richard nienhuis wrote:
This is all very black magic. Most of the information is internal to
Tek, Lecroy and Agilent. Also in order to keep parasitics down
sampling heads etc need to be made tiny. This also doesn't include
the complexity of making a pulser capable of produsing perfectly edged
pulses fast enough to drive the sampling head.
There are two main ways to accomplish the sampling in scopes of this
caliber. First, it is important to point out that DSOs have two
different specs. Sample rate and bandwidth. They are related, but
separate. High bandwidth is usually accomplished by sampling a waveform
multiple times at slight offsets. For example, if I have a 1GS/s ADC,
Nyquist says 500MHz is my theoretical limit and as was pointed out, you
really need way more than 2 samples per period to get a recognizable
waveform. Let's say, for the sake of math that we want 10 (This is what
Tek uses). That limits us to 100MHz. To get 1GHz bandwidth, you adjust
your sample timing. Instead of sampling every 10ns you sample every
11ns. This increases your effective resolution on a repeating waveform
but a factor of 10, giving you an effective 1GHz bandwidth. Note this
only works for repetitive wave forms. For non repeating wave forms, you
are limited to 100MHz. Though for single event triggers you have a
resolution of 1ns.
To increase actual bandwidth, you have to have multiple ADCs each
sampling a a slight time offset. To get the necessary ten samples per
period for a 1GHz waveform you need ten 1GS/s ADCs each sampling 0.1ns
apart.
A 1 Ghz scope might be doable. But I can see it being several hundred
in parts alone.
If you could put together a 1GHz DSO for $1k in parts you could probably
sell more than you could ever make. A low end Tek DSO (TDS2014) is
going to set you back around $2kUS and that buys you a 4 channel 100Mhz
- 1GS/s DSO. A 4 channel 1Ghz - 5GS/s Tek DPO4104 has a list price of
$14kUS.
Patrick M
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