[time-nuts] Performance verification for time counters
kb8tq at n1k.org
Wed Nov 29 18:41:44 EST 2017
Needless to say *demonstrating* this 0.001 db sort of gain flatness on a repeater
out to crazy low frequencies is a bit involved. It *is* a great gig if you happen to be
a consultant …
> On Nov 29, 2017, at 6:36 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 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
> 10800 channels.
> 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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