ozQRP DDS VFO 2, Construction Manual

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DDS VFO 2 Construction Manual – Issue 1 Page 16
7 C
ONFIGURATION
7.1 U
NDERSTANDING OPTIONS
Master Clock
The DDS VFO requires a master clock to generate the output frequencies. The firmware can
accept three clock frequencies of 50MHz, 80MHz or 100MHz. The 50MHz clock rate is the
default setting while the other options allows ‘over clocking’ of the DDS chip to generate higher
frequencies if required. See section 10 for more details.
IF Offset
When the IF Offset is set to Yes , the LCD will display the desired transmit frequency while the
actual DDS signal frequency will be equal to the difference between the IF and transmit
frequencies.
For example, the MST3 transceiver employs a 10MHz IF frequency. If the desired transmit
frequency is 7.1MHz then the DDS VFO frequency would be 10 – 7.1 = 2.9MHz.
The relationship between the IF frequency, DDS VFO frequency and transmit frequency when
used with the MST3 is shown below:
Band IF frequency DDS VFO frequency Transmit frequency
80M 10MHz 6.5MHz 3.5MHz
40M 10MHz 3MHz 7MHz
20M 10MHz 4MHz 14MHz
17M 10MHz 8MHz 18MHz
Note that the DDS VFO firmware assumes that the DDS VFO frequency is always
below the IF frequency.
If the IF Offset is set to No the DDS output frequency will be the same as the displayed
frequency. This is the setting selected when the DDS VFO is to be used as a general purpose
signal generator or with a Direct Conversion receiver.
IF Freq High and IF Freq Low
To generate an accurate output frequency the DDS VFO must know the exact IF frequency. To do
this the frequency must be measured using a frequency counter and the value entered at
configuration. If not the default values will be used and the displayed frequency may not
accurately match the transmit frequency.
Also, the MST3 moves the IF frequency when changing sidebands by about 3KHz. So that the
displayed frequency and the actual transmitted frequency are matched it is necessary to enter
both upper and lower IF frequencies. Using these values the DDS VFO firmware automatically
adjusts the output frequency when sidebands are changed.