Almost all frequency counters have an internal source which is a potential
means of injection locking an external reference if the isolation between the
internal source and the external source is inadequate. High impedance taps on a
single terminated line ensure that the isolation between such internal sources
and the shared line is limited by the isolation afforded by the internal source
selection gating/switching of each device.adding or removing a tap invariably
changes the phase shift between the source and each of the other receivers.The
minimum isolation required can be estimated from the maximum acceptable
frequency shift, the resonator Q and internal reverse isolation between the
source output and the resonator Q.
Frequency distribution systems like the Spectracom 8140 with wide range ADC
tend to degrade the source phase noise significantly with respect to non agc
distribution systems.
Bruce
On Sunday, 4 January 2015 9:41 PM, Bill Hawkins <b...@iaxs.net> wrote:
Friends in Time,
There's been a large amount of discussion about distribution amps on
this list.
People may be using them just because that's what's done. So I ask you:
What are we trying to isolate? The destination devices do not generate
an interfering signal, n'est ce pas?
The receiving devices do not need to have 50 ohms input impedance if the
source cable is properly terminated, no?
If I use high impedance receivers tapped off a terminated line, how is
this different from 10 base T?
Yes, there will be cable delay between receivers, but how were you going
to avoid that with your distribution amp?
Put another way, why do counters like the Racal 1992 allow you to choose
50 ohm or high impedance at the input?
Please, no "take it on faith" audiophile answers.
HNY.
Bill Hawkins
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