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

[Bob Bruhns]

The amplifier amplitude linearity problem is universal and well known, and there is plenty of coverage of constant amplitude systems that are FSK-PSK hybrids, with and without transition softening.  This is the idea behind Minimum Shift Keying (MSK) and Gaussian MSK (GMSK).

If you send square-wave data to an FM modulator, you get FSK.  For controlled bandwidth, the first order of business in FSK is to maintain continuous carrier phase.  Let the transitions increment phase, but not cause sudden jolts as they would if you simply switched between two separate oscillators.  This carrier phase continuity is natural for the usual variable-reactance FSK.

FSK modulated by square waves is the same as PSK modulated by triangle-waves.  The frequency shifts cause gradual carrier phase rotation after each transition.  By carefully controlling the frequency shift vs. symbol rate, carrier phase can be made to rotate between specified phase angles during each bit time.  Then the receiver can use carrier phase information to make bit decisions, and the performance closely approaches that of PSK.  This is called MSK because the frequency shift represents the minimum that could produce the desired phase shift during the bit time.

FSK can be demodulated with a simple frequency discriminator, but sophisticated phase-synchronous demodulation is also possible.  There is an advantage when the receiver can track and lock to the carrier phase.  When carrier phase transmits useful signal information, the receiver can be more synchronous, and the performance of FSK closely approaches that of PSK.

This phase shift does not have to be 180 degrees per bit.  It could for example be 90 degrees, and you can have Quadrature MSK.  QMSK is very close to QPSK in performance and behavior, but amplitude is constant, so power amplifiers do not cause increases in bandwidth.

In GMSK, the data is passed through a low pass filter before being applied to the frequency modulator.  By softening the transitions this way, GMSK reduces transmitted bandwidth because it produces greatly diminished side-currents farther from the carrier.  At the transitions, phase  begins to rotate slowly, with the rotation accelerating until the phase gets halfway to the final position, then the phase rotation begins to slow down, and it gradually approaches and reaches the next-bit phase position.  Again you can apply Quadrature GMSK, etc.

Very sophisticated systems make deliberate use of pre-transition and post-transition modulation waveform overshoots to achieve faster transitions in minimum bandwidth.

  Bob Bruhns, WA3WDR


Andre' Kesteloot wrote:
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> Talbot Andrew wrote:
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> >  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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