ozQRP DDS VFO 2, Руководство по монтажу

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DDS VFO 2 Construction Manual – Issue 1 Page 11
4 T
HE
DDS VFO
AND THE
S
UPERHET
When the DDS VFO is used in a Superhet transceiver, the relationship between the displayed
frequency, the DDS output frequency and the actual transmit frequency can be confusing. This
section contains an example to help illustrate the relationship. The example assumes a DDS VFO
connected to a MST operating LSB on the 40M band.
The balanced modulator in the MST takes the nominal 10MHz carrier (BFO) signal and mixes
this with the audio signal when you speak. The output of the balanced modulator is comprised
of an upper sideband (USB) and a lower sideband (LSB) signal centered on the carrier
frequency. The carrier is not present at the output (suppressed) and so we use the term Double
Sideband Suppressed Carrier modulation or simply DSB. The DSB signal is applied to the crystal
filter.
The filter’s upper skirt is at 10MHz, and so the LSB is passed by the crystal filter, while the USB
is blocked. The LSB signal is then mixed with the output of the DDS VFO to obtain the final
transmit signal.
We will assume the DDS VFO is configured for an IF frequency of 10MHz, and display frequency
of 7.100MHz.
The DDS computes the output frequency to be 10.000MHz - 7.100MHz = 2.900MHz.
The final transmitted signal will be 10.000MHz - 2.900MHz = 7.100MHz.
Now let's assume that we want to move the carrier frequency 300Hz higher. This could be due
to a slightly higher crystal filter response or after testing we find the USB is not suppressed
sufficiently.
The new carrier frequency will be 10.0003MHz, however the DDS VFO will still be programmed
for an IF frequency of 10.000MHz and generate an output frequency of 2.9MHz. As a result the
displayed frequency will be 10.000MHz - 2.900MHz = 7.100MHz, but the actual transmit
frequency will be: 10.0003MHz - 2.900MHz = 7.1003MHz.
This creates an error situation where the LCD displayed frequency and the actual
transmit frequency are not the same.
To fix this we need to measure the actual suppressed carrier frequency and re-configure the
DDS VFO IF frequency parameter.
Using this example we would now configure the DDS VFO with IF frequency of 10.0003MHz.
The DDS output frequency will now become 10.0003MHz - 7.100MHz = 2.9003MHz, and the
final transmitted signal will be 10.0003MHz - 2.9003MHz = 7.100MHz.
The displayed frequency now correctly matches the transmitted signal frequency.