ozQRP MDT, Construction Manual

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MDT Construction Manual – Issue 2 Page 6
3 DSB
TRANSMITTER
Figure 1 shows how a Double Sideband signal is generated. The mixer used here is not to be
confused with an audio mixer that combines, for example, microphones. The mixer here is more
correctly called a multiplier, where the inputs are multiplied in the same way as in a
mathematical equation. When multiplying sine waves there are two main outputs and these are
the sum and difference of the frequencies of the input signals.
The first input to our mixer is from the VFO or carrier oscillator. The second input is audio from
the microphone amplifier.
The dominant outputs of the mixer are the sum and difference frequencies, that is, the sum and
difference of the carrier and audio frequencies. In this case, 7.101MHz (7. 1KHz)
upper sideband, and 7.099MHz (7.100MHz – 1KHz) lower sideband.
The important thing to note is that only the sidebands are present at the output of the mixer as
the carrier and audio signals have been suppressed by the action of the balanced mixer.
The diagram in Figure 1 at top right shows the DSB output signal in the time domain, or how it
would be seen on an oscilloscope. Note the overlapping envelope shape that follows the audio
waveform. The diagram at bottom right shows the DSB output signal in the frequency domain
and how it would be seen on a spectrum analyser. The horizontal axis is frequency and the
vertical axis is amplitude. The dotted vertical line in the middle indicates the suppressed carrier
frequency.
By contrast, if this was a SSB transmitter, there would be a crystal filter placed after the mixer
and one of the sidebands would be filtered out. However, it would then be necessary to add
another mixer to move the SSB signal onto the wanted transmit frequency.
Figure 1 DSB generation