[time-nuts] Spectracom 8140 distribution amplifier and the Taps- some application info

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jul 27 19:12:39 UTC 2011


walter shawlee 2 wrote:
> There have been many questions posted here about the units, here are a 
> few answers I have collected:
>
> 1. the 8140 can drive up to 25 total downstream 8410T taps on the 
> combined outputs. The mopdels are frequency specific, and are not 
> easily changed, 10Mhz is the most common. The front selector provides 
> TTL divided waveforms from the input rear clock.
>
> 2. yes, the taps are needed, as the output of the 8140 itself is a 
> sinewave and +12VDC.  using the tap allows long cable distances, and 
> provides the extracted, buffered sinewave. long cable runs of RG58 up 
> to 500 feet are possible, with T-taps all along the way, and all 4 
> outputs can have long runs. the manual says a capactively coupled 50 
> ohm load is meant to terminate the line, but I have not found it to be 
> required so far, as the sine wave drive produces very little ringing.
>
> 3. yes, it works WITHOUT the taps, if you add a 0.47uF good quality 
> cap in series with each BNC output, run distance is then short into 50 
> ohms.
>
> 4. YES, taps are different, if you have a 10Mhz source, you must have 
> 10Mhz taps (8410T10). beware of the 8410TTL10, it has a TTL 10 Mhz 
> output, not a sinewave.
>
> 5. the operating quality of the system is excellent, do not be 
> deterred by it's modest appearance, it works just as well as the 
> racal, fluke and hp distribution amps I have and use. Late ones are 
> much better made (they have black, not blue vinyl tops), if you can 
> score one.
>
> 6. Phasing: my measurements show approximately a 190 degree phase 
> difference between the input sine, and the output waveform at 
> 10Mhz...not an issue for me, but maybe for others?
>
> 7. The system checks for DC output shorts (excessive loading) and loss 
> of input clock, these drive the red front indicator (output fault). An 
> option can also drive a sonalert.
>
> 8. the taps are not generally repairable, as they are potted with 
> foam. bummer.
>
> 9. System details, and downloads are here at Spectracom:
> http://www.spectracomcorp.com/ProductsServices/TimingSynchronization/TimeandFrequencyDistribution/8140FrequencyDistributionSystem/tabid/133/Default.aspx 
>
>
> 10. Note, there are no schematics for Taps, if anybody has some to 
> send me, I'd really appreciate it, just to satisfy my curiosity.
>
> so, if you find one in good condition, it is a great addition to your 
> lab, for distributing your master reference all around the shop area 
> to counters, spectrum analyzers, generators, etc.  it works with 
> literally all common clock sources, from 0.3Vrms to 2Vrms, should be a 
> sine wave.
>
> regards,
> walter
>
> Walter Shawlee 2, President  -  Sphere Research Corporation
> 3394 Sunnyside Road, Kelowna, B.C.,  CANADA    V1Z 2V4
> URL: http://www.sphere.bc.ca   E-Mail: walter2 at sphere.bc.ca
> Phone: +1 (250) 769-1834             Fax: +1 (250) 769-4106
>             --Home of the Slide Rule Universe--
> WS2:    If wishes were fishes, we'd all be covered in cod.
> WS2:  We're all in one boat, no matter how it looks to you.
>
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Unfortunately it has a relatively high phase noise floor (around 
40dBc/Hz worse than the current state of the art) common to most 
distribution amplifier designs (eg the SRS distribution amplifier) using 
AGC or limiting.
The HP5087 has a somewhat lower phase floor but it is still 
significantly worse than the current state of the art.

Bruce



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