by Paul Harden, NA5N


         DC voltages, AC voltages, time period and frequency


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.  


Today's scopes have 500MHz bandwidths or higher.  Likely your scope is

much less than that.  A limited bandwidth scope is still very useful

to the QRPer.  Say the bandwidth of your scope is 5MHz.  This does not

mean you can't see a 7MHz (40M) signal ... it just means that the

calibration of the scope is no longer valid.  The peak-to-peak value

of the display is not correct and much smaller than it really is, and

the sweep rate may be in error.  But still, you may likely beable to

resolve individual cycles higher than the cited bandwidth to a certain

degree, and make the gain and phase measurements that follow (since

they are based on RATIOS).

Most of the examples in this article explore many regions of a QRP

rig without the benefit of any great bandwidth.  Experiment with your

scope to learn its limitations.





It is assumed you have your scope relatively calibrated as described

in Part 1, and familiar with the front panel controls.  For the sake

of the following discussions (since illustrations can not be

included), it is assumed the scope has 4 vertical and 4 horizontal



Say you want to check the T-R switch (Transmit-Receive) in your QRP

rig.  Usually this will be a transistor (or inverter gate, such as in

the 38-Special). The key line goes to the base, which is pulled HI to

some positive voltage (on key UP), and goes LO to ground when the key

is DOWN (or closed).

Setup your scope for DC voltage at 2v/div. and a slow sweep speed

(say 100mS/div).  Set the trace so the bottom graticle (division line)

is 0v.  Place the scope lead on the T-R switch transistor base.  Say

the trace deflects two divisions.  This would be 4vdc bias on the

base.  Now close the morse code key.  The trace should drop to 0v.

The purpose of the T-R switch is to generate a POSITIVE voltage on

key down, which is taken from either the collector or the emitter

(depending upon the circuit configuration).  Say it comes off the

emitter.  Move the scope probe to the emitter.  Now you should have

about 0v with the key UP, and with the key DOWN, the voltage should

jump to some positive voltage, often +12v.  In this case, the trace

will go off the top of the screen.  Change the scope to 5v/div.

Re-verify that 0v is the bottom graticle.  On key DOWN the trace

jumps up 2 divisions.  The key DOWN voltage is thus +10v.  If the

emitter is "stuck" at +10v on both key up and down, the transistor

is not switching.  If the base signal above is correct, then likely

the transistor is bad.  

While this test could be done with a DVM, the integration time is

slow requiring long key downs to get the proper voltage.  A scope

will also show you how clean the switching is, or if there is an AC

voltage (or RF noise) riding on the T-R voltage.

Scopes are thus good DC voltmeters, with about a 5% reading accuracy.


Here is where a scope pays for itself by making AC voltage (and

frequency) measurements.  You must remember that AC voltage displayed

on a scope is PEAK-TO-PEAK VOLTAGE, while a voltmeter or DVM measures

AC voltage in RMS (root mean square).  RMS voltages read on a DVM will

be ABOUT 1/3rd the peak-to-peak voltage (Vpp) shown on a scope.

Or specifically,

    Vrms = .707 x Vpeak = 0.5(.707 x Vpp) = .35 x Vpp

If the signal on your scope      -----|--**--|------|--**--|------|

looks like that in the quasi-         | *  * |      | *  * |      |

illustration, at 2V/division,         |*    *|      |*    *|      |

then the signal would be              *----- * -----*------*------*

4V peak-to-peak (4Vpp), or            |      |*    *|      |*    *|

1.4Vrms if read on a DVM or           |      | *  * |      | *  * |

voltmeter.                       -----|------|--**--|------|--**--|

                                      VERT: 2V/DIVISION

For example, let's measure the output voltage and frequency of the

sidetone oscillator in your QRP rig.  Setup the scope for 1v/div, AC

volts, and a sweep speed of 1mS/div.  Connect the scope probe to the

audio output of your rig and set the volume control on key DOWN so the

audio sinewave is 2 division peak-to-peak.  This would then be 2Vpp

AC, and should look similar to the illustration above.


With this same waveform, we might as well see what frequency our

sidetone or transmit-offset is at.  Most operators prefer the side-

tone to be about 700Hz.  With the same setup as above, trigger the

scope for a stable waveform and the time base sweep to display 2 or

3 cycles.  Center the waveform on the center horizontal graticle so

the sinewave goes one division above, and one division below the

center graticle.  Now move the HOR POSition so the first "zero

crossing" of the sine wave is on the first or second vertical

graticle.  With this setup, zero-crossing would be where the sine

wave crosses the center horizontal graticle.  Now measure the time

it takes to make one complete sine wave, from one zero-crossing

(sine wave going positive) to the next positive going zero crossing.

Say one complete sine wave takes one and half horizontal divisions.

At 1.0mS/div., this would be 1.5mS per cycle.  Frequency is the

reciprocal of time, such that the sidetone frequency is:

    f = 1/t = 1/1.5mS = 667 Hz

(Sidetone frequency is the tone heard on key DOWN).

This may be a little low to your liking.  To raise it to 700Hz,

calculate the period of 700Hz, which is t = 1/f = 1/700 = 1.4mS.

At 1.0mS/div, you can adjust your XMIT OFFSET on key down until 

zero-crossings (or the positive peaks) are 1.4 divisions apart.  

This will be 700 Hz.  (The XMIT OFFSET is not adjustable in all

rigs ... such as the 38-Special.  In this case, it usually

requires changing the value of a capacitor on the XMIT MIXER and

usually discussed in the instruction manual).

QUALITY OF THE WAVEFORM is another feature of a scope that is

unsurpassed, since your are "seeing" the waveform in real time.

For example, say the audio output from your QRP rig is not a

clean sine wave, that is, it has a slant to it, or the rise time

takes longer than the fall time.  This could be due to improper

time constant on the audio amplifier coupling capacitors or

improperly biased amplifiers.  Or, say the audio output sine wave

is flattened at the top, looking sorta like a square wave then a

sine wave.  This would be a raspy sounding sidetone, and due to

the audio power amplifier being overdriven and in gain compression

(clipping).  You should beable to see this effect by turning the

volume control to its maximum level, overloading the output audio 

amplifier (unless your QRP rig has anemic audio like some).

The o-scope is an invaluable tool for detecting and diagnosing

such distortions and impurities in the signal quality.  The audio

output of a QRP rig, whether the sidetone or an on-the-air signal,

should be a fairly pure sine wave.  If not, something is wrong,

from a poor product detector action, poor filtering after the

product detector, poor coupling capacitors, or severe noise being

introduced into the audio at some point.

By tuning in an on-the-air signal and plotting the Vpp at different

audio frequencies, you can plot the filter response of your QRP

rig.  This will be discussed in detail in a later section.  You can

also adjust your BFO on the product detector for the maximum Vpp of

the received signal for centering the signal in the filter passband.

Note how these important tests and adjustments (sidetone, filter

response and setting the BFO) are a few of the things that can be

done on a scope with a very limited bandwidth ... since you're not

looking at anything beyond the audio range.

Once you get comfortable making the above voltage, time and frequency

measurements, you might want to go through your QRP kit with the

schematic and record the various DC and AC voltages and waveforms at

pertinent locations in the circuit.  This will be a great aid in the

future should your rig develop a problem.  NOTE however, that signal

levels from the receive mixer through the IF crystal filters are VERY

weak and can not not be seen on even an excellent scope.  The main

exception to this would be the local oscillator (LO) drive on pins

6 and 7 on a NE602.  They are usually in the order of 100mVpp.

!!! NOTE: You can't hurt anything by probing around the circuit of

    your QRP rig.  The biggest mistake made by beginners is to let

    the ground lead come loose and drag along the tops of components,

    which can short out the power supply or damage a component ... 

    OR when putting the scope probe on an IC pin, to slip and let

    the probe touch two pins at once.  This will short out the two

    pins, which in some cases, could cause damage to the IC.  For

    example, on a NE602, measuring the Vcc voltage on pin 8.  A slip

    to Pin 7 (the OSC input) could destroy the internal oscillator

    if the pin 8 to 7 short persisted a second or two.


72, Paul Harden, NA5N


To part 3

Many thanks to Paul for allowing me to use his work

Frank G3YCC

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