Remote Education Test Case – Class C Amplifier Characterization

Remote Education Test Case – Class C Amplifier Characterization

The Keysight Smart Bench Essentials is a fantastic combination of test instruments for any educational lab. Collaboration between students and between educational facilities has become standard practice and the global pandemic has heightened the need for remote learning. This test case explores the potential of the Keysight Smart Bench Essentials package for remote learning.

Keysight provides a software package to meet this emerging need, PathWave Lab Operations for Remote Learning. Unfortunately, the remote learning package is not included with the Smart Bench Essentials. Network based access to the instruments is delivered through Keysight Pathwave BenchVue apps and built-in web interfaces, and this access can be extended to remote students, although with challenges.

I expected the web interfaces would provide the best method to access the equipment remotely. As I noted throughout my review, Keysight’s implementation of the web-based instrument control provides an experience very close to sitting at the bench.

I hold a Certified Information Systems Security Professional (CISSP) designation, and my first thoughts went to security – exposing any device to the Internet can be extremely risky and I wanted to avoid a situation where the new equipment could get hacked. Additionally, the instruments do not directly support encrypted connections. To mitigate this, I restricted several ports on my network firewall to allow only one specific external IP address, then temporarily implemented port forwarding on the firewall to advance traffic destined to those ports onward to port 80 of each internal instrument. I then enhanced the intrusion detection on the external ports and changed the device's passwords. While this solution was not perfect, it did limit exposure sufficiently for the test. A more secure, scalable, and encrypted solution would use a Virtual Private Network (VPN), and I certainly would have implemented one if I needed sustainable remote access or had more than one distant student.

Remote testing was successful except for one thing, the oscilloscope’s control software uses Virtual Network Computing (VNC) which requires an additional port. Further, the Keysight implementation of VNC does not use the standard VNC port so I dusted off my network analysis skills to determine which port VNC was listening on and added it to the list (port 5850).

Testing remote access by myself would defeat the purpose of this test, so I reached out to Instructorman who kindly offered to help me with this test case. Mark provided a lab design for a Class C amplifier (with tuned reactive load) which he had developed and taught years ago. The idea is that I would choose a different value for capacitance and Mark would use the Keysight Smart Bench Essentials instruments to determine the value from the comfort of his home. 

Class C amplifiers use a resonant LC load in the collector circuit. They are usually biased in cut off to increase efficiency. The circuit we used had -1.5 VDC of bias on the base relative to the grounded emitter. This bias arrangement keeps the transistor deep in cutoff when there is no input signal present. A transistor in cutoff will not draw appreciable collector current. If a sine wave of sufficient peak amplitude is applied to the input, the transistor can be brought out of cutoff into conduction. While the base-emitter junction is in forward bias, collector current can flow into the tuned tank circuit. The idea is to have the transistor turn on briefly at the peak of the input signal to pump energy into the tank circuit, then allow the transistor to turn off and have the tank circuit "ring". Think of pushing someone on a swing set. The person pushing from behind only inputs energy into the system briefly at the peak. For maximum swing amplitude and efficiency the pusher needs to time each push to occur at the resonant frequency of the swing. In a class C amplifier maximum output swing occurs when the input signal injects a pulse of energy into the tank exactly at the resonant frequency. When this condition is achieved, the input and output signals are 180° apart.

The Class C Amplifier lab made use of all four instruments including two outputs from the EDU36311A, one connected in reverse to provide the negative base emitter cutoff bias. First the circuit was setup with known values so we could verify it was working. It was not. I fried two NPN transistors before I made appropriate use of the current limiting features in the Keysight EDU36311A power supply to avoid further carnage. After double checking all connections, I found a substitute transistor, tested again and it was working.

We manually adjusted the EDU33212A waveform generator until the circuit produced the best gain at approximately 180° phase difference. The keypad enabled large changes and the dial helped make finer incremental adjustments easily, I settled on 45.8kHz.

Once validated, I proceeded to change the C2 50nF capacitor value to one that only I knew and reset the equipment (4.64nF). At that point I was completely hands-off, and Mark took over remotely. Mark retested everything, found the new resonant frequency, and calculated that the capacitor’s new value would be close to 5.68nF. Mark was close! He attributed the difference to the value of the inductor which may not be tight to 220µH.

A 3rd setup used 16nF as the value for C2, Mark's calculation was close again, 19.5nF using a frequency of 24.278kHz:

Mark’s observations of the Keysight Smart Bench Essentials remote access capability:

• Successfully proved that remote operation can be used to complete a basic lab task
• Lost connection to the waveform generator and power supply several times, however the connection recovered
• VNC on the oscilloscope was ‘rock solid’, the virtual front panel is very nice
• Occasionally clunky, lagging between control change and response, but workable

I then closed the holes in my firewall and reset C2's capacitance to the original 50nF. For an final test, I used the oscilloscope’s frequency response analysis to produce a plot of the circuit’s phase and gain characteristics.

The frequency response analysis of the EDUX1052G provides a much faster way to determine the circuit’s resonant frequency. The plot clearly shows that maximum gain of 9.23dB is achieved at 46.43kHz, almost exactly 180° phase difference.

In summary, the Keysight Smart Bench Essentials instruments can provide an effective means for remote access, however it is crude without a unified software package. Remote control works well using the web interface. The oscilloscope’s VNC panel works particularly smooth even over the Internet. Network security is critical and should be addressed before presenting the equipment onto the Internet – a VPN can provide an encrypted solution.