Comparison of timing residuals (TOAs subtracted from a model of the pulsar’s expected TOAs) obtained using a single template for each frequency band (left), and using a frequency-dependent template (right), plotted against ν on a logarithmic scale. The residuals from the static templates show significant deviations from white noise. These systematic errors are mostly removed with the frequency-dependent templates and the residuals appear straighter, and agree on a single value of DM (apart from those of PSR B1133+16, see text for details). “Gaps” in the frequency coverage are from subbands which have been removed because they contain strong RFI. The pulsars used are (from top to bottom) B0329+54, B0809+74, B1133+16 and B1919+21.