[time-nuts] A small piece on HP's hydrogen maser in 1968

[Bob Camp]

Hi

One of the more common explanations for the 18 GHz “upper limit” is that the broad water vapor absorption peak at about 23 GHz made systems less practical as you went up from 18. I suspect the same water issues make certain types of parts more difficult to fabricate.

Bob

On Feb 26, 2014, at 9:01 AM, Jim Lux <jimlux at earthlink.net> wrote:

> On 2/26/14 12:44 AM, Hal Murray wrote:
>> 
>> richard at karlquist.com said:
>>> Solid dielectric cable and connectors of 3.5 mm size are mode limited to 18
>>> GHz.  That is why there is so much stuff rated at 18 GHz as opposed to 16 or
>>> 20 GHz.
>> 
>> Thanks.  That's what I was looking for.
>> 
>> Wiki says that SMA works to 18 GHz and the 3.5 mm is good for 34 GHz.
>>   http://en.wikipedia.org/wiki/SMA_connector#Variations
>> 
>> 
> 
> And, as pointed out earlier, the market is smaller, so volumes are smaller, and driving the price down from being able to change to truly mass production is harder.
> 
> There's also a manufacturing tolerance issue.  The higher you go, the tighter the mechanical tolerances get.  I suspect there is a huge amount of tooling out there for SMA connectors and other things of that size where the machining tolerances are "good enough" for SMA and 18GHz, but not "good enough" for higher.
> 
> That drives all sorts of things.
> 
> THere's also semiconductor parts.  Lots and lots of stuff in the under 12-13 GHz range that are inexpensive.  A fair amount up to 18-ish, and then it sort of falls off.
> 
> There, it's driven by market, which in turn is driven by international allocations.  DBS satellites at 12-13 GHz is a high volume market, so there's lots of things like MMIC low noise amplifiers.  Likewise anything around 2.45 or 5.1-5.8 GHz.  You see a big break in RF equipment model capability at 3GHz and 6GHz, and I suspect that's driven by the desire to test cellphones and wifi and BT (<3 GHz) and 802.11a/802.11n, WiMax, etc at <6GHz.
> 
> Parts that are cheap and easy to use lead to interesting products like the SignalHound spectrum analyzer, but I don't expect to see a 50GHz SignalHound any time soon. ($900 for 4.4 GHz, $2k for 12.4GHz).  You could probably *build* a front end converter for a signal hound fairly inexpensively, but the parts for, say, 32 GHz would cost as much as the Signal Hound backend.
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