Am 25.04.2012 18:24, schrieb Stefan Schäfer:
Hi Wolf,
Am 25.04.2012 18:02, schrieb wolf_dl4yhf:
Much easier and more reliable to build a dedicated hardware,
clocked by
a 10 MHz reference, using a PLL, DDS, or similar.
Really easier?
Definitely easier than building a control loop (Regelkreis) which
involves the soundcard's input, output, and a bunch of
system-dependent latencies. Ok, the loop doesn't need to be fast,
but it will never give the amount of phase coherence as ...
(1) A "good" soundcard (like yours, presumably) where ADC + DAC use
truly identical clocks,
(2) A hardware solution as suggested (and already implemented) by
Andy G4JNT.
The DDS doesn't need to be an IC in TSSOP (or even worse, BGA) case,
a resistor ladder network as a simple DAC, plus some smart machine
code does the job. Just clock the PIC (or similar micro) with the 10
MHz from the OCXO, GPS unit, or whatever you have available.
Don't get me wrong, if the reference-locked soundcard-based solution
works, there's no reason why it shouldn't be used. But if it doesn't
work, instead of spending time and money to test different
soundcards, try a simple hardware-based solution.
For higher frequencies, there are very affordable ARM-based
microcontrollers available, which are able to multiply the
externally provided "crystal" clock to frequencies in the range of a
few hundred MHz, which can then be divided down through a
programmable digital timer/counter with a very good precision (used
for PWM and motor control). After initialisation, the CPU itself is
not involved in the generation of the PWM signal. The good news is
that some of these Cortex-M based microcontrollers are available as
homebrew-friendly development boards ("LPCXpresso" starter kits).
If there was a bit more time to spend, I would definitely give this
a try since I use similar (but larger) microcontrollers at the QRL.
Cheers,
Wolf.
|
|
