[time-nuts] Practical considerations making a lab standard with an LTE lite

Charles Steinmetz csteinmetz at yandex.com
Sun Nov 23 19:22:33 EST 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,


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