[time-nuts] A silly question ...

Dana Whitlow k8yumdoober at gmail.com
Fri Sep 28 11:30:26 EDT 2018


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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