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Subsections

4 Receivers

4.1 The 1420-MHz receiver

Sensitivity at 1420 MHz is primarily determined by the low-noise HEMT amplifier, which has a noise temperature, $T_{\rm R} \approx 35$ K (for 16 amplifiers built, 28 K $< T_{\rm R} < 38$ K and $\langle T_{\rm R} \rangle=33.4$ K). The uncooled 3-stage amplifier is an enhanced version of the amplifier described by Walker et al. (1988). It uses source-inductance feedback to achieve a good input match (Weinreb et al. 1982).

After filtering to suppress the image band, the signal is converted to an intermediate frequency (IF) band of 12.5-47.5 MHz. IF signals are conveyed from each antenna to the central building by coaxial cables. No compensation is attempted for changes in IF phase due to cable-length changes with temperature; physical matching of cables provides adequate stability.

The Local Oscillator (LO) system (Landecker & Vaneldik 1982) delivers signals of controlled frequency and phase to the mixers at each antenna. To control phase, the system uses the "round-trip'' design, in which the electrical length of the cable connecting the central electronics to the antenna is measured by sending a signal out to the antenna and back. If the cable electrical length changes by $\epsilon$, the returning signal changes its phase by the equivalent of $2\epsilon$. Closing the feedback loop requires a division by 2; consequently there is an ambiguity of $\pi$in the output phase. This difficulty is overcome by operating the round-trip system at half the frequency of the required LO signal. At the antenna, the output is doubled to produce the LO signal near 1390 MHz. The tuning range of the LO system is given in Table 1; it accommodates observations of Hi in the Galaxy and in nearby extragalactic systems.

The interferometer fringe rate is reduced to zero for each baseline by rotating the phase of the LO signal for each antenna relative to the reference antenna (Antenna 5 at the physical centre of the array - see Fig. 1). Phase switching in a Walsh-function pattern (Granlund et al. 1978) is also applied to the LO signal to reduce spurious correlation from crosstalk between receiver channels. The switching pattern has 16 steps of 5.6 s in the telescope visibility-averaging time of 90 s.

Compensating delays are inserted in the IF signal paths to equalize path lengths, from the incoming wavefront, through each antenna to the correlator. Stripline paths are used for short delays and coaxial cable for longer delays, selected by PIN diode switches. Cable delay sections are equalized for attenuation and dispersion. This is an enhancement of the earlier system described by Landecker (1984). The minimum delay step of 3.91 cm ( $\approx 0.13$ ns) is determined by the specification that the delay error should produce a phase difference across the spectrometer band of less than 0.1$^\circ$. Delay system specifications are given in Table 3.


   
Table 3: 1420 MHz delay system specifications
Delay medium: stripline and coaxial cable
Frequency range: 10 to 50 MHz
Delay step: $2^{-8}\lambda_{30} = 3.91$ cm
Total delay: $64\lambda_{30} = 640$ m
Amplitude accuracy: $\pm0.02$ dB (rms)
Phase accuracy: $\pm1$$^\circ$(rms)

Residual gain fluctuations due to imperfect delay-cable equalization or unequal switch losses are removed by an automatic gain control (AGC). The AGC measures signal levels at the delay-system input and output, and keeps the net gain of the delay system constant. A second gain monitor spans the entire signal path, from antenna feed to correlator output: small modulated noise signals ( $\Delta T_{\rm A} \approx 1$ K) are injected into each antenna feed, and are demodulated from autocorrelation outputs (see Sect. 5.1) to give continuous measurements of system gain and system temperature.

Since this is a spectroscopic telescope, the reception frequency must be varied to account for diurnal and orbital motion of the Earth, as well as for the radial velocity of the source under observation. The total reception band, of width 35 MHz, is kept centred on the Hi band selected for observation by continuously tuning the first LO. The continuum reception band is divided into four sub-bands distributed on either side of the spectroscopy band, as illustrated in Fig. 2. In the course of an observation, continuum band centres vary as the LO is tuned, but the imaging software accounts for this slight change. The five reception bands are translated to fixed locations in the IF band.

  \begin{figure}
\includegraphics[width=8.8cm,clip]{H2136F2.PS}\end{figure} Figure 2: The arrangement of continuum and spectroscopy bands near 1420 MHz. S is the band used for Hi spectroscopy, and A, B, C, and D are continuum bands. The system LO is varied to keep the Hi information centred in the IF at 30.0 MHz. The band allocated to radio astronomy extends from 1400 to 1427 MHz; part of band C and all of band Dare outside the protected band. The overall width of the spectroscopy band can be varied from 0.125 to 4 MHz (see text)

The four continuum bands are defined by IF bandpass filters and are converted to baseband using fixed second LOs. A quadrature replica of each band is created using a phase-shifted second LO. The spectrometer band is converted to lower frequency in a single-sideband mixer. The overall bandwidth of the spectrometer is selectable with six options from B=0.125 MHz to B=4.0 MHz in multiples of 2. Before digitizing, the band is sharply defined by a six-pole elliptic-function bandpass filter spanning B to 2B. Use of this "quasi-baseband'' allows better image rejection in the single-sideband mixer.

4.2 The 408-MHz receiver

The 408 MHz receiver is designed to use as much as possible of the band assigned to radio astronomy (406.1-410 MHz), while suppressing out-of-band communications signals. Several stages of filtering are distributed between gain stages in the receiver, and a total bandwidth of 3.5 MHz is achieved.

A single coaxial cable carries both LO and IF signals between the focus of each antenna and the central receiver building. The LO phase is controlled by a feedback system which keeps the phase of the 30 MHz IF signal arriving at the centre of the telescope completely independent of the length of the interconnecting cable (Veidt et al. 1985). All subsequent signal processing is carried out in digital electronics or in software (see Sect. 5.3). A minor drawback of the LO-IF system is the generation of an unwanted fixed-frequency signal at the centre of the IF band; this is removed prior to correlation by a narrow bandstop filter.


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