Use an Oscilloscope, RF Demodulator and RF Sampler to Check Exciter and Amplifier Modulation for Low Distortion and Near Perfect Amplifier and System Linearity Wipe the dust off of that old scope... Use it... You need it !!
First of all, let me state right up front that I am certainly no expert on measuring the complex components of a modulated RF signal. The following paragraphs are suggestions based on the information and practice that are tried and true for many.
One thing is for sure; if you take a little time to set up your scope properly, and you can achieve a decent pattern with the tests that follow, the likelihood of being overmodulated or out of linearity will greatly be reduced! Additionally, the chances of your 3rd and 5th order IMD (Inter-Modulation Distortion) products will be lower as well.
Your SSB / AM audio modulation envelope and amplifier / system linearity should be greatly improved if you can adjust your equipment to produce the classic scope patterns for the type of modulation being used. The RF Sampler and RF Demodulator circuits discussed in the writing are valuable tools when used with your oscilloscope.
May I also say that as far as I am concerned, learning how to use an oscilloscope to measure your modulation quality should be required learning for any amateur radio license, regardless of mode or frequency! An oscilloscope is an instantaneous RF measuring tool that can show details about a signal that a mere watt-meter will NEVER show. So, dust off that old scope and let's get started...
The following tests and procedures were made available courtesy of CleanRF Systems and are based on the excellent "Splatter View" combination (seen below) consisting of an RF-Demodulator and RF-Sampler that compares both pre and post linear amplifier signals as viewed on your oscilloscope. For details on the "Splatter View" visithttp://www.cleanrf.com/.
Identifying the problem:
The possibility of your 3rd and 5th order IMD (Inter-Modulation Distortion) products will be exponentially reduced; assuring that your occupied bandwidth will be directly proportional to your transmitter's audio passband, not a mistuned nonlinear amplifier.
All oscilloscopes represent a voltage displayed visually near the speed of light in real time. When used in conjunction with the model RF-D and series RF-S, great details about your signal can be acquired that a mere watt meter would never be able to display!
Note 1: Your oscilloscope's ability to measure RF will be limited by the vertical amplifier's maximum bandwidth. A 30 MHz oscilloscope or higher is therefore recommended to easily cover the entire HF radio frequency spectrum.
APPLICATION #1
Step 1 - Making the Connections:
To interface the "Splatter View" between the exciter, amplifier and antenna, use the high quality UHF male to male connectors and shielded jumper cables provided.
Connect the model RF-D (RF Demodulator) directly to the output of the transmitter, via the UHF male to male connector provided. (Choose either SO-239 connector as they are bi-directional)
Connect the series RF-S (Variable RF Sampler) directly to the output of the amplifier, via the UHF male to male connector provided. (Choose either SO-239 connector as they are bi-directional)
Connect one of the BNC ends of your 6’ jumper cable to the BNC connector on the model RF-D (RF Demodulator) and the other end to the Horizontal (X) input of your oscilloscope.
Connect one of the BNC ends of your 6’ jumper cable to the BNC connector on the series RF-S (Variable RF Sampler) and the other end to the Vertical (Y) input of your oscilloscope.
See the wiring illustration "Figure 1" below.
Figure 1
Note 2: The RF Demodulator's 1/4" TRS jack may be used as an AM audio modulation monitor!
Use with stereo headphones or stereo line-level unbalanced input to your mixer or stereo amplifier.
(1/4" TRS stereo plug required for line out application)
Step 2 - Initial Oscilloscope Setup:
Next, find the controls on your oscilloscope labeled "Position" for both the Horizontal and Vertical axis. Adjust the Horizontal Position control so that the horizontal sweep trace is centered horizontally. Adjust the Vertical Position control so that the horizontal sweep trace is centered vertically.
Focusing may be required. We will refer to this as establishing your "Base Line". See "Figure 2" below.
Figure 2
Establishing Initial Baseline Adjustment
Now, set your oscilloscope's controls for the following:
- Vertical (Y) mode selector to channel 2
- Vertical (Y) voltage scale 2v / Div for starters
- Both Horizontal (X) and Vertical (Y) channel coupling selectors to "DC" coupling
1 - Setting Oscilloscope for Envelope Monitoring
- Turn off channel 1 (X) by moving coupling selector to the grounded position.
- Turn time sweep control for 2mS / Div for starters.
- Turn Trigger section to (X) channel.
- An added advantage of using the trigger selection, is the ability to synchronize your horizontal modulation envelope sweep, regardless of changing voice or data modulated frequencies, in either SSB or AM envelope monitoring.
2 - Calibrating Oscilloscope and Power Output for Envelope Mode:
Tune up your exciter (transmitter) if necessary, and then your amplifier to its rated output within legal limit operation (1500w PEP). (The linearity tests discussed later in "Application 3" will determine if you tuned your amplifier properly)
With a continuous carrier established at the desired power level, adjust the "Vertical (Y) Voltage Gain" control on your oscilloscope so that the centered horizontal sweep trace expands 2 divisions above and below the baseline established in "Figure 2" on the oscilloscope's display. You should be able to make this adjustment within 10 seconds. Calibration is now complete and you can proceed to the mode you will be using (SSB or AM). See "Figure 3" below.
Figure 3
Oscilloscope Calibrated for Full Carrier Power Used
SSB Modulation Envelope Pattern:
When using the SSB or AM modes, adjust your audio gain so that your RF envelope peaks expand to the established plus and minus 2 divisions previously calibrated for, shown in "Figure 3". A well modulated SSB modulation envelope will look like "Figure 4" and an overmodulated SSB pattern is also represented in "Figure 5", both shown below.
Figure 4
Proper SSB Voice Envelope Pattern at Full Modulation / Drive Power
Figure 5
Incorrect SSB Voice Envelope Pattern Showing Overmodulation / Drive Power
AM Modulation Envelope Pattern:
When using AM, first calibrate an unmodulated carrier to the plus and minus 1 division level shown in "Figure 6". To achieve a fully 100% modulated envelope, adjust your audio gain so that the envelopes expand 2 divisions above and below the baseline. See "Figure 8" below. This would indicate full PEP modulation at 4 times the unmodulated carrier level. See "Figures 6, 7, 8, 9a and 9b" below.
Figure 6
AM Unmodulated Carrier Calibration
Figure 7
Undermodulated AM Envelope
Figure 8
100% Modulated Symmetrical AM Envelope
Figure 9a
Overmodulated "Low-Level" AM Envelope (Balanced Modulator)
Figure 9b
Overmodulated "High-Level" AM Envelope (Plate Modulation)
Figure 9c
AM Asymmetry - Correct Phase (Pos 110%, Neg 95%)
Figure 9d
AM Asymmetry - Inncorect Phase (Pos 95%, Neg 110%)
APPLICATION #2
Using Your Oscilloscope as an Instantaneous Peak Reading Power Meter:
Using the outlined techniques so far, you also have an excellent instantaneous peak reading power meter. If, for example, your initial adjustment on your oscilloscope was relatively based on 500 watts (measured with an existing in line meter) at the plus and minus 2 division marks on your oscilloscope, then the SSB or AM modulation envelope, as observed above, will be exactly 500 watts PEP whenever the peaks reach these divisions or any relative power level observed and set with this method.
This same method can be used with trapezoid linearity monitoring as well.
Refer to "Application 3" below.
APPLICATION #3
Trapezoidal Pattern Linearity Signal Monitor System Self Diagnostic Test:With this method, two sources will be used to feed both the Horizontal (X) and Vertical (Y) inputs of the oscilloscope, simultaneously comparing the output signal of the transmitter with the output signal of the linear amplifier.
!!!WARNING: THE MODEL RF-D (RF DEMODULATOR) MUST BE CONNECTED TO THE TRANSMITTER'S OUTPUT - NEVER TO THE AMPLIFIER'S OUTPUT. FAILURE TO COMPLY WILL DESTROY THE DEMODULATOR AND VOID YOUR WARRANTY!!!
At this time, set your oscilloscope controls as follows:
Set oscilloscope to "XY" mode. (No horizontal sweep, small focused dot will appear - See "Note 3" below)
Most oscilloscopes will have an "XY" button. Others may provide the "XY mode" via the "A and B Sec/Div" knob. Some older oscilloscopes will not have an "XY mode" selection at all because the "X" input is the "External Horizontal Sync" input. When this is the case, simply select the "External" position on the oscilloscope's "Sweep Range" selector.
Vertical voltage scale 2v / Div for starters. - See "Note 3" below.
Both Horizontal (X) and Vertical (Y) channel coupling selectors to "DC" coupling.
Note 3: Move the focused "dot" to the far left side of the screen, then while applying voice or data modulation, adjust the voltage selection controls so that the trapezoidal pattern fills half of the screen horizontally, and plus / minus two divisions vertically.
The Horizontal (X) input of the oscilloscope will be fed by the model RF-D (RF Demodulator) via the transmitter.
The Vertical (Y) input of the oscilloscope will be fed by the series RF-S (Variable RF Sampler) via the linear amplifier.
System Self Diagnostic Test:
With your amplifier turned off or in stand by, transmit voice or data and check to see that produced is a perfect trapezoidal pattern. This is a self diagnostic test, and you must see a perfect trapezoidal pattern as you are comparing the original signal to itself. If the trapezoid looks like "Figure 10" below, then you are ready to turn your amplifier on and in the operating position to check for its linearity. If you did not produce a perfect trapezoid with the amplifier in the off or standby position, then you should contact me directly.
Adjustment of voltage scale will be required to achieve resolution - See "Note 3" above.
If your amplifier is linear, the oscilloscope pattern will look like a near perfect trapezoid (Sideways Elongated Triangle) with sharp and distinct angles during voice or data transmissions. See "Figure 10" below.
Figure 10
Linear Trapezoid Modulation Pattern
If your amplifier has a linearity issue, the trapezoid's widest end, opposite the point, will be flattened on the ends, or some other property of the trapezoid will be distorted with a lack of distinct angles.
See "Figures 11, 12 and 13" below.
Figure 11
Nonlinear Trapezoid Modulation Pattern
Figure 12
Nonlinear Trapezoid Modulation Pattern
Figure 13
Nonlinear Trapezoid Modulation Pattern
More often than not, a nonlinear trapezoidal pattern is the result of a mistuned amplifier, particularly with the load control being set with insufficient loading. To test this, simply advance the load control slightly. With more loading of the amplifier, you will more than likely see the widest side of the trapezoid edges become nice and crisp as seen in "Figure 10" above.
If the resulting patterns displayed on the oscilloscope are well formed, your unintelligible distortion products will be significantly reduced, resulting in a one to one ratio between your audio frequency and radio frequency bandwidth. This is a step in the right direction and is more than most hams ever attempt to do in analyzing their on-air signal purity.
Conclusion
Even though these scope measuring techniques will not be as complete in determining IMD, THD and more complex forms of distortion as a true RF Spectrum analyzer, if the resulting patterns displayed on the scope are well formed, your distortion products will more than likely be well within FCC specs and guidelines for good engineering practice. It is a step in the right direction in solving common distortion problems and is more than most hams ever attempt to do in analyzing their on-air signal purity.
Have fun, and may you find "Scoping Out Your Audio" to get the full scope of things as rewarding and educational as I have...
