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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