[time-nuts] A silly question ...

Bill Byrom time at radio.sent.com
Fri Sep 28 08:04:45 EDT 2018


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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