We developed a single sideband SIS receiver for 0.85 mm wavelength and tested it on the 30m Telescope. The single sideband receiver noise of 48 K is only three times above the quantum limit of heterodyne receiver sensitivity. Spectral line observations are speeded up by a factor of two to three with the new receiver.
The receiver noise may be expressed as a number N of incident photons per second and unit bandwidth, doubling the output signal of the receiver . N may be used to compare different low noise receivers operating in different bands. The same value is obtained by comparing the receiver noise with the quantum limit: . In our previous work we reported on a four photon noise double sideband SIS receiver with a broad band fixed-tuned SIS mixer  used at the 30m telescope in 1993-1995. The current is about a factor of three above the quantum limit.
Figure 1: SIS receiver module with the mixer block, the waveguide coupler for local oscillator power injection and the mixer backshort drive.
In the new receiver the upper sideband is rejected by the tuning of a single backshort in the SIS mixer. Compared to a quasi optical image band filter this technique allows one to reject the image band noise with a minimum increase in receiver noise. In order to simplify the sideband rejection we increased the IF band central frequency from 1.5 GHz to 4 GHz. The new 4 GHz IF amplifier has been developed by D. Gallego at the Centro Astronomico de Yebes (Spain). It has a 1 GHz bandwidth and 7 K noise temperature. In the SIS mixer we are using a two junction array with individual inductive tuning. The individual junction area is about m and the critical current density is about 7 kA/cm . A general view of the SIS receiver is presented in Fig. 4.
Figure 2: SIS junction Current - Voltage characteristics with and without LO power at 340 GHz.
Figure 3: DSB receiver operation in the laboratory.
The new receiver single sideband noise temperature is 48 K at 330 GHz while the double sideband noise is about 30 K around 320 - 340 GHz (Figs. 2, 3).
The receiver was used at the 30m telescope on March 4-11, 1997. With the new receiver the minimum radio telescope system noise in the reference plane out of atmosphere is about 400 K (Fig. 6). The receiver sideband rejection at 330 GHz was measured using the CO(3-2) spectral line of Orion-IRC2 as a reference source (Fig. 6). The gain in observing time with the new receiver is a factor two or three, depending on the atmosphere.
Figure 4: SIS receiver: general view
Figure 5: SSB receiver operation at the telescope. The system noise is 415 K ( scale). The peak-to-peak noise is 35 mK after 5 min of integration (spectrum kindly communicated by F. Combes)
Figure: The receiver sideband rejection at 330 GHz, measured using the CO(3-2) line in Orion-IRC2 as a reference. Two spectra were measured with and without upper sideband rejection. The spectral lines coming from the upper sideband, as CH OH at 338.4 GHz, are attenuated by about 7 dB (spectrum spectrum kindly communicated by F. Combes).
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