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Old 30th May 2012, 08:02 AM   #21
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Quote:
Originally Posted by DF96 View Post
[snip]If you are really worried about jitter then your jitter reduction circuit needs to be next to the DAC chip, as I said earlier.
Indeed. Eevn with a perfect adapter, there's always an opportunity for jitter introduction towards the DAC itself. Jitter reduction should be done right at the DAC chip, otherwise it doesn't make a lot of sense.

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Old 30th May 2012, 12:56 PM   #22
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The DAC being used (Anedio D2) has very good jitter reduction characteristics. I am just attempting to reduce the input jitter (from a Tascam CD200). Regards
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Old 30th May 2012, 01:48 PM   #23
DF96 is offline DF96  England
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The DAC receiver will deal with any high frequency jitter from the connection. That leaves low frequency jitter from the clock in the transport. To reduce this you either need a better clock, or a crystal-controlled PLL which goes down to low frequencies - I assume this is what your jitter reducer is. I think you are solving a non-problem: the jitter reducer deals with LF jitter from the transport and the DAC deals with HF jitter from the cable. I don't see any reason to worry about the cable.
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Old 30th May 2012, 01:55 PM   #24
AndrewT is offline AndrewT  Scotland
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I thought jitter needed to be minimised in the last digital stage before conversion to analogue.
Is this wrong?
Or, does there come a point where jitter has become so bad from the earlier stages and transmission deficiencies, that it cannot be corrected later?
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Old 30th May 2012, 02:14 PM   #25
DF96 is offline DF96  England
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Jitter has to be minimised by the last stage. Two ways to do this: maintain low jitter throughout (not fully possible), or correct for jitter using a PLL. A decent crystal oscillator will have low jitter, although it could get worse at lower frequencies. A cable can introduce high frequency jitter, which is why S/PDIF receivers have a PLL to reduce HF jitter. They can't reduce LF jitter because at lower frequencies the PLL simply follows the signal - it has to do this in order to lock on to the incoming data stream. The PLL will of course introduce its own jitter, but provided it reduces the incoming jitter by a sufficient amount then you are still better off with a PLL than without it.
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Old 30th May 2012, 04:53 PM   #26
RJM1 is offline RJM1  United States
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Or TNC connectors we use them for radio altimeters ( 4.3 GHZ ) 50 ohms are good to at least 11GHZ, 75 are only good to about 1 GHZ. http://www.spectrum-et.org/new_web2/...Series-TNC.pdf

Last edited by RJM1; 30th May 2012 at 05:02 PM.
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Old 30th May 2012, 05:07 PM   #27
AndrewT is offline AndrewT  Scotland
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Why should a 25ohm difference make a 10GHz difference?
Or, is it different connector types that account for the bandwidth differences?
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Old 30th May 2012, 05:30 PM   #28
DF96 is offline DF96  England
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If a TNC is a TNC (i.e. unlike BNC there is only one version), then mismatch would limit the bandwidth away from the design impedance. Is there only one TNC? 75 ohms is not much used for RF. I suspect that it only because of video that there is a 75ohm version of BNC.
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Old 30th May 2012, 05:34 PM   #29
RJM1 is offline RJM1  United States
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The75 ohm version has the same dimensions as the 50 ohm connectors and some compromises where made to the insulator size in the 75 ohm version. In other words they would have to have a different inside diameter to outside diameter ratio to be optimized for 75 ohms. From the Wiki. Most TNC connectors are 50-ohm type even when used with coaxial cable of other impedances,[citation needed] but a 75-ohm series is also available, providing a good SWR to about 1 GHz.[2] These can be recognized by a reduced amount of dielectric in the mating ends. They are intermatable with standard types. They seem to have an air gap in addition to the insulation.

Last edited by RJM1; 30th May 2012 at 05:42 PM.
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Old 30th May 2012, 05:38 PM   #30
DF96 is offline DF96  England
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So the 75ohm version of TNC has much poorer VSWR. At microwaves, it is not just the connector itself but the transition from cable to connectors and back again. Join two perfect 50ohm cables of widely different diameters and you will still get a reflection at the join.
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