[time-nuts] VLF Phase-tracking receivers.

[Jim Lux]

On 8/10/14, 9:26 AM, Kenneth G. Gordon wrote:
> On 10 Aug 2014 at 6:24, paul swed wrote:
>
> Hello again, Paul. Thanks for replying. Please see below.
>
>> On iPhone
>> Yes but those stations are fsk so the offsets an issue.
>
> As I understand it from back in the 1970s when I was first working on this
> sort of thing, the modulation is not FSK, but "MSK" (whatever that is) at
> something like 25 to 50 Hz, and the Navy installed the necessary equipment
> in the 1970 time-frame to assure that the transmitted signals are
> phase-stable. The same thing, apparently holds true for GBR and some of
> the other stations like those. Russian, for instance.
>



MSK = Minimum Shift Keying... Run the bits through an appropriate filter 
(usually a Gaussian) before feeding them into the modulator.  The 
"minimum" comes from the fact that the frequency deviation (or phase 
shift) is the smallest you can have and still decode the bits, so it has 
very high spectral efficiency (e.g. maximum bps/Hz).

The well known G3RUH 9600 bps packet modem is close to being a GMSK 
modem: deviation is 3kHz for 9600 symbols/sec and uses an 8 tap FIR 
filter in the modulator.

Bluetooth uses GMFSK


> I have no idea what, exactly, has occurred in this area since then though.
>
>> W