PART 2 - LET'S MAKE SOME MEASUREMENTS
DC voltages, AC voltages, time period and frequency
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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.
NOTE ON LIMITED BANDWIDTH SCOPES.
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calibration of the scope is no longer valid.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.
!!! IF POSSIBLE, SPEND THE MONEY TO GET A GOOD SCOPE PROBE AND MAKE
MEASUREMENTS WITH A GOOD GROUND CONNECTION TO GET THE MOST OUT OF
THE BANDWIDTH YOU HAVE.
BASIC MEASUREMENTS.
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
divisions.
DC VOLTAGES.
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.
AC VOLTAGES.
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.
AC FREQUENCY MEASUREMENT.
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.
END OF PART 2
72, Paul Harden, NA5N
NA5N@Rt66.com
To part 3Many thanks to Paul for allowing me to use his work
Frank G3YCC