DDS VFO Construction Manual – Issue 1
Page 11
5
T
ESTING
Before applying power check the board over one more time. Look for solder bridges and
components in the wrong way. A moment spent here may save a lot of frustrating time later on.
Once you are satisfied connect the DDS VFO to a power supply between 8 and 15V DC. If the
power supply has current limiting set this to about 200mA.
Apply power and check that the LCD backlight turns on. Also check with a multimeter that the
output of the regulator is between 4.75 and 5.25V DC. The power supply current should be
around 100mA, but this will depend on the exact LCD you have installed and it’s backlight
characteristics.
If the readings are way outside this range turn off immediately and look for problems.
At this stage you may or may not see anything on the LCD. Adjust the contrast trimpot until the
characters are clear and easy to read.
Turn the power off then on again and check that the first thing displayed is the firmware version
number. This is then followed by the operating display.
As the DDS VFO has not been configured the LCD will display default settings. These are
Transmit frequency of 7.1MHz and a 1KHz Step. The default IF frequency is 10MHz with a Minus
offset so using an oscilloscope or a frequency counter verify that the DDS is outputting a 2.9MHz
sine wave signal of around 300mV pk-pk.
6
C
ONFIGURATION
6.1
U
NDERSTANDING OPTIONS
IF offset
Consider a transmitter using a Superhet mixing arrangement. The transmit frequency is a result
of the IF frequency being mixed with a VFO frequency. If for example the IF carrier frequency is
10MHz and the desired transmit frequency is 7.1MHz then the VFO frequency can be either:
1.
10 – 7.1 = 2.9MHz using a Minus offset
2.
10 + 7.1 = 17.1MHz using a Plus offset
It is important to understand that when the DDS VFO is configured with an IF offset the
frequency displayed on the LCD is not equal to the DDS output frequency.
If however, the VFO DDS is to be used as a general purpose signal generator or with a Direct
Conversion receiver, the IF offset can be disabled. In this case the displayed frequency will be
the same as the DDS output frequency.
