LF: Clear spots around 74.55 kHz

["Markus Vester" <markusvester@aol.com>]

Here's a slightly wider view of DCF77 "phasenoise" around 74.55 kHz http://dl.dropboxusercontent.com/u/26404526/74k_131001_2001.png

 

You can see some "sweet spots" around 74557, 74552 and 74540 Hz, wheras the vicinity of 74549 seems more affected. Not sure how much this pattern might evolve over longer periods of time, with changing BCD code bits for hour, day, month etc. 

 

The strong line on 74540 is a locally generated carrier to check frequency drift of my FiFi-SDR receiver.

 

Best 73,

Markus (DF6NM)

From: Markus Vester

Sent: Tuesday, October 01, 2013 8:35 AM

To: rsgb_lf_group@blacksheep.org

Subject: Re: LF: 74.550kHz Sep 29/30

Here are clearer shots of the DCF77 sidebands from the morning:

http://dl.dropboxusercontent.com/u/26404526/dcf77_prn_sidebands_131001_0605.png

http://dl.dropboxusercontent.com/u/26404526/74k_131001_0600.png

 

Notes:

- the RX antenna is resonant around 75 kHz, which emphasizes the PRN sidebands below the DCF77 carrier. The fifth and sixth lobe are still visible. In reality, the upper sidebands are slightly stronger. This is probably due to an offset (or a 75 kHz notch) in the transmitter antenna matching, which happens to help us now.

- the Swiss time signal HBG on 75 kHz is no longer on air.

- there is an RTTY signal at 73.6 kHz which could be CFH. 

- the 1 Hz lines are surrounded by a somewhat regular fine structure, consisting of 16.6 mHz spaced sub-lines. This is probably due to parts of the BCD timecode and weather information data which are repeating or similar in consecutive minutes.

 

Best 73,

Markus (DF6NM)

From: Markus Vester

Sent: Monday, September 30, 2013 11:26 PM

To: rsgb_lf_group@blacksheep.org

Subject: Re: LF: 74.550kHz Sep 29/30

Hi Bob, LF,

 

last night my improvised grabber http://dl.dropboxusercontent.com/u/26404526/df6nm_74kHz.jpg indeed showed weak and slightly fuzzy traces on both your QRG's, and on all other integer Hz frequencies as well.

 

These are presumably artifacts from DCF77 which is only about 160 km from here. In addition to the well known AM timecode, it also carries  pseudorandom phase modulation, which has been proposed in the 80ies to provide higher resolution timing (albeit orders of magnitude worse than Loran or GPS). The resulting sidebands extend a couple of kHz on either side of the carrier, with pronounced minima around multiples of the chip rate 77500/120 = 645.833 Hz, see

http://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/pdf/5_1988_Hetzel_-_Proc_EFTF_88.pdf (page 358). The same code sequence is repeated every second, so in theory the spectrum would consist of sharp 1 Hz spaced lines. However, additionally the sign of the sequence is alternated with the disseminated timecode bits, producing some widening or "fuzzyness" of the lines.

 

Attached is a spectrogram which was taken tonight on the resonant antenna. Between statics, you can still see the fourth sideband lobe which is centered near 74.6 kHz. The spectral gaps are on 

 74916.666 Hz,

 74270.833 Hz,

 73625.000 Hz,

with small and sharp central lines, presumably caused by slight inbalances or nonlinearities in the transmitter.

 

By these criteria, if you have the choice I would recommend to operate somewhere near these gaps, but not exactly in their middle, and also preferably not exactly on integer Hz frequencies ;-)

 

Best 73,

Markus (DF6NM)

 

 

From: Bob Raide

Sent: Monday, September 30, 2013 8:02 PM

To: rsgb_lf_group@blacksheep.org

Subject: RE: LF: 74.550kHz Sep 29/30

Hartmut;
You make it look easy!!! 
That station on 74.548 is WG2XRS/5, Dex in South Carolina.  I am sure you will see him when condx favor his area.
Really great capture-this time I have my transmitter running correctly.  I thought I better drop down .1 hz as I was seeing sigs on .550 and .549 on Vester's grabber but then remembered you have directional ant and the sigs come from east of you and are not a problem.  I can go to 74.549 from now on.
Thanks for all your good work! Bob  WG2XRS/4 NY
 

> Date: Mon, 30 Sep 2013 17:33.:59 +0200
> From: h-wolff@gmx.de
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: 74.550kHz Sep 29/30
>
> Here are the captures taken last night:
>
> https://dl.dropboxusercontent.com/u/50178231/74550Hz-2013-09-30-QRSS60Screen.JPG
>
> https://dl.dropboxusercontent.com/u/50178231/74550Hz-2013-09-30-QRSS120Screen.JPG
>
> XRS4 was just below 74.549 kHz and very faint traces of another station
> on 74.548. Only visible on the QRSS120Screen.
>
> --
> 73
> Hartmut
>
>