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VK2ZRG > TECH 27.09.08 08:44l 50 Lines 3140 Bytes #999 (0) @ WW
BID : 2668_VK2ZRG
Read: GUEST
Subj: Interesting day at ATNF
Path: IZ3LSV<IK2XDE<F5GOV<F4BWT<YO6PLB<CX2SA<VK7AX<VK2TGB<VK2IO<VK2WI
Sent: 080927/0326Z @:VK2WI.#SYD.NSW.AUS.OC #:2676 [Sydney] $:2668_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : TECH@WW
VK2ZRG/TPK 1.83d Msg #:2668 Date:27-09-08 Time:3:26Z
I had an interesting day at ATNF last Wednesday. ATNF stands for Australia
Telescope National Facility, a division of CSIRO. The people at ATNF make the
"front end" of the receivers for the Australia Telescope at Narrabri, and for
the Parkes radio telescope, amongst other things.
Travelling by shanks pony and City Rail, I arrived at the Epping address at
morning tea time. After a brief guided tour of a small part of the facility
(where I saw some cryogenically cooled front ends and at least a photograph
of the SETI front end for the Parkes Dish), it was down to the business of the
day.
This was to show them (the people at ATNF) how to retune some amplifiers that
I made for ATNF in 2002 when I was working in the Department of Astrophysics at
the University of Sydney. These amplifiers had a centre frequency of 680 MHz and
were used for pulsar detection work. 680 MHz is the middle of the UHF television
band, so when a new digital TV transmitter began operation from Mount Canobolos
(an extinct volcano 13 km SW of Orange in NSW) 80 km to the East of Parkes, it
was necessary to shift the observation frequency upwards by 64 Mhz.
My amplifier design uses Ku band HEMT's (High Electron Mobility Transistors).
At UHF these HEMT's have a lower noise temperature, lots of gain and a very
high input impedance. To get a reasonable 50 ohm input match it is necessary
to use source degeneration. This is achieved by adding inductance, in the form
of a microstrip line, to each of the source leads. Doing this makes the circuit
into a nice little SHF oscillator and the trick is to prevent SHF oscillations
while maintaining low noise UHF operation.
The people at ATNF attempted to retune the amplifiers a number of times but
couldn't get the required performance at the new frequency. From what I gather
in talking to them, they contemplated buying some commercially made amplifiers
to replace the amplifiers that I made. Apparently the commercial amplifiers
had a noise temperature some two to three times the 15 Kelvin noise temperature
of my design and had a very wide bandwidth (and cost $5000 to boot).
I had no real problem in moving the operating frequency by 64 MHz (I had
after all made 400 LNA's for MOST, Molonglo Observatory Synthesis Telescope,
at 843 MHz). A lot of the time involved in the job was to find chip capacitors
of a suitable size. I had used 2 by 1.25 mm size capacitors in my design but
the ones ATNF had were much smaller. After changing two capacitors, the source
inductance, and one gate coil the job was done on the "spare" amplifier.
They seemed reasonably happy with the performance at the new frequency; it now
remains to fetch the four operating amplifiers back from Parkes for retune.
I hope they'll have no more problems retuning these ones. No doubt I'll hear
from them if they do strike trouble.
It's nice to know ones skills are still of some use and that the amplifiers
I made will back at work on the Parkes radio telescope in the near future.
73's de Ralph VK2ZRG
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