In addition to nonlinear issues with output amplifiers, filters have poor performance when improperly terminated. This can lead to harmonic distortion and that can be a problem. You want the duty cycle to be exactly 50%. See:https://tf.boulder.nist.gov/general/pdf/1437.pdf -- Bill Byrom N5BB
On Fri, Sep 28, 2018, at 10:30 AM, Dana Whitlow wrote: > Hi, > > There is one other issue that can bite you if you fail to properly > terminate the output of a source: > > Depending on the source's design, an essentially unloaded output > can have a substantially higher voltage swing than expected (by > 2X if the source impedance is actually 50 ohms), possibly leading > to the output stage's going into clipping, which can in turn distort > the timing, possibly even in an unstable manner. > So if you want to play the "unterminated game", at least take a > look at the waveform to be sure it's still a clean sinewave. I've > noticed such distortion on my PRS-10, for example, although I've > seen no evidence of unstable timing results. But in this arena, > it generally pays to be fussy. > > Dana Whitlow > > On Fri, Sep 28, 2018 at 7:06 AM Bill Byrom > <[email protected]> wrote:> >> On Thu, Sep 27, 2018, at 11:55 AM, Dave B via time-nuts wrote: >>> Triggering a dual beam 'scope (Tek 465) from the TB on Ch1, and >>> having>>> the output of the OCXO on Ch2, the resulting display on Ch2 of >> course>>> drifts in relation to the static waveform on Ch1. (Both nice >>> sinusoids.) >> The Tek 465 analog cathode ray oscilloscope was/is a very flexible >> instrument. But this flexibility allows you to set up the >> instrument in>> ways which will not allow this commonly used oscillator >> comparison >> technique to work correctly. Since you are interested in these >> instruments, here are some details about setting up the >> instrument for>> such comparisons. >> (1) The Tek 465 is not a dual beam oscilloscope. Dual beam >> oscilloscopes>> (such as the Tektronix 556 and 7844) use a special CRT >> which >> incorporates two independent electron guns. Each electron gun >> assembly has a set of vertical and horizontal deflection plates. >> There are two vertical amplifiers (one for each electron gun) and >> two horizontal sweep systems (one for each electron gun). If >> you had>> a dual beam oscilloscope you could compare oscillator#1 to >> oscillator#2 while simultaneously comparing oscillator#3 with >> oscillator#4. It's like having two independent oscilloscopes >> sharing>> the same CRT display. >> (2) The Tek 465 single beam oscilloscope can display two traces on >> the>> display using one of two methods:(a) Chopped trace display: >> This mode>> works well at low sweep rates (such >> as 1 ms/div) but causes trouble at fast sweep rates (such as 1 >> us/div). The displayed trace is switched between Channel 1 and >> Channel 2 at a fixed rate of about 500 kHz.(b) Alternate trace >> display: This mode works well at high sweep rates >> but is hard to see at low sweep rates. The scope alternates between>> >> displaying one sweep of Channel 1 and one sweep of Channel 2. >> (3) The trigger source setting is crucial to using this technique to>> >> compare oscillators. The technique does not require you to display>> two >> channels. What is important is that you display one oscillator>> while >> triggering on the other oscillator. The trigger source can >> be set to:(a) CH 1: The Channel 2 display will drift if the two >> signals have a >> varying phase relationship.(b) CH 2: The Channel 1 display will >> drift>> if the two signals have a >> varying phase relationship.(c) NORM (normal): The trigger >> system gets>> input from the channel being >> displayed at that moment. So in chopped trace display mode the >> trigger is rapidly switched between CH1 and CH2, and in alternate >> trace display mode the trigger alternates between CH1 and CH2 on >> alternate sweeps. In all cases, you should not use NORM trigger >> source with both channels displayed when comparing oscillators!(d)>> EXT: >> You apply the trigger signal to the external trigger input >> connector. This works well well when comparing oscillators. If you>> use >> alternate trace display mode and an external trigger, you can >> compare oscillator#1 (on CH 1) to oscillator#0 (on the external >> trigger input) while you are also comparing oscillator#2 (on CH2) >> oscillator#0. So you could compare two oscillators (one on CH1 and>> the >> other on CH2) to a GPSDO (on the external trigger input). >> (4) When comparing oscillators, the fractional frequency difference >> (such as ppm Parts Per Million or ppb Parts Per Billion) you can >> measure depends on the oscilloscope sweep rate. What you are really>> >> measuring is the drift of the time delay between the edge (or zero>> >> crossing of a sine wave) of one signal relative to an edge or zero>> >> crossing of another signal. The relationship is: >> Fractional difference = (observed timing change) / (measurement >> interval)Here are some examples: >> Fractional difference in ppm = (time delay drift in us) per second of>> >> observation timeFractional difference in ppb = (time delay >> drift in ns)>> per second of >> observation time >> (5) As you can see in my previous section, you need a very fast sweep>> >> rate (small time/div) to measure small fractional frequency >> differences. This means that for a small fractional frequency >> difference with a moderately low measured oscillator frequency >> (such>> as 1 MHz), you may not see any edges for a long time when you >> use a>> small time/div. The Tek 465 has a delayed timebase, and you can >> use>> this feature to move the signal edge (or zero crossing) onto the >> screen. You can then watch the signal for a few seconds to >> determine>> the timing drift rate. If the edge is drifting at 10 ns per >> 10 >> seconds, the fractional difference is 1 ppb (1 part in 10^9). >> If the>> displayed oscillator edge is drifting to the left (earlier in >> time),>> the displayed oscillator frequency is higher than the reference >> oscillator you are using for the trigger. If the displayed >> oscillator edge is drifting to the right (later in time), the >> displayed oscillator frequency is lower than the reference >> oscillator you are using for the trigger. >> (6) If the edge rate is not very fast (such as when you are measuring>> >> sinewave signals), the waveform edge you see at a fast sweep rate >> will appear to be nearly horizontal (spread out across many >> divisions). You normally want to measure the displayed signal >> at the>> midpoint of the peak to peak voltage swing. For a sinewave >> this will>> be the zero crossing, and for a square wave this will be the >> 50% >> point on the edges. You can get better resolution on >> determining the>> edge timing by increasing the vertical gain (reducing >> the >> volts/div)>> setting on the oscilloscope. But you probably only want to >> increase>> the gain so the signal is off the screen by a factor of 2 to 5, >> because too much gain may result in overdrive recovery problems in>> the >> vertical amplifier. The trigger signal (on a display channel or>> external >> trigger input) gain should also be increased to get lower>> jitter >> triggering. >> (7) The Tek 465 input impedance (of CH1, CH2, and the external >> trigger>> input) is 1 M ohm in parallel with about 20 pF. If you are >> using 50>> ohm cables, it's best to use 50 ohm feedthrough terminators on >> the>> two connectors to which the oscillators are connected. With low >> frequency (no higher than around 10 MHz) sinewave sources a lack of>> >> proper termination doesn't cause many problems, but if a signal has>> fast >> edges (small values of risetime/falltime) an improper or >> missing termination can result in reflections. This can cause >> distortions in the waveform near the rising and falling edges which>> >> add jitter and cause unstable triggering of the scope. So it's good>> >> engineering practice to properly terminate the cables at the >> oscilloscope BNC connectors.-- >> Bill Byrom N5BB >> Tektronix Application Engineer for past 31 years. >> First used the Tek 465 about 42 year ago. >> >> _________________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _________________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com> and follow > the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
