by Paul Harden, NA5N



NOTE: This is a text version of an article appearing in the Summer

1997 issue of "QRPp."  The article contains numerous illustrations and 

photos of oscilloscopes displays, which unfortunately can not be 

included in a text file.  


Phase relationships between two signals at the same frequency can be

measured with 2-5 degree accuracy with a scope, although more suited

for a dual-trace scope.  The REFERENCE signal is applied to CH. 1 and

the signal to be phase measured to CH. 2.  For proper phase measure-

ments, ensure your dual trace display is in the CHOPPED mode, not

ALTERNATE mode.  (Alternate mode can effect the triggering position

for the second, or CH.2 sweep).

There are many methods to do this.  One is to stretch out the signal

so it takes 4 full divisions, so each division is 90 degrees of

phase.  By measuring from a common point on one signal to the other

(zero crossings or from the peaks), the phase can be measured.

For example, say you are making a phased array antenna system, in

which one feedline must cause a 90 deg. delay.  You calculate the

electrical length for a quarter wavelength [L=(246/f) x velocity

factor] and cut the coax to that length.  You are now working on

blind faith that you have exactly 90 degrees.  With a scope, you can

measure it fairly accurately by injecting a signal into one end with

a signal generator (at the frequency of interest) and a 50-ohm load

on the other.  Connect the scope CH.1 to the coax input (signal

generator end) and CH.2 to the load end and measure the phase.  

Trigger the scope and move the horizontal position and/or the time

base vernier so the positive peak of the CH.1 sinewave is on the first

vertical graticle line and the

second positive peak is on the        :         :     +117 degree        

fourth vertical graticle, as      --->:         :<--- Phase Shift

shown in the illustration to          :         :     based on PEAKS

the right.  Now measure the           **-----|------|------|-----**

phase by noting where the             |  *   |  :   |      |   *  |

first positive peak on CH.2           |    * |  :   |      | *    |

occurs.  Say it occurs about     CH.1-|------*------|------*------|

1.3 divisions to the RIGHT of         |      | *:   |    * |      |

the CH.1 positive peak. Since         |      |  :*  |  *   |      |

one division is 90 degrees,           |------|-@@@-***-----|------|

using this method, then               |     @|  : @ |      |      |

1.3 div. x 90 deg. = 117 deg.         |   @  |      @      |      |

YOUR DELAY LINE IS TOO LONG!     CH.2-|-@----|------|-@----|------|

Cut off an inch or two at a           @      |      |   @  |      |

time until the CH.2 peak is           |      |      |     @|     @|

one division from the CH.1            |------|------|------|-@@@--|

peak (or on the 2nd vertical           DUAL-TRACE PHASE MEASUREMENT

graticle as shown in the

illustration) for precise tuning of the delay line.

Another method is to make the CH.1 signal to be two divisions high,

and center it between the two divisions, such that the zero-crossing

points are on the middle graticle line.  Where the CH.1 sinewave 

signal crosses zero going positive is the 0 deg. REFERENCE; the 

positive peak is 90 deg.; the 

negative going zero crossing is

180 degrees, etc.  For CH.2 to be        0   90 degrees

90 degrees delayed from CH.1, the        :     :

CH.2 sinewave should cross zero,         :     :

going positive, right under the          |---**|-----|-----**----|

90 degree peak of the CH.1 signal.       |  *  *     |    *| *   |

If the CH.2 zero crossing is farther     | *   |*    |   * |  *  |

to the right from the CH.1 positive      |Z----|-Z---|--Z--|---Z-|

peak, the phase shift is MORE than       *     |  *  | *   |    *|

90 degrees.  Back to the example of      |     |   * |*    |     *

the coaxial delay line, you would        |-----|----**-----|-----|

cut an inch or two at a time until        Z=Zero-crossing points

the CH.2 zero crossing is directly

underneat the CH.1 positive peak

(the 90 degree point).

And still yet another method of comparing the phase between two

signals on a dual-trace scope is to accurately measure the period

it takes for one complete sine wave on the CH.1 reference channel.

Say it is 140nS (that would be 7.14 MHz, by the way).  Now say the

CH.2 signal is 50nS delayed from the CH.1 signal.  The phase shift

would be:

    Phase = 50ns/140ns x 360 degrees = 129 degrees


One thing you must remember is how to "read" phase shifts on an

oscope.  When comparing two signals as described above, remember

that if the CH.2 signal peak is to the RIGHT of the CH.1 peak, then

the CH.2 signal is OCCURING LATER IN TIME than the CH.1 signal,

because time is traveling from left to right.  If the CH.2 peak is

say 90 degrees to the LEFT of the CH.1 peak, then the CH.2 signal

occured in time BEFORE the CH.1 signal.  This would then be a -90

degree phase shift, or 270 degrees.  Think about this carefully

before you start cutting the coax on that delay line!


Phase measurements can be made on a single trace scope as well.

First, connect the REFERENCE signal, using a BNC "T", to both the

VERTICAL INPUT to the scope and the EXTERNAL TRIGGER and select

EXTERNAL as the trigger SOURCE.  Adjust the TRIGGER LEVEL so the zero-

crossing occurs at the beginning of the trace on the first vertical

graticle.  Now remove the reference signal from the scope's vertical 

input (but NOT the external trigger input) and connect the signal to 

be phase measured to the vertical input ... WITHOUT altering the time 

base or trigger level.  The sinewave of the signal to be tested should 

be on the CRT, with the trace being triggered from the external

trigger input, or the reference signal.

The sinewave now on the CRT      

likely will not have it's zero-          |----**-----|-----|**---|

crossing starting at the first           |   * |*    |     *  *  |

vertical graticle as the reference       | :*  | *   |   :*|   * |

signal did, but some place else.         |-*---|--*--|---*-|----*|

On the illustration to the right,        |*:   |   * |  *: |     *

the zero crossing occurs about           * :   |    *| * : |     |

0.3 divisions to the RIGHT.  This        |-----|-----**----|-----|

can now be converted to the phase        : :             :      

angle in degrees.  In the             -->: :<--.3 div.   :

illustration, one complete cycle           :<----------->: 2.5 div.

takes 2.5 divisions, and the phase

delay from the reference is 0.3 div.         SINGLE TRACE SCOPE

The phase shift is therefore:                PHASE MEASUREMENT

  Phase shift = 0.3 div/2.5 div. x 360 

              = 0.12 x 360 = 43 degrees

The SINGLE-TRACE scope method is a little easier to do if you make

the sine wave of the reference to be 4 divisions for one cycle, thus

making 90 degrees per division.  The phase angle can be guestimated

a little quicker with the signal to be phase measured.

It is noteworth to mention that the above examples, measuring the

phase through a delay line at 7 MHz, would require an oscope with a

20MHz or higher bandwidth, if for no other reason, then just assure

that the time base is fast enough to display 1 or 2 sinewave cycles

on the CRT.  If at your fastest sweep speed, the 7MHz signal is

displayed as many cycles, then obviously the accuracy that you can

determine the phase angle will be highly degraded.


If your scope has a limited bandwidth of only a few MHz or less,

there are still useful phase measurements that can be performed.

One interesting experiment is to measure the phase shift of the

audio signal at different frequencies as it travels through the

stages in a CW audio filter. This is done by putting the input to

the CW audio filter on CH.1 and the output on CH.2.   What is the

phase shift of the wanted vs. unwanted frequencies?  Recall that an

audio filter works by cancelling out (180 degree phase shift) the

unwanted signals, while re-enforcing (0 degrees) the frequencies you

wish to pass.  There will only be one audio frequency for which

there is a 0 degree phase shift.  This will be the "pole frequency"

of the active filter, or the frequency you wish to have the maximum

gain.  For CW QRP rigs, this should be around 700 Hz.

And finally, on a limited bandwidth scope, the phase angles of higher

frequencies can be determined by applying the reference signal to the

vertical input and the signal to be phase measured on the external

horizontal input.  This will form a lissajous pattern, the angle or

tilt will signify the phase angle.



This is the last part of this series of articles on OSCOPES posted

to QRP-L.  There will be another series on scope measurements posted

in the future (many months) that will include some advanced 

techniques, such as measuring sideband rejection, tuned circuits,

filter responses, group delay, VCO phase noise, etc.  This will be

the contents of part 2 of the oscope article for the Winter QRPp.

I haven't written it yet or made hardcopies of the scope displays.

But following publication in QRPp, I will convert it to a text file

and post it to QRP-L as I did this one.

PS - making those waveform illustrations really sucked swampwater!

72, Paul Harden, NA5N


Many thanks to Paul for allowing me to use his work

Frank G3YCC

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