

DARPA have created first solid state receiver with gain at 0.85THz - p4bl0
http://www.darpa.mil/NewsEvents/Releases/2012/07/31.aspx

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maaku
For reference, at 0.85THz light travels only 350um ( _micro_ meters) in one
clock cycle.

Unbelievable.

~~~
antoko
Can you explain why that is of any significance? I understand the relationship
between frequency and wavelength I'm just not certain how that makes the
process more complicated. (I'm sure THAT it does because DARPA are researching
it, I'd appreciate some explanation of HOW)

~~~
jengel
I am not a hardware guy, so I'll shy away from ignorantly answering the
question about hardware significance. My limited understanding, however, is
that equipment that operates in the higher frequency ranges is exotic and
currently quite expensive. That's why you primarily only see satellites
utilizing the >6 GHz range (RADAR and microwave backhaul links being two other
applications in the higher frequency range [1]). One of the problems in the
higher-frequency range is increased attenuation due to oxygen and water
absorption [2].

Why would the military interested in higher-frequency communications? I can
think of a couple of reasons of the top of my head:

1) There has been significant pressure over the last couple of years from
Congress (lobbied by commercial carriers) to reallocate spectrum from the
federal government and to auction it to commercial carriers [3]. With
increasingly advanced communications systems and waveforms being deployed in
the military which require significantly more bandwidth, that means they have
to do more with less. Shannon's law tells us that higher frequencies are
capable of more bandwidth than the lower frequencies, which means high-
frequency RF could transfer a lot more information than low-frequency RF
systems [4]. This is important given the military's push towards buzzwords
like "sensor fusion".

2) High-frequency communications systems are point-to-point rather than
broadcasting over a wide area. Think of how light propagates from a laser
pointer versus a light bulb. Low-frequency broadcasts, such as TV stations,
propagate over a wide area of land from a single antenna. High-frequency
microwave communications, such as the white cylindrical drums you see on cell
towers, are directional and require the communication antennas to more or less
be pointing directly at each other. There are three advantages of these point-
to-point antennas: A) higher bandwidth, as discussed earlier, B) much lower
likelihood of detection by your enemy, since your communications are targeted
rather than broadcasted, and C) lower probability of interception by your
enemy, because they would have to place an antenna directly in the path
between your two links in order to capture your signal.

3) RADAR systems can be significantly more accurate in the higher frequencies,
allowing more precise targeting and identification of targets. This has major
applications both within and outside the military. The Doppler Radars are an
example of a non-military application that provides weather information about
the US [5]. The more accurate they can make the RADARS, the better they can
distinguish between cloud formations and therefore provide more accurate
weather predictions.

(Disclosure: I was previously involved in the battles between commercial
carriers and US Federal Government regarding reallocation of spectrum. The
statements in this comment are my own beliefs and should not be construed as
the beliefs of my current or previous employers.)

[1]
[http://www.ntia.doc.gov/files/ntia/publications/2003-allochr...](http://www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf)
[2] <http://www.mike-willis.com/Tutorial/PF5.htm> [3]
[http://broadband.about.com/od/wireless/a/Ntia-Continues-
To-I...](http://broadband.about.com/od/wireless/a/Ntia-Continues-To-Identify-
Wireless-Spectrum-For-Broadband.htm) [4]
[https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...](https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem)
[5] <http://radar.weather.gov/>

~~~
hatcravat
> Shannon's law tells us that higher frequencies are capable of more bandwidth
> than the lower frequencies, which means high-frequency RF could transfer a
> lot more information than low-frequency RF systems [4].

Almost: Shannon's law sets a limit on the amount of information that can be
transmitted through a channel with a given signal-to-noise ratio and a given
_BANDWIDTH_ (it's bandwidth * log(s/n) ). Now, it happens to be the case that
systems operating at higher frequencies _often_ (nay, usually) do have higher
bandwidth for a number of reasons, but Shannon's law doesn't care what carrier
frequency your channel uses.

~~~
jengel
You're right. Thank you for the clarification!

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Groxx
Does this mean there have been _non_ -solid-state receivers at this frequency?
Or is this a use of 'solid state' I'm not familiar with?

~~~
klodolph
I don't know much about this area. I know that there are vacuum tubes that
operate in this frequency range.

<http://en.wikipedia.org/wiki/Backward_wave_oscillator>
<http://apl.aip.org/applab/v93/i14/p141108_s1?isAuthorized=no>
[http://www.sron.nl/index.php?option=com_content&view=art...](http://www.sron.nl/index.php?option=com_content&view=article&id=44&Itemid=111)

It looks like the existing receivers at this frequency range are of the
heterodyne type, possibly without any RF gain. A heterodyne receiver mixes the
RF signal with a local RF oscillator, which results in a lower frequency IF
signal which is easier to work with.

At these frequencies, it looks like the local oscillator is a "backward wave
oscillator" or a "quantum cascade laser", and the mixer is some kind of
esoteric non-linear device like a "hot electron bolometric mixer", whatever
that means.

The highest frequency amplifier I know of is the "traveling-wave tube"
amplifier, which goes up to something like 50 GHz. It's a crazy vacuum tube
device and it's found on satellite transmitters. Making these things solid-
state would be awesome because the tubes take up so much space and power on
the satellites.

Summary: There are vacuum tubes that operate in this range, and there are
receivers without gain in this range.

~~~
JonnieCache
_"backward wave oscillator"_

 _"quantum cascade laser"_

 _"hot electron bolometric mixer"_

 _"traveling-wave tube"_

Computer scientists, take note: start naming your ideas like this and you will
get more grant money.

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zitterbewegung
Could you create a communications system with these solid state receivers ?
I'm thinking about a communication network with these things? Or is that
impossible.

~~~
klodolph
Well, right now, the article only mentions a receiver. So you're half-way
there, you just need to build a transmitter.

~~~
ChuckMcM
An oscillator with a loose wire [1] is a transmitter :-)

[1] [http://sci-
toys.com/scitoys/scitoys/radio/am_transmitter.htm...](http://sci-
toys.com/scitoys/scitoys/radio/am_transmitter.html)

~~~
klodolph
And do please show how to build an 0.85 THz oscillator...

~~~
fooandbarify
AFAIK, an oscillator is necessary for building a receiver. In other words,
this is apparently a solved problem.

~~~
copper
Well, you do get vacuum tubes that operate up to a terahertz - they don't
count as solid-state devices, though.

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DigitalSea
This changes everything.

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mhb
I hope the TSA doesn't hear about this.

