146.5 MHz reduced bandwidth DATV   Tx exciter

Jean Pierre's DATV MiniTioune software, with its real-time constellation and MER displays has been quite an incentive to investigate the World of digital TV, both amateur and satellite broadcast. Add to this Evariste's Raspberry Pi software to generate the I/Q drives necessary to drive a suitable transmitter exciter and it's hard not to have a go, particularly on the new NOV allocation just above 2m, centred on 146.5 MHz.

As with most digital transmitters, the architecture used is the Weaver one, ie, phasing. Folding of the modulation about the channel centre (the digital equivalent of an analogue pilot tone) ensures that the unwanted sideband components appear under the main signal itself and not in the adjascent channel. In the digital World this amounts to no more than a code conversion of the digital transport stream.

Since only a single frequency in one band only was required, the LO drive at 146.5 MHz was derived from a 12 MHz xtal multiplied by 12. Also, a fixed set of filtering, suitable for a symbol rate of 333 kS was used to simplify the design.

Circuit:

reduced bandwidth tx cct

exciter - top   exciter - bottom


Tx spectrum - 333 kS and two-tone modulation
 
333kS symble rate signal spectrum
two tone RBW tx spectrum

This set-up is currently been fed into another BDG814 feeding an 1800 MHz cellular device to give me 10W (20W PEP) output:

10W amplifier