[time-nuts] A silly question ...

Bill Byrom 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:
> 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
> <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
>>> 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.
>> 
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