[time-nuts] Papers on timing for lunar laser ranging
brooke at pacific.net
Mon Jul 17 13:24:42 EDT 2017
The Quantic Timing GPS receiver makes use of patent 5440313which results in a 48 bit DAC. How does this idea compare?
PS the above patent cites 4582434 i.e. the Heathkit GC1000 HFDO.
-------- Original Message --------
> On Sat, 8 Jul 2017 11:42:44 -0700
> Tim Lister <listertim at gmail.com> wrote:
>> Forgive the ignorance, but why is there a large disparity between ADC
>> and DAC capabilities ?
>> For example, Linear Technology sell a 24 bit ADC for ~$7 but an 18 bit
>> DAC is $30-50...
> Much simplified, it boils down to it being easier to measure voltage
> differences by averaging than keeping a voltage constant.
> E.g. in those >20bit ADC's you will usually find a delta-sigma ADC,
> usually 3rd to 5th order with a 1.5 to 5 bit ADC/DAC inside. The ADC
> and DAC can be laser trimmed to be in the order of 0.1% of their
> ideal values. With a few additional tricks you can get the most of
> the remaining non-linearity out. These tricks also help to remove
> errors due to DC-offsets in the signal path. But the biggest
> improvement comes from averaging over many "samples" to get the
> white noise out. If you look at the usual sample rates at which
> those ADC reach their "full" performance, it is around
> 1-30 (output) samples per second.
> On the other hand, on a DAC you need to keep the output voltage
> stable. You can do the same delta-sigma approach as with the ADC
> with much the same result, but you have one big problem:
> it is not easy to build an analog low pass filter that has a corner
> frequency down at 10Hz. This means, you have to work at a much higher
> frequency to have a low pass filter that can be realized (let's say 1kHz
> if you are building a discrete filter, higher if it's integrated).
> But that means that you have several orders of magnitude more (white) noise.
> Additionally, a lot of people expect to do a couple of 1000 samples
> per second at least, to have a usefull DAC. But that contradicts the
> need to have a narrow band low pass filter to get the noise out.
> Attila Kinali
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