Hi Tom, yes, the GPSTCXO has a pretty good typical phase noise above 100Hz or so for being "just" a TCXO, competitive to or even a bit better than some good SC-cut OCXO's. Compare that to the Trimble Mini-T for example which has a noise floor spec of only -145dBc/Hz according to the Trimble datasheet.. But there is a big difference between -73dBc/Hz at 1Hz, and -100dBc/Hz at 1Hz. These are certainly not similar numbers, not remotely.. That's what makes the difference between a low cost TCXO and a high-performance SC-cut double oven OCXO. In fact you can probably expect an OCXO that is specified at -105dBc/Hz at 1Hz to cost twice as much as one specified at "only" -100dBc/Hz at 10MHz.. There is also some specmanship here, because of the difference between max noise and typical noise. Max noise means all of the units shipped must be at or better than x, typical means the 1-Sigma noise numbers of a number of production units should be at or better than x. Some may be way better, some may be worse. There will be a cost difference between the two for the same number. In terms of signal buffering, I think this thread is sometimes slightly over-complicating the issue, take a handful of fast single CMOS gates especially if you already have a CMOS source, then run them at 5V, and then do some more or less complicated low pass filtering to generate 50 Ohms 10MHz sine waves. You can probably even get some low-cost Mini Circuits BNC type low pass filters of-the-shelf to do the job for you. But layout and power supply design is of critical importance to achieve the best noise performance, I would not use a simple LM78M05 regulator, I would use a low noise LT or Analog Devices unit because any low-frequency 1/f type power supply noise is going to be modulated right onto your signal unless you do a true differential design (way too complicated). With the above mentioned design you can assume to achieve better than -130dBc/Hz noise at 1Hz, and a floor of about -162 to -165dBc/Hz for the amp. That matches the Crystek VCXO performance you will get pretty well. All that said, in my experience there are really few applications where you truly need a -100dBc/Hz at 1Hz noise performance in a radio transceiver, other than for maybe Radar type applications that analyze data down to a 1Hz offset from the carrier.. But this is time nuts after all, so every dB matters. Bye, Said In a message dated 9/26/2013 23:15:16 Pacific Daylight Time, tom_min...@att.net writes:
Thanks for all your thoughts on the subject. Let me play back what I have learned and how it may apply to my challenge. One of my first applications is to use a 10MHz output to phaselock a VCXO master clock in a radio transceiver. The VCXO is the Christek CVHD-950 which has a noise floor of -164dBc and is -86dBc at 10Hz. The source I want to use is the Jackson Labs GPSTCXO which has a noise floor of -155dBc and is -73dBc at 1Hz and 103dBc at 10Hz. i did a quick survey of the phase noise specs on various Jackson products that claim to be ultra low phase noise and found similar numbers. One was -100dBc at 1Hz but only -145dBc at 100KHz. Another was down -90dBc at 1Hz and -160dBc at 100KHz. It would appear that even the best parts I could find quickly would not merit the fancy analog gizmo and that a good stiff logic buffer would work. Next I went to IDT to find the best logic buffer I could find. I am looking at the IDT 74FCT38072 2 channel clock driver for PPS. It can drive about 50mA if needed with 1nS rise and fall times. The one I am looking at for 10MHz is the ICS553 4 channel clock driver. This one is good for 25mA drive and they actually give a typical output impedance spec of 20 Ohms. With a 3.3V supply, it has 1nS rise and fall times and a little faster with a 5V supply, 0.7nS and 35mA drive. To make a sine wave should I use one of the 4 ports on the 4 port driver to input to the filter or should I try to hook the filter input directly to the clock driver input? Are there tried and true 10MHz filter circuits or is that a non issue? After the filter would come the video amp set up for a 50 Ohm drive and into a splitter. That sound simple enough. What am I missing? Tom _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
