2 mm SIS Receiver

Project Team: Peltonen, Oinaskallio, Rönnberg, Sjöman, Urpo

Metsähovi Radio Observatory has received a 2 mm SIS VLBI receiver as a permanent loan from Instituto de RadioAstronomia Milimetrica (IRAM), Spain. This receiver has been already used successfully for VLBI measurements at IRAM for around 10 years. The receiver consists of a 3 K cryostat with a waveguide SIS mixer block (non-contacting backshort as a tuning element) and a corrugated feed horn, Figure 3.

Figure 3: 2 mm receiver installed to the antenna of Metsähovi Radio Observatory.

The IF signal centered at 3.95 GHz with 500 MHz bandwidth is amplified with a HEMT preamplifier at 15 K ambient temperature. A motorized LO coupler of Mach Zender design is used to couple the required LO power into the mixer block, Figure 4.

Figure 4: LO system mounted to the 2 mm receiver.

The local oscillator power is derived from an InP Gunn oscillator (J.E.Carlstrom design) followed by a Schottky diode frequency doubler. For the signal (sky frequency) of 147.05 GHz the required Gunn oscillator frequency is 75.5 GHz when LSB operation is preferred. For the phase locking we use 16th harmonic of a Rohde Schwarz synthesizer giving reference frequency of 4.7125 GHz. With a second down converter we transfer the 3.7$-$4.2 GHz IF band to the standard VLBI band of 0.5$-$1.0 GHz using a Miteq phase locked 3.2 GHz cavity stabilized oscillator as a LO source.

First operational tests for the receiver were conducted with supervision help of Mr. Santiago Navarro and Dave L. John. The quasi-optical feed system (to convert the F/D ratio of 9.73 to 4.074) was designed by Mr. Kaj Wiik. This system consists of 3 plane mirrors and 2 elliptical mirrors together with rotating chopper beam-switch. Also a quarter wave polarizer from IRAM can be installed between elliptical mirrors, Figure 5.

Figure 5: Quasi-optical mirrors in front of the 2 mm receiver.

For optimum performance the four main parameters of the receiver which should be optimized are mixer bias current, mixer backshort position, LO coupler and the pressure of liquid helium. With the optics in front of the receiver it is very easy and quick to measure the Y-factor of the system simply removing/replacing the hot load and liquid nitrogen load is kept in a fixed position. It was noticed that the receiver is very sensitive to the LO feed power level. For that reason the Gunn oscillator, harmonic mixer and tunable waveguide attenuator were installed in a temperature controlled enclosure. The measured noise performance of the system with optics (without polarizer) is shown in Figure 6.

Figure 6: Noise temperature of the 2 mm SIS receiver as a function of LO feed.

It was found that the IF level peak with LO power is very tightly bound to the sensitivity of the receiver. Let us define the I-factor i.e. the relative IF level increase when first the LO power is turned off and then increased to reach the first IF maximum. If the I-factor is around 6 dB the noise temperature of the receiver is close to minimum. Due to this analogy between I- and Y-factors the sensitivity of the system can be roughly estimated (without cold load) measuring only the I-factor with 5.5$-$6 dB being acceptable values. For optimum noise temperature 130 K was measured with optics (with polarizer 145 K) and in front of the receiver feed horn 120 K noise temperature was recorded respectively.