Dan Werthimer wrote:
Can you transmit the RF or IF through analog fibers, and then locate the
digitizers in
an electronics lab, where they are easy to service, upgrade, keep cool,
shield from the receivers, ...?
We can, in principal. The receiver being built first for Canberra by
Steve Smith in Sandy's group at Caltech re-uses much of the GAVRT design
and technology. 17-27 GHz will be converted to baseband in five 2-GHz
bands. The conservative assumption is that we will clone this for
Goldstone. Of course, technology is always changing.
At the ATA we use analog transmission
over fiber.
Each ~15 GHz bandwidth fiber driver/receiver pair cost $2500. All the
digitizers
and digital backends are in the same room, making it easy to upgrade and
service.
40 x $2500 = $100K is about the cost of the receiver. This is also the
reason why Sandy is bringing down only half of the IFs that he generates
in his GAVRT receiver. Do you think that, given the receiver design, we
would want to upgrade the ADCs?
Currently, the design is to bring the baseband out to connectors on the
receiver box, to be connected with hardlines to the ADC box. Steve
understands the need for shielding well. He works elbow-to-elbow with
Glenn, after all, and they have tested both their analog and digital
gear together in close proximity.
ATNF digitizes at the receiver on the AT so it can be done, carefully.
The fastest current CASPER digitizers is 6 Gsps (8 bit); we plan to develop
a 20 Gsps digitizer over the next year or so. Their are a some 80 Gsps
digitizers,
but we don't have any plans to use them yet, as we can't handle the data
rate.
When you finish the 20 Gsps digitizer, we may want to revisit the
receiver design. Currently, with available mixer chips, we cannot cover
the whole 17-27 GHz band but we could break it into two.
Jouko's recommendation seems like a good idea. Fiber cables are cheap.
If I understand it correctly, the single mode fibers will support analog
if we need to go that way.
Best regards
Tom
Best Wishes,
Dan
Tom Kuiper wrote:
During the upcoming Goldstone 70-m down-time, we have an opportunity
to put fibers dedicated to radio astronomy in the antenna. The
receiver we envision would, when fully built up, have 40 2-GHz ADCs.
To be able to use our signal processing equipment with other antennas
as well, we would want to put it in the Signal Processing Center,
which is about 0.5 km away from the receiver. I assume that we would
have a 10 GBe switch in the antenna. Could someone give me an
estimate of how many fibers of what kind would best serve our needs?
An alternative scheme would be what GAVRT DSS-28 does, which would be
to bring 40 IFs to the SPC and digitize there. That way the ADCs
could serve other antennas. However, this appears to me to be more
expensive and more problem prone, and ADCs are cheap.
Thanks and regards
Tom
