On Mar 24, 2016, at 3:06 PM, G3YYD wrote:
> But avoid the $5 "bargain".
At one time, I started writing a review of various sound cards, but I switched
to writing demodulators right into an SDR, so I never bothered to finish the
white paper.
With an SDR, you don't need a sound card.
However, I still have all the data that I collected.
The five representative soundcards that I imeasured were the microKeyer II,
SignaLink USB, Griffin iMic, a FireStudio mobile (best soundcard I ever owned;
I could pass a 3 kHz passband to it and it never buckled) and a sub-$10 Amazon
special (Syba SD-CM-UAUD).
The Amazon special wasn't too terrible in terms of measurements. The second
harmonic (important if you want to use it as a skimmer) only breaks the -80 dBc
mark when the signal is within 2 dB of clipping. The noise floor was only 2 dB
worse than the much more expensive MicroKeyer II, but the 5th order IMD was
high enough that I made a note of it -- again, IMD is not nice if you want to
demodulate multiple RTTY signals.
However, the real deal breaker is that the copy of the cheap sound card I got
from Amazon was unstable (breaks into oscillation by itself) when the source
impedance is higher than about 200 ohms! I ended up finishing my measurements
using a source impedance of 91 ohms.
In general, if you don't do multiple signal demodulation over a 3 kHz passband
(really the only way to operate PSK31; 3 kHz is not quite wide enough for RTTY
-- you need an SDR), cheap soundcards should work with narrowband RTTY.
73
Chen, W7AY
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