LF: Re: [rsgb-lf-group] FW: JT9-5 Drift

[Andy Talbot <andy.g4jnt@gmail.com>]

OOPS

I forgot that!   Careless!

Was using the period for JT9-1

In which case my speculation about soft the decision decoder must be applicable, at least to some extent.

To work with a drift of several tone-spacings; quite impressive.

Andy

www.g4jnt.com

On Mon, 7 Jan 2019 at 13:34, Rik Strobbe <rik.strobbe@kuleuven.be> wrote:

Hello Andy,

the drift is given as Hz/min, so for JT9-5 the drift can be as high as 1Hz over the (almost) 5 minute TX cycle.

The decoder must somehow be able to correct the drift within certain limits.

73, Rik  ON7YD - OR7T

---

Van: rsgb-lf-group@groups.io <rsgb-lf-group@groups.io> namens Andy TALBOT <andy.g4jnt@gmail.com>
Verzonden: maandag 7 januari 2019 14:13
Aan: rsgb-lf-group@groups.io
CC: rsgb_lf_group@blacksheep.org
Onderwerp: Re: [rsgb-lf-group] FW: JT9-5 Drift

 

The JT9-5 tone spacing is 0.29Hz, so those drift figures correspond roughly to a bit less than one complete tone spacing in the transmission period.  Which makes intuitive sense.  The error correction allows it to "lose" the few symbols at the end where it has drifted too far, while those at the start are near enough to work normally.

(Pure speculation) The soft decision process in the decoder algorithm will base its metric on how far the received signal is away from where it ought to be, so a tone  that has drifted into an adjacent bin may still have a quite strong weighting applied. 

BTW Rik, you still appear to be copying these posts to the now-dead Yahoo Group

Andy

www.g4jnt.com

On Sun, 6 Jan 2019 at 10:48, Rik Strobbe <rik.strobbe@kuleuven.be> wrote:

Dear all,

Roger, VK4YB, asked me about JT9-5 frequency stability requirement.

As this might be interesting for others too, my answer below.

73 & HNY

Rik  ON7YD - OR7T

---

Van: Rik Strobbe
Verzonden: zondag 6 januari 2019 11:43
Aan: Roger Crofts
Onderwerp: Re: JT9-5 Drift

 

Hello Roger,

as JT9-2 and JT9-5 signals are resampled to JT9-1 to be decoded by WSJT decoder engine it all depends on the drift tolerance of that decoder.

For JT9-2 the frequency stability must be 2.222 times better than for JT9-1.

For JT9-5 it must even be 5.926 be better.

As I couldn't find any information about the required JT9-1 stability I did some brief tests with JT9-5:

- At -5 dB level a drift up to 0.35 Hz/min seems acceptable.

- At -15 dB level it is 0.26 Hz/min.

- At -25 dB level it is 0.24 Hz/min

- At -30 dB level it is 0.22 Hz/min

- At -34 dB level (more or less the JT95 limit) it is 0.20 Hz/min

But as said this was just a brief test, these numbers should be handled with some caution.

Also keep in mind that this numbers represent the combined TX and RX drift (according to Murphy TX and RX drift will always be opposite).

If using a transverter the TX drift is the combined drift of the driver TX and the transverter.

Furthermore I assumed that the drift was linear over time, for frequency jumps it might be different.

Another frequency instability source will be the ionosphere, but there is little we can do about that (but the more stable TX and RX the more room for ionospheric instabilities).

Based on the above I would suggest to aim for a LO stability of no worse than 0.1 Hz/min, 0.05 Hz/min would be even better for weak signal DX (and that is what JT9-5 is all about).

Tests over the past weeks have shown that JT9-5 works very well for 136 kHz DX (N1BUG was copied by numberous EU stations).

On 472 kHz K3MF has been reported in Europe (as far as I know by G0LUJ and myself).

73, Rik  ON7YD - OR7T

Virus-free. www.avg.com

_._,_._,_

---

Groups.io Links:

You receive all messages sent to this group.

View/Reply Online (#8446) | Reply To Group | Reply To Sender | Mute This Topic | New Topic

Your Subscription | Contact Group Owner | Unsubscribe [andy.g4jnt@gmail.com]

_._,_._,_