[time-nuts] NAA experiments as a reference
kb8tq at n1k.org
Sun Sep 14 11:23:09 EDT 2014
Remember that one model for FSK is a pair of AM signals that “just happen” to represent an FSK waveform. With normal FSK if you tune to one side, you will get a nice AM carrier and sidebands. With MSK it’s a bit more complex since they to do some cute stuff to cut down the bandwidth. It’s still got a pair of spikes in the spectrum at the upper and lower shift frequencies.
Probably the best way to get a timing signal is to recover the modulation and then “subtract” it from the original in software. That would be crazy with a high speed signal. With something like this - a pc plus sound card probably can do the job. More or less - “that must have been a 1 = do this to the history buffer, that must have been a zero = do that to the history buffer”. You would need to know a bit about how they filter their data stream for it to work well. The “proof” that you had it right would be a nice clean sine wave communing out of the history buffer. Spit it out the audio output port on the sound card and take a look …..
No I haven’t thought real hard about all the implications of doing that, there might be a gotcha in there somewhere. I’m assuming that they have a pretty predictable clock on their data. Beyond that I don’t think there are any other weird assumptions. I’d probably do the “de-fsk” by a phase modulation on the buffer. There might be a practical bump in the road there.
On Sep 14, 2014, at 10:20 AM, paul swed <paulswedb at gmail.com> wrote:
> OK I am use to traditional fsk that typically had a far wide shift then the
> What you say would match what the tracor book says and the system is
> designed for.
> I still do not see why if I offset the LO to -150 hz I get a useful display
> to judge timing. I am using the lissajous method.
> On Sat, Sep 13, 2014 at 11:25 PM, Jim Lux <jimlux at earthlink.net> wrote:
>> On 9/13/14, 7:13 PM, paul swed wrote:
>> If NAA is transmitting 200 baud then I would expect the MSK carrier to be
>>> +/- 100 Hz. Not +/-50 Hz.
>> I'd expect the total shift to be half the baud rate: 100 Hz..
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