or removes the sharp corners or steps in the
waveform.
The Digital to Analog converter output can be any desired
Voltage, limited only by the switching circuits in the converter.
For example, the bits in the digital word could be used to switch
Voltages of 100, 50, 25, 12.5, and 6.25 Volts, so that the digital
word 00101 would produce a D/A converter output of (25 + 6.25)
= 31.25 Volts, instead of (1.00 + 0.25) = 1.25 Volts as in the
example. The bits in the digital word can also be used to switch
Radio Frequency signal Voltages on and off to produce a varying
or amplitude modulated RF signal.
Digital amplitude modulation simply takes the digital words and
turns on and off units of RF Voltage instead of
DC Voltage to
create amplitude modulation. Then the transmitter output band-
pass filter removes the steps to provide a high quality analog
amplitude modulated signal.
888-2339-002 a-3
WARNING: Disconnect primary power prior to servicing.