Paul wrote:

So what you have is a 100 Hz signal that comes and goes according to
the mark/space timing.  (Or if you didn't filter the other frequency
out you'd have 100 Hz and 300 Hz signals representing mark and space.)

Chapters and volumes have been written about carrier recovery with
FSK signals.  Read up and you'll find more than enough possible
solutions to make your head spin.

Given what you have so far -- a 100 Hz signal that comes and goes
according to the mark/space timing -- you could try using the 100 Hz
as the reference to a PLL with a very long time constant (compared to
the symbol rate), much the way that color televisions used to keep
their color subcarriers locked to the video signal's color burst, or
a synchronous AM detector rides out carrier fading.  I'm not saying
that will be the best you can do with the incoming NAA signal -- for
that, read up on carrier recovery -- but it is the next logical step
on your current path.

Also of note, since the MSK signal is FSK, not binary (180 degree)
PSK, the frequency multiplication didn't do anything magical for you
(like removing the modulation, which it would have done with a BPSK
signal such as WWVB).  What it did for Tracor was produce the 100 Hz
signal needed by the rest of the box, when the existing LO was
adjustable only in 100 Hz increments and the mark and/or space
frequencies would, therefore, have been 50 Hz using the existing
LO.  (Doubling also widened the separation of the mark and space
frequencies from a delta of 100 Hz to a delta of 200 Hz, thus making
the filtering a bit easier -- but that is not the primary reason for
it.)  Finally, once you are no longer tied to the Tracor 900 and its
existing 100 Hz IF and detector chain, you are free to use whatever
LO frequency you want.

Best regards,

Charles

Charles I literally just sat down to do some math. What you say is the same
thoughts I have. The information I have on NAA says that for 200bit msk its
a total of a 100 hz shift +/-50 Hz. That makes no sense I would think it
would be at least +/- 100 Hz. They had in the past run a 100 bit msk. I
somewhat wonder about the documentation accuracy in the tracor manual and
the net. Hey it has to be true on the internet.
Agree the whole tracor d-msk-rs goal is to recreate a 100 Hz signal for the
PLL. The signal is only there when its on one of the signals so again you
are right and that was expected.
I had very high hopes that without going to the next step I would get a
clue as to the quality of the underlying NAA reference to better understand
if the next step had value. The jitter is so high the answers not apparent.
Regards
Paul
WB8TSL

On Sat, Sep 13, 2014 at 12:56 AM, Charles Steinmetz csteinmetz@yandex.com
wrote:

On 9/13/14, 7:33 AM, paul swed wrote:

As I recall, MSK is FSK with the shift being half the symbol rate. I
assume they're doing gaussian minimum shift keying GMSK.  The mod index
is typically 0.5 with gmsk.

Thats what I am trying to understand. How good is good. Is it a useful
replacement for wwvb. Certainly kicks butt in signal strength.
Regards
Paul
WB8TSL

On Sat, Sep 13, 2014 at 2:03 PM, Poul-Henning Kamp phk@phk.freebsd.dk
wrote:

Well it doesn't make any sense but by using a LO of + or-150hz I do get a
stable signal that at least allows me to get a sense of the stability of
the carrier. I am not using the tracor d-msk-r to see this. In fact I need
to relook at it may have an issue it does not seem to be doubling.
A big change I made is to use a Krohn Hite 5910c arb gen that actually has
an amazing stability all by itself. Have never had a manual for it. Though
I see  the b exists. Its set to 100 hz and putting out a nice clean sine
wave.
So the math is still nuts.
Re-read the tracor documentation and they injected the LO 100 Hz low. So 24
Khz needs a LO of 23.9 Khz. The d-mask-r did not require any change to this
setting.
If NAA is transmitting 200 baud then I would expect the MSK carrier to be
+/- 100 Hz. Not +/-50 Hz.
Regards
Paul.

On Sat, Sep 13, 2014 at 7:18 PM, paul swed paulswedb@gmail.com wrote:

On 9/13/14, 7:13 PM, paul swed wrote:

I'd expect the total shift to be half the baud rate: 100 Hz..

OK I am use to traditional fsk that typically had a far wide shift then the
baud.
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.
Regards
Paul
WB8TSL

On Sat, Sep 13, 2014 at 11:25 PM, Jim Lux jimlux@earthlink.net wrote:

Hi

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.

Bob

On Sep 14, 2014, at 10:20 AM, paul swed paulswedb@gmail.com wrote:

I am able to phase track NAA using the following method:

Reception using active whip antenna input into HP 3581C selective vm, tuned
to 24 KHz using its 300 Hz bandpass filter.
Output is taken from "Restored" connection on back panel and input into the
squaring chip AD835, set up on a breadboard.  If the output of the AD835 is
looked at with a dynamic signal analyzer, I see two monochromatic signals at
48.1 KHz and 47.9 KHz.  I am able to phase lock on either of these using the
Tracor 599J if they are input into it.  The lock seems to agree with the
tracking I got with WWVB over the 15 minutes or so that I let it lock.  I
have also looked at some of the other Navy MSK signals, but this morning
they were too weak.  I could see hints of the monochromatic signals with NLK
but there were too weak to lock on.

John, KA5QEP

-----Original Message-----
From: paul swed
Sent: Saturday, September 13, 2014 6:18 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] NAA experiments as a reference

Thats what I am trying to understand. How good is good. Is it a useful
replacement for wwvb. Certainly kicks butt in signal strength.
Regards
Paul
WB8TSL

On Sat, Sep 13, 2014 at 2:03 PM, Poul-Henning Kamp phk@phk.freebsd.dk
wrote:

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and follow the instructions there.

On 9/14/14, 8:23 AM, Bob Camp wrote:

For GMSK, it looks more like a flattop with no ears.  Straight MFSK has
the ears.

There's several ways to demodulate MSK/GMSK.  One way is to use a
discriminator followed by an appropriate matched filter.  Typically,
there's some sort of symbol timing tracking loop that depends on there
being transitions periodically to keep the loop going.  A costas loop
might work for carrier tracking, if that's what you want to do. It would
have better SNR performance because it's coherent, while most
discriminator approaches are incoherent detection.

there's quite a few gnuradio demodulators out there, and that might be a
good way to get started.  At least the source code will give you the
algorithm, if you don't want to bring in all the rest of gnuradio (which
can be somewhat of an ordeal).

The big paper everyone cites on timing recovery is from Mueller and
Mueller  (actually, an umlaut u in the last name).
http://www.mathworks.com/help/comm/ref/muellermullertimingrecovery.html
describes the simulink block.

this might also be interesting:
The MSK-Type Signal Timing Recovery block recovers the symbol timing
phase of the input signal using a fourth-order nonlinearity method. This
block implements a general non-data-aided feedback method that is
independent of carrier phase recovery but requires prior compensation
for the carrier frequency offset. This block is suitable for systems
that use baseband minimum shift keying (MSK) modulation or Gaussian
minimum shift keying (GMSK) modulation.
http://www.mathworks.com/help/comm/ref/msktypesignaltimingrecovery.html

You might look at the venerable G3RUH 9600 baud modem implementation,
which is a form of MSK.  DPLL for symbol timing.

On Sun, Sep 14, 2014 at 10:20:48AM -0400, paul swed wrote:

Consider that with the LO offset 150 Hz you will have a 100 Hz

beat note with the mark tone and a 200 hz beat note with the space
tone - more or less exactly right on if the MSK signal carrier is right
on (which it is) and the signal is true MSK (which is is) and the 150
Hz LO is nearly right on (which it presumably is).

And the 200 Hz and 100 Hz are of course related and coherent

with each other (the phase of the mark and space tones is precisely
related to each other because of the nature of true MSK).

This means the resultant waveform should be stable seen in

a sweep triggered by your local clock at 100 Hz and also in a lissajous
pattern with the LO.

--
Dave Emery N1PRE/AE, die@dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either."

David thanks what you say seems to be true. I could see the stable 100 Hz
and the 200 Hz was a typical 2 X lissajuo pattern within the 100 Hz pattern.

They used a LO 100 Hz below because the synthesizer was reasonable to
build. Adding the 50 Hz would have been messy at that time. They have a PLL
runing at 10 X the final LO frequency and it steps in 100 hz increments
after its been divided.
The LO is at 23.9 KHz and the demod out was 50 Hz and 150 Hz that was
doubled to 100 and 300 Hz. But the signal passed through a 100 Hz BPF
leaving just the 100 Hz. I should be able to see that 100 Hz against the
100 Hz reference but its not nearly as nice looking as the -150 Hz LO.
Regards
Paul
WB8TSL

On Sun, Sep 14, 2014 at 2:52 PM, David I. Emery die@dieconsulting.com
wrote: