PC-Based Oscilloscopes and RF Waveform Monitoring

1. Introduction:

2. System Setup:

Start the PicoScope and WaveNode software. Connect the RFView port(s) on your sensor to the channel(s) inputs on the PicoScope with coax. You may need to insert an isolation transformer if you have a DC voltage potential between the sensor and the oscilloscope. These small ground isolation units are exceptionally good for decoupling the grounds between the PicoScope inputs and other station equipment. If required, the TISO-1  ground isolation units are available from WaveNode (shown in Fig #2 as the small boxes at the PicoScope inputs). All Spectrum analyzer views have the averaging feature turned on from the Options menu in the oscilloscope software.

The RFView port uses a toroid coupler to sample the RF current in the transmission coax line. The coax current is converted to a voltage that is sampled at the RFView port. A 6-foot section of small 50-ohm coax connects the RFView port to the oscilloscope inputs. The signal output at the RFView port is about 100 millivolt Peak-Peak with 100 watts forward power. The isolation transformers step up this signal to 150 millivolt.

The PicoScope Model 3206 provides oscilloscope and spectrum analyzer capability with adequate bandwidth to monitor all HF frequencies. The specified sampling rate of this unit is 200 megasample/sec and the memory buffer is 1 Million samples. The deep memory buffer is especially useful when viewing the modulation waveform of the RF. This allows viewing the modulation waveform without any detection circuitry to capture the RF amplitude waveform. The deep memory buffer is a feature typical of high-end dedicated oscilloscopes costing far more than this unit.

Start your setup with both Oscilloscope and Spectrum Analyzer windows opened, and observe a continuous RF output on 7.100 mHz. Figure #3 shows the expected response. Figure #2 shows the WaveNode indicated power. Note both Peak and average powers are equal when transmitting a continuous, un-modulated output.

The same transmitter has now been set to SSB mode. The modulation is a normal speech pattern. Notice the PC oscilloscope allows the modulation envelope to be monitored, and the oscilloscope memory buffer is deep enough to sample the waveform fast enough to capture the RF envelope even when the horizontal timebase has been reduced to 2 millisec/div. This is very important, and highlights the importance of high sampling rate (200mHz) combined with deep memory buffer. Figure #4 shows the indicated WaveNode peak and average power levels.

Figure #5 shows the same waveform as viewed on the PC Oscilloscope. Notice that the average power is roughly 20% of the peak power in the RF envelope. This is fairly typical of SSB operation, and varies depending on speech pattern and compression of the audio signal before the final amplifier. The WaveNode peak/hold detection circuit is designed with ~ 50 microsecond risetime, which is more than adequate to capture voice peaks, ALC time constant problems and key clicks.

Figure #6 is typical of CW operation. The entire Windows screen is shown in this view so the reader can view how the wattmeter and oscilloscope can be organized on the screen. Note the “clean” rise and fall of the leading and trailing edge of each bit. This is very important to reduce key clicks and splatter, which will make the signal occupy unnecessary bandwidth.

Also, notice the amplitude of the signal is maximum at ~1.8millisec, after which the signal is reduced to a steady-state level. This is the action of the ALC circuitry in the transmitter, and serves to protect the final amplifier to a safe output level.  The ALC time constant is approximately 1 millisecond, also to prevent splatter and excessive transmit bandwidth.

There are points of 0 power, indicating no hum or carrier leakage from the SSB exciter. The operator should not infer that splatter due to Intermodulation components is not present. The spectrum analyzer does not have sufficient resolution to view intermodulation components, but harmonic energy out of the linear amplifier is viewed. This is a good indicator that no out-of-band radiated energy is within good operating practices. High 2 harmonic energy (~14 MHz in these views) might indicate a damaged or weak side of a Class AB Push-Pull amplifier. Higher 3 harmonic energy can indicate overdriving of the amplifier input.

In Figure #11, the Picoscope capture oscilloscope and spectrum analyser data for two coax sensors. In this configuration, one sensor is at the input of a linear amplifier, and the second sensor is monitoring the amplifier output. The blue trace is the input, the red is the output. The transmission mode is SSB on 7.250 MHz.

Correlating what the operator views on the oscilloscope with on-air signal reports can provide a lot of information about the transmitted signal quality. Make hardcopy prints of your signals on the oscilloscope and keep them for reference later when problems are suspected.

Impedance = Z = Ei/Ii

VSWR = E max / E min = (1 + |P|) / (1 - |P|)

where P = reflection coefficient = (Z - Z0) / (Z + Z0)

Return Loss in dB = 20 log |P|

2. VSWR = ZL / ZO or ZO / ZL for resistive loads only.

% Modulation = (( PEP - PC) / PC) *100%

Carrier Power PC = 100 W, PEP = 400 W.

Figure #14. Definition of Saturation Curve