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2. System description

Pulsar polarimetry has been performed for the strongest pulsars in the declination range of the Effelsberg 100-m radio telescope using receivers centered at 1.4, 1.7, 4.85 and 10.45 GHz. The dispersive properties of the interstellar medium (ISM) tend to degrade the milliperiod resolution attainable with the pulsar data acquisition system at Effelsberg, PUB86 (Kramer 1995). Typically, between tex2html_wrap_inline1825, dispersion smearing is significant for most of the pulsars in our sample, and therefore, a device was employed to compensate for this effect. As dispersion smearing scales inversely with the square of the frequency, observations at frequencies higher than 1.7 GHz were usually performed without the use of a dedispersing device. Dedispersion is handled by an on-line signal processor called the Pulsar Signal Entzerrer, (PSE). This device is basically a four unit, 60 channel tex2html_wrap_inline1827 667-kHz filterbank providing 40-MHz total bandwidth in each unit. The output of each channel is detected and converted to a digital signal by a fast A/D converter. After a time delay selectable by the user to counteract the effect of dispersion smearing, the outputs of all channels are summed, and the total 40-MHz bandwidth is then recorded by the PUB86. To provide both high time resolution polarimetry and profiles with high signal-to-noise ratio (snr), wide bandwidths were needed. Therefore, two different types of polarimeters were employed. When dispersion smearing significantly degraded the resolution of our profiles, an adding polarimeter was used. This device consists of a network of passive hybrids that produce a maximum bandwidth of 40 MHz. Where dispersion was not significant, a multiplying polarimeter was used to handle bandwidths of up to 500 MHz. This is a correlation device which derives the linear component of the incident radiation by correlating two nominally circular inputs.

Differential complex gains introduced by various components in the signal path, as well as system imperfections, alter the polarization state of the incoming radiation. We present here a full gain analysis of both polarimeters and describe the procedure followed to monitor gain variations and then account for them during the off-line reduction. We will also refer to the cross-coupling correction; an effect which tends to be stable with time. However, gain variations demand dynamic monitoring if high-precision polarimetry is required.


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