> -----Original Message----- > From: time-nuts-boun...@febo.com [mailto:time-nuts- > boun...@febo.com] On Behalf Of bob grant > Sent: Monday, February 06, 2012 12:41 PM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Why a 10MHz sinewave output? > > > Some sine-wave outputs are not very symmetrical, in that the rising > edges > are much more sinusoidal in shape than the falling edges. > > I guess my question is really about what type of input circuity and > drive > level are most common and which signal shape would provide the lowest > jitter. >
Most line receiver chips are fine for squaring up sine-wave signals that come in on a 50-ohm connection at +5 to +15 dBm. If you need fast 10 MHz edges, they are pretty easy to generate on demand without adding significant jitter. A two-transistor differential amp works well enough in almost every case. I think the answer to the "why sines?" question is a mixture of several points others have raised. 1, 5, 10, and now 100 MHz sine waves have all been used for reference-frequency distribution. Equipment that needs to be locked to a common reference does one of two things with that reference signal as soon as it enters the enclosure: they either fan it out for use where it's needed internally, or they phase lock their own internal oscillator to it. Before digital hardware became so prevalent, these applications -- either miscellaneous RF circuits or analog PLLs -- both tended to expect sine wave signals. Also, the signal would have originated in a crystal oscillator or similar source, where it would naturally be available as a sine. So it would have taken extra circuitry to square it up, potentially extra circuitry to filter it at the destination, and better cabling to transmit the harmonic-rich signal. Differential signaling was not yet in common use, and not entirely free to implement... and EMI is always a concern even with 10 MHz sines. If you don't use good double-shielded cabling for 10 MHz distribution, your lab environment will be full of 10 MHz energy from your house clock that is difficult to keep out of sensitive circuits. Things would be even worse if we were at risk of 'broadcasting' harmonics from 10 MHz to daylight. These days you might do things differently if starting from scratch, but there is so much infrastructure designed to generate, carry, and use 5/10 MHz fundamentals that these have become an entrenched standard. If there's a trend away from 5/10 MHz, it seems to be towards 100 MHz fundamental distribution. -- john _______________________________________________ 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.