[time-nuts] A silly question ...
time at radio.sent.com
Fri Sep 28 19:22:55 EDT 2018
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:
> 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
> <time at radio.sent.com> 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
>> 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.
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