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#22
October 10th 14, 11:01 AM
posted to rec.radio.amateur.homebrew
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Frequency accuracy in older RXs
Rob wrote in
: Lostgallifreyan wrote: Peter Able stuck@home wrote in news:ks2dncC5fJrIAqrJnZ2dnUVZ7s- : Fine, but first find a microcontroller with such a fast (minimum 60MHz) DAC. Even if you do you'll spend many times the cost of a simple microcontroller plus DDS chip system - and you won't rival the cheaper system's performance until your DAC can work at several times 60MHz. Good point. Almost certainly easier to just send step size from CPU to the accumulator in fast hardware. The step size needs changing far less often than it needs stepping. That is what the external DDS chip is doing. When you require only FM-modulated signals and no sinewave, existing microcontrollers can do it using their onboard timers and programmable clock dividers. For example, the Raspberry Pi has been turned into FM-broadcast and amateur radio shortwave FSK/ASK transmitter, the RF signal appears (as a square wave) directly on a GPIO pin. Just filter and amplify, or when you don't ca just connect a random wire as an antenna to transmit a couple of mW (and harmonics). An off-topic question, but very interesting at least to me... Do DDS chips exist with 128 or even 256 phase accumulators onboard with the step size adjustment being capable of matching the speed of the stepping itself (though taking external control of local phase modulations between accumulators), and allowing mixing of all outputs, perhaps in user-selected groups based on binary fractions of the total accumulator count? I ask because if they do it might be possible for me to convert my phase mod synth code to dedicated hardware without resorting to very fast CPU's... While the rates are audio only, the huge parallel array gets demanding of CPU time as it is. I suspect the answer to all that may be 'no' without custom VLSI chips because of the relatively complex paths between accumulators needed for a phase mod synth of N operators per algorithm, but maybe DDS chips come in enough varieties to surprise me.  |
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