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Introduction
Introduction
There are a number of low-noise amplifier + filter devices available useful for radio astronomy. These are available from Nooelec (https://www.nooelec.com/store/sdr/sdr-addons/sawbird.html) as Sawbird LNAs. The gains and noise figures are reasonable. The passband characteristics are suitable for a couple of transition line targets - namely the HI 1420 MHz line and the OH maser 1612/1665/1667/1720 MHz lines.
The bandpass characteristics of three Sawbird LNAs are given below - the +HI, +IR and +GOES models are examined.
For excitation a broadband noise source is used and the bandpass output displayed on a spectrum analyser. The broadband noise source used has an output at a sufficiently low level such as to not overload the LNAs in the Sawbirds. If a higher level source was used, an appropriate amount of attenuation would need to be inserted between the source and the Sawbirds to avoid overload.
Sawbird +HI (1420 MHz)
Sawbird +HI (1420 MHz)
The passband is not particular flat. However, the 3 dB bandwidth (~70 MHz) is much wider than required for the spread of HI line velocities likely to be encountered. For example - a velocity range of ± 250 km/s translates to about 2.4 MHz at 1420 MHz . This bandwidth corresponds to about 1/10th of a division (1 division = 25 MHz) on the horizontal frequency scale. - 1/30th of the 3 dB bandwidth. Therefore the inflections of the passband ripple are not seen - but some local slope will be seen.
Sawbird +IR (OH 1612 MHz)
Sawbird +IR (OH 1612 MHz)
Nominally used for observing the Iridium satellites - which use the L-band frequency ranging from 1616 to 1626.5 MHz to communicate directly with ground-based users. Also in this band are the GLONASS satellites (1598 to 1605.5 MHz).
Our target OH 1612 MHz transition line is sandwiched between the two.
This OH line - generally speaking - is emitted from some of the strongest 1.6/1.7 GHz masers.
Sawbird +GOES (OH 1665, 1667 and 1720 MHz)
Sawbird +GOES (OH 1665, 1667 and 1720 MHz)
Nominally used for receiving the GOES weather satellite, this Sawbird can be used for the other three 1.6/1.7 GHz OH transition lines - 1665, 1667 and 1720 MHz.
These three transition lines - again generally speaking - are emitted at a lower flux than 1612 MHz.
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