[Lf] [Fwd: LF: DDS Board supplies and PSK sidebands]

[Andre' Kesteloot]

Talbot Andrew wrote:

>  Thanks to the recent RadCom article, the first batch of
> DDS boards has now sold out and a second set have been
> ordered from the PCB manufacturers.    We bought the
> entire stock of AD9850 chips from one UK supplier (13) and
> these are being mounted on the PCBs at this moment.We hope
> to produce a version of this frequency source using the
> AD9851 chip soon.  Although deliveries of this chip are
> slow, they should arrive by March 2001.  The AD9851
> differs from the AD9850 chip by operating at a higher
> clock frequency, up to 180MHZ, BUT has an internal x6
> multiplier allowing it to be driven with a 20MHz clock
> signal.  This removes one problem users have found, that
> of trying to find packaged oscillators in the 100MHz
> region.   One of these boards as the source would solve
> the 100Hz step problem on the "New PC based 1750 Meter
> Transmitter" mentioned in earlier EMails.   Complete kits
> and made up boards can now be supplied on request. Contact
> HF-INST.CO.UK     tel    (0)1420 590000I have been
> considering the key click problem when using BPSK with
> switching transmitters.  Traditionally, amplitude shaping
> is used to limit the transmitted bandwidth of PSK signals,
> but this is not a valid solution for class E Txs . Dave's
> idea of going part way there by ramping the PSU to the
> driver chips to generate Pulse Width Mod being only a
> temporary solution.  PWM of the drive waveform does not
> linearly control the amplitude of the RF component which
> varies with pulse width in a SIN(X) / X manner.Has anyone
> any experience of systems that ramp the phase slowly in
> lieu of the amplitude ?    My feeling is that this will
> give a similar result to amplitude shaping, but the text
> books don't cover PSK bandwidth control using a constant
> amplitude waveform. Andy G4JNT
>
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