LF: DDS Board supplies and PSK sidebands

["Talbot Andrew" <ACTALBOT@dera.gov.uk>]

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 590000

 

I 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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