[GPS_Standard] 10MHZ SMD TCXO output boost ?

[Dimitri.p]

At 02:14 PM 8/17/2012, Dave Platt wrote:
>On 08/17/2012 01:52 PM, Dimitri.p wrote:
> > Hi everyone.
> > I attempted to use the CRYSTEK CVT32 TXCO  (a tiny SMD unit) and
> > although it presents about 1V Pk-Pk to the input of U1, it doesn't
> > seem to be enough since there is no output on pin 1 of U1.
> >
> > Regarding U1, (in the interest of full disclosure), I am not using
> > the LTC1485 (Digikey was out of stock) but the SN75LBC176P from Mouser.
> >
> > I'll try adding a a 74AC14 as a buffer in between the output of the
> > TCXO and  the input of U1 and in the meantime maybe someone can
> > either shed some more light or suggest a better arranegment :)
>
>I used that same part, and wasn't initially able to latch onto
>the output from an Efratom oscillator having a similar P-P
>voltage.
>
>The problem turned out not to be the amplitude of the signal, but
>the fact that the oscillator's output was DC-coupled into the
>SN75LBC176P.  One of the differential receiver's inputs is
>grounded, the other is fed from the oscillator.  Unless the
>oscillator can swing at least 50 millivolts or so *below*
>ground, it won't exceed the receiver's hysteresis range, and
>the receiver will stay in one state.
>
>The data sheet for the CVT32 pretty strongly indicates that
>this is what's going to happen... the oscillator itself has
>no negative supply and cannot swing below ground.
>
>The fix (for me at least) was easy - just add a .1 uF capacitor
>between the oscillator output, and the receiver's A input.
>It's probably be a good idea to have *some* resistor in the
>R1 position (either the 51R termination resistor suggested
>in the article, or a 4k7 or 47k pulldown) just to ensure that
>the "receiver" side of the DC-blocking cap is at ground potential
>on average.
>
>Add such a cap and (if necessary) a pulldown, and your oscillator
>will be swinging the differential receiver's pin about 500 mV
>below the other (grounded) input, and you should get a good
>output.
>
>The CVT32 data sheet "test circuit" shows just this sort of
>.1 uF / 10k-to-ground circuit, probably for just this very
>reason.
>
>The other option is to leave the oscillator DC-coupled into
>the differential receiver, but bias up the second differential
>input to about 0.5V so that it's right in the middle of the
>oscillator's common-mode range.  This seems like a lot more
>trouble... you'd have to cut traces on the board and pooge
>in a couple of additional resistors.

Thanks for the swift reply!!
The 10MHZ signal ridiong on almost 5 volts didsn't "look" right!
I was using the 0.1 capacitor and a 10K resistor, Your message 
prompted me to lower the input R to U1 p6 , so I tacked on a 2K and Success!!!

So if the U1 doesn't produce output ,  lower the input R is the 
morale of the story
Thanks again

Dimitri - KF6QEX