reducing jitter of serial ntp refclock

Question

I am currently trying to connect my DIY DC77 clock to ntpd (using Ubuntu). I followed the instructions here: http://wiki.ubuntuusers.de/Systemzeit.

With ntpq I can see the DCF77 clock

~$  ntpq -c peers
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+dispatch.mxjs.d 192.53.103.104   2 u    6   64  377   13.380   12.608   4.663
+main.macht.org  192.53.103.108   2 u   12   64  377   33.167    5.008   4.769
+alvo.fungus.at  91.195.238.4     3 u   15   64  377   16.949    7.454  28.075
-ns1.blazing.de  213.172.96.14    2 u    -   64  377   10.072   14.170   2.335
*GENERIC(0)      .DCFa.           0 l   31   64  377    0.000    5.362   4.621
 LOCAL(0)        .LOCL.          12 l  927   64    0    0.000    0.000   0.000

So far this looks OK. However I have two questions.

1. What exactly is the sign of the offset? Is .DCFa. ahead of the system clock or behind the system clock?
2. .DCFa. points to refclock-0 which is a DIY DCF77 clock emulating a Meinberg clock. It is connected to my Ubuntu Linux box with an FTDI usb-serial adapter running at 9600 7e2. I verified with a DSO that it emits the time with jitter significantly below 1ms. So I assume the jitter is introduced by either the FTDI adapter or the kernel. How would I find out and how can I reduce it?

Answer 1

Part One:

• Positive offsets indicate time in the client is behind time on the server.
• Negative offsets indicate that time in the client is ahead of time on the server.

I always remember this as "what needs to happen to my clock?"

• +0.123 = Add 0.123 to me

• -0.123 = Subtract 0.123 from me

Part Two:

Yes the USB serial converters add jitter. Get a real serial port:) You can also use setserial and tell it that the serial port needs to be low_latency. Just apt-get setserial.

Bonus Points:

Lose the unreferenced local clock entry. NO LOCL!!!!