Xmit

XMIT - New tools for old jobs

This repository has design files and (soon) code for a direct synthesis digital mode transmitter optimized for use on amateur bands. It should work well up to 2m. The novelty here relative to other systems like the QDX lies in a few areas:

a) the use of 74LVC2G157 to generate differential drive for a differential driver such as the THS6232 which can directly drive most balanced antennas without a balun at up to about 5W. This design avoids the need for fancy gate drive or paralleling FETs in favor of SMT packaging everywhere.

b) the use of SMT inductors and SMT analog switches such as the 74lvc2g53 or 74lvc1g66. This change is not earth-shattering, but it allows a much tighter all-SMT design with as much as 80dB suppression of harmonics. The high degree of suppression is achieved by using a filter tuned to put the response zeros directly on the harmonics and careful filter layout to suppress parasitic coupling.

Transmit filtering

The design of the transmit chain uses one of a number of filters to reject harmonics of the main frequency. One of these filters is selected using analog switches to suit the desire transmission frequency. These filters are designed to have minimal attenuation in each of several amateur bands. Since these bands are relatively narrow, passband flatness is not a grave concern. The zeros of the filters are positioned near the second or third harmonics of the carrier to suppress these frequencies very strongly. Higher harmonics have nearly 75dB of suppression.

The response curves for the filters designed so far are shown below.

Filter response curves