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

Said Jackson saidjack at aol.com
Tue Nov 25 11:17:57 EST 2014


The increased current for the driver will cause heating near the crystal in both the CMOS driver and the 3.0V LDO as the LDO has to convert the excess voltage into heat. This may or may not affect the crystal.

One could certainly try, this is why I initially said its certainly possible, but up to the individual to decide. Adding an external buffer is so simple that I just did not think it would be worth it..


Sent From iPhone

> On Nov 25, 2014, at 4:12, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> Hal wrote:
>> > So driving 50 Ohms inputs is not optimal here, 1M inputs are much better for
>> > this purpose.
>> That only works if you have a (very) short connection to the next stage.
>> Things get interesting if you have, say, 10 feet of unterminated coax.
> Thinking that the output was a sine wave, I previously suggested testing to determine what its actual impedance is and to proceed accordingly.  Said pointed out that it is not a sine output, but rather 3v CMOS.  Still, I think it is worthwhile to test to see what the actual output capability is.  For example, most HC and AC CMOS outputs will source and sink 20-25mA.  The Fairchild "advanced CMOS family characteristics" document says:
>> All SSI and MSI devices (AC, ACT, ACQ or ACTQ) are
>> guaranteed to source and sink 24 mA. 74AC/ACTxxx
>> devices are capable of driving 50 [ohm] transmission lines.
> Some of the newer CMOS logic is similar, including Fairchild TinyLogic UHS (NC7xZ series), LCX, and LVC devices.  Now AFAIK, we do not know what CMOS device is used for the TCXO output -- and it may well not be any of these.  Testing will provide a definitive answer, and it may show that there are better options than a 1M termination.
> Of course, the TCXO output is used internally to the LTE Lite (and may be used internally to the TCXO itself), so one cannot count on having all of the rated device output current available to drive an external load.  Avoid anything that pulls the output logic levels very far down (logic high) or up (logic low), say by more than 200mV (such as a termination resistance that is too low), or materially distorts the output wave shape (such as a Tee or Pi filter, which one might consider to convert the output to a sine wave and match it to coax).
> To test, one would use a voltage divider from the logic supply voltage to ground, with the TCXO output feeding the center point of the divider.  (See attached diagram.)  I will be very surprised if it will not drive 10k + 10k with ease (already MUCH better than 1M), and 1k + 1k is a distinct possibility [NOTE: in some cases, this scheme works best if the resistor to the positive supply is about 50% higher than the one to ground, for example 1.5k + 1k].  You may even find that it will drive 100 + 100 (or 150 + 100) without problems, in which case it should directly drive 50 ohm coax.  With any of these, best performance in the final installation will be achieved with the termination resistors at the far end of any wire, PC trace, or transmission line longer than a few inches.  [Note that the divider scheme is the right way to terminate CMOS logic for analog uses at any impedance -- to terminate in 1M ohm, one would use 2M + 2M, although at that level it matters less.]
> Because the CMOS device is a saturated switch, the TCXO and LTE Light power dissipation will not increase by a significant amount with the increased load current.  The logic supply will need to source some extra power, but only 45mW even for the 100 + 100 ohm output network.
> If the gods are truly with us, we may even find that the TCXO output will source and sink sufficient current to drive a Tee network if the circuit is designed properly -- say, a divider with 150 + 150 ohm resistors (or 220 + 150) feeding a series 10nF capacitor and 200 ohm resistor to a Tee network using 10uH/50.5pF/10uH -- which would drive a 0dBm sine wave into terminated 50 ohm coax with harmonics below -40dBc.  (See attached diagram.)  This requires peak currents from the CMOS output of +/- 5mA.  But don't count on this until you test and verify, and don't be surprised if the TCXO output will not support it.  [If one can live with a sine output of < 0dBm, the divider resistors and the series resistor can all be increased in value until it does work.]
> Best regards,
> Charles
> "All electronics is analog."
> <CMOS_output_circuits.png>
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