ozQRP MDT, Construction Manual

Page: 5 / 42

MDT Construction Manual – Issue 2 Page 5
2 DSB
VS
SSB
Why DSB? The answer is simple. A DSB transceiver is less expensive, less complicated and
easier to build and align than a SSB transceiver. This is due mainly to a DSB transceiver not
having a crystal filter, IF amplifier and multiple mixers that are required in a SSB design. Note
that in a DSB transceiver the receiver is more often referred to as a Direct Conversion (DC)
receiver.
For the first time builder or for a small and cheap rig, DSB is ideal. Over the years countless
amateurs have started out this way.
While a DSB rig has many advantages for the home builder, there are some things to consider.
Firstly, a DSB transmitter occupies twice the bandwidth of a SSB transmitter. On a quiet band
this does not cause any problems but on a crowded band it may not be as easy to find a free spot
to operate without interfering with nearby stations. Secondly, the Direct Conversion receiver
has equal response to both sidebands. This means you hear signals on both upper and lower
sidebands simultaneously. This results in a slightly higher noise level and the possibility of
hearing two separate stations at the same time.
There are, however, a couple of nice advantages when a Direct Conversion receiver is used with
a DSB transmitter. Firstly, you can operate with SSB stations using Upper Sideband (USB) or
Lower Sideband (LSB) without having to change controls or move frequency. Secondly, being
able to hear both sidebands means that you can check for other stations on both sides of your
frequency before transmitting and avoid interfering with them.