[time-nuts] Practical considerations making a lab standard with an LTE lite
Charles Steinmetz
csteinmetz at yandex.com
Mon Nov 24 00:22:33 UTC 2014
Said wrote:
>The 10MHz units have a different RF output than the 20MHz units. The
>20MHz units have a 50 Ohms series-terminated and buffered RF output,
>while the
>10MHz units have the TCXO output drive the MMCX connector directly without
>series impedance matching. Both drive the line with 3.0V CMOS levels. This
>means the cable on the 10MHz unit should be kept as short as possible, and
>that impedance matching for maximum power-transfer is not required nor
>desired. The suggestion that Charles made for checking the impedance by
>progressively loading the output more and more is valid for Sine
>Wave outputs, but
>not for CMOS outputs as implemented on the LTE Lite.
Absolutely correct -- I did not anticipate that anyone would make
unbuffered logic levels available to the external world.
In that case, I'd put a logic-level line driver immediately at the
unit (by immediately, I mean with a small breakout card that plugs
directly onto the LTE's MMCX connector with no intervening
cable). For example, all 6 outputs of an HC14 or AC14 hex inverter
connected in parallel, or a dedicated line driver chip like an
HC365/366 or AC240/244/540/541.
The buffer should be inside the enclosure with the LTE, and I would
also add a T-network filter to convert the logic-level square wave
into a sine wave. This would confine all of the fast logic
transitions inside the shielded box, where they can do the least mischief.
For the T-network, I like 10uH/50.5pF/10uH, others like
1.5uH/310pF/1.5uH. Both draw ~ +/- 35mA from a 5v logic
output. Make sure your buffer can supply this current, and feed the
T-network through 10nF and 50 ohms in series. You'll get a 1Vrms
(13dBm) sine wave into 50 ohms (675mVrms with 3v logic). H3 is down
40dBc with the 1.5uH network and 60dBc with the 10uH network. [Note
that the apparent source impedance is > 50 ohms, so the open-circuit
voltage is more than double.]
Best regards,
Charles
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