Next Generation Time/Frequency Standards May Require Provisions Preventing Vertical Displacement
On 09/30/2010 06:14 AM, jimlux wrote:
Magnus Danielson wrote:
Doppler effects is much more important, and it's effects is being
treated regularly, such as when talking in the GSM phone while driving
the car...
Hmm.. I think crystal oscillator frequency variation in the phone is a
bigger factor. Let's say you're zipping down the road at 200 km/hr
(55m/s), texting your friends. That's about 0.2ppm Doppler or around 400
Hz (for a 2 GHz carrier).. as noted.. the XO probably has 1ppm (at
best.. more like 10ppm)
On why there is a big market for small but stable TCXOs at 26 MHz.
An even bigger problem in a mobile environment is multipath, which is
far worse than the Doppler.. The effective propagation path distance
could easily change 1000 meters in a fraction of a second.
Depending on system, different approaches to handle fading and multipath
is done.
Cheers,
Magnus
Magnus Danielson wrote:
On 09/30/2010 06:14 AM, jimlux wrote:
Magnus Danielson wrote:
Doppler effects is much more important, and it's effects is being
treated regularly, such as when talking in the GSM phone while driving
the car...
Hmm.. I think crystal oscillator frequency variation in the phone is a
bigger factor. Let's say you're zipping down the road at 200 km/hr
(55m/s), texting your friends. That's about 0.2ppm Doppler or around 400
Hz (for a 2 GHz carrier).. as noted.. the XO probably has 1ppm (at
best.. more like 10ppm)
On why there is a big market for small but stable TCXOs at 26 MHz.
how stable? I'm sort of curious, I wonder what sort of temperature
range cellphones are expected to really work over.. (not necessarily
what they're specified for, but what the designers see as the "sweet
spot").. It's not like people carry their phones in pocket on the back
of a backpack in -40 weather.
I wonder if they're like pager receivers in some sense (e.g. they're on
all the time, waiting for a call)
And, as the phone heats up as you transmit, how much does the frequency
change?
It's a real cost sensitive huge volume market, so the specs for a
cellphone reference oscillator could be highly tailored to a specific
application.
On 9/30/2010 8:43 AM, jimlux wrote:
how stable?
The parts are generally ~ 1 ppm over temp and another ppm or two aging.
I'm sort of curious, I wonder what sort of temperature range
cellphones are expected to really work over..
depends on the vendor to some extent. Not all standards spec an
operating temp range. The bottom end is usually -20 or -40 C though not
at full spec, and -40 is pretty rare.
(not necessarily what they're specified for, but what the designers
see as the "sweet spot").. It's not like people carry their phones in
pocket on the back of a backpack in -40 weather.
Most phones are not specified to operate this cold. Even standards that
specify operating temperature range are often not fully complied with.
I wonder if they're like pager receivers in some sense (e.g. they're
on all the time, waiting for a call)
No, a lot of effort is spent in letting them spend most of their time in
standby and only wake up every second (or few seconds.) Most modern
pagers do the same thing. They indicate to the receiver when they will
be sending data, and when they will be sending device addresses
(generally called the paging interval for cellphones and pagers both.)
And, as the phone heats up as you transmit, how much does the
frequency change?
Once the phone is participating in the network it's locked to it. At
that point the only thing we care about in the TCXO is short term (~
1sec) drift and that's WAY better than 1 ppm. If you are "transmitting"
you are still receiving several times a second and getting frequency
offset updates at the same rate. It's closed loop.
It's a real cost sensitive huge volume market, so the specs for a
cellphone reference oscillator could be highly tailored to a specific
application.
Yup, and stamped out in the millions per month so they are really
cheap. Well under a buck. Most are just a slab of silicon, a slab of
quartz, and a package. All oscillator functions and compensation are
provided in a custom bit of CMOS with EEPROM or fuse programmable
compensation memory. I didn't think this was possible, but I saw one
yesterday in a GS that was in an SSOT-23 package. Sheesh.
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