“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 15

This continues a series of blog posts on David Rutledge’s text, “The Electronics of Radio”, that I am studying while building the NorCal 40B transceiver. This series of posts will not be a review of the book, nor is it a assembly manual. Rutledge presents a series of problems at the each chapter that aid in understanding electronics and building the 40M QRP CW transceiver. I am going to try to go through all of these problems and document them here. All of these are titled similarly, so search for them that way. For what its worth, most people will want to skip these posts, they are really for my own self-education on electronics and may not make a lot of sense unless you have Rutledge’s book.

[The links to all problem solutions as I go through them will be posted here.]

Note: In order to tackle this problem, you need to modify the placement of the function generator and the additional 1KΩ resistor to the D7 hole that is in continuity with the long winding of the T1 transformer. Also, check which hole of Q6 is in continuity with the long winding of T1 and add the 200Ω resistor to that. Remember I am building the NorCal 40B, while the book refers to the NorCal 40A.

I wanted to confirm that my Koolertron signal generator outputs voltages in peak-to-peak. When I attached the 5V 7MHz signal to my Fluke 45 multimeter, I only received an output of 30 mV which was not the 1.78V RMS that I was expecting. In this experiment I learned that the Fluke 45 is only rated for frequencies up to 1 MHz. When I decreased the frequency of the Koolertron to 300 kHz, the Fluke measured 1.707 V, which is in keeping with what I was expecting, and confirmed that the Koolertron indeed references Vpp for its voltages. The endless nuances of electronic test equipment is fascinating.

I need to address one topic here. When I measured the output voltage of the transformer, I used the same ground as I did for the function generator. I don’t want to get into it here, but you can read these old blog posts to learn why it can be a problem (this one, and this one). Namely, a transformer acts to isolate one side of a circuit from the other, but when you test it with an oscilloscope and a function generator connected to mains power, the ground becomes the same on both sides of the circuit (i.e. in my case, my home’s earth ground), and therefore the primary and secondary sides of a transformer are no longer isolated from each other. Just something to keep in mind, and while I have no doubt that it effects these measurements, for this exercise I am not going to worry about it.

A.

The measured output voltage is 136 mV (i.e., 68 mV doubled).

The results below are the output measurements of the transformer circuit with a 1x oscilloscope probe into a 50Ω parallel termination with the oscilloscope set at the 1MΩ. The results should be one half of the open circuit voltage given the voltage divider effect of the 50Ω termination, thus 136 mV.

Similarly when using the 50Ω internal termination, the oscilloscopes yields the same results.

B.

I believe that the reason the circuit analysis below does not match the measured findings above is because the primary and secondary portions of the built circuit share a common ground. The circuit analysis calculations are based on formulas that presuppose isolation between each side of the transformer.

C.

The 3-dB low frequency cut-off equals the frequency at 0.707 of the voltage.

The maximum voltage of the transformer is around 85.5 mV * 2 = 171 mV at 30 MHz.

(0.707 * 171) / 2 = 121 / 2 = 60.4 mV

The low-frequency cut-off, where the voltage is cut to 70.7% of the highest output voltage (i.e. 121 mV across an open circuit, or 60.4 mV on the oscilloscope), is at 4.4 MHz.

D.