title: reducing jitter of serial ntp refclock
body: <p>I am currently trying to connect my DIY DC77 clock to ntpd (using Ubuntu). I followed the instructions here: <a href="http://wiki.ubuntuusers.de/Systemzeit" rel="nofollow">http://wiki.ubuntuusers.de/Systemzeit</a>.</p>
  
  <p>With ntpq I can see the DCF77 clock</p>
  
  <pre><code>~$  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
  </code></pre>
  
  <p>So far this looks OK. However I have two questions.</p>
  
  <ol>
  <li>What exactly is the sign of the offset? Is .DCFa. ahead of the system clock or behind the system clock?  </li>
  <li>.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?</li>
  </ol>