[time-nuts] Performance verification for time counters
phk at phk.freebsd.dk
Wed Nov 29 18:36:01 EST 2017
In message <409346F4-60E1-4CAE-8602-84FB1C061DB7 at n1k.org>, Bob kb8tq writes:
>>> The HP3336 with its outstanding level-control is a much
>>> overlooked bargain for this kind of stuff.
>> I looked for the manual, and it seems to have ROM feeding values to a DAC.
>> Is that not DDS?
That is for the level control, not for generating the signal.
The level control has a custom HP-chip with two matched thermal
converters, so you can compare the power of two signals with it.
One signal is the RF output, the other comes from a DAC.
The 3336 is as far as I know one of the the most precise instruments
when it comes to amplitude, and it was built specifically for the
last two generations of the Carrier Frequency Coax telephone network
(L4 and L5) where up to 10.800 4kHz telefone channels were stacked
over each other in a single coax cable.
Because the top frequency were near 70 MHz, they needed a repeater
for every mile of cable which is up to 4000 repeaters for a call
across the US.
If every one of those repeaters had a systematic dip of 0.01dB at
the same frequency, That would amount to 40dB attenuation in total,
so the amplitude tolerances were almost insane, in order to keep
the amount of equalizing manageable.
Google: bstj53-10 site:archive.org
It's an amazing story...
The HP3336 and the HP3586 level meter go together to measure these
lines: The 3586 measures the received signal and when it is done
it tells the 3336 to step 4kHz up to the next channel. The crucial
trick is that the 3336 produces the exact same output level for all
See: HPJ May 1980.
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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