“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 8

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.]

Yay! The first components have landed on the NorCal 40B’s PCB! Here is the trimmer cap and inductor at the receiver antenna.

So far, I am greatly enjoying this process, even though I know I am not getting all of the problems solved perfectly. I wish there was a comprehensive answer book that could be referenced after attempting these problems. I would say I am putting one together here, but quite frankly a lot of my answers are incorrect. One of the things that frustrated me with Problem 8 is that the bandwidth I calculated in Part D was significantly different from what I measured in Part C. I would have sworn I did both correctly, which is the root of the frustration. Oh well, I know I could spend many more hours working through each of these problem sets, but I would like to keep the pace of this build respectable.

A.

The function generator is set to a 7MHz 1Vpp sine wave.

The first measurement is taken across both the inductor and capacitor. The capacitor is adjusted in order to find the minimum input voltage which is 37.8 mV with the 10X probe (so approximately 0.378 V). The waveform is quite ratty looking.

B.

The set-up is rearranged so that there is now a 50Ω termination at the oscilloscope.

As well as a different circuit configuration. The signal looks a lot better on the o-scope, and measures 37.6 Vpp with a 10X probe (thus 0.376 V) this time with the capacitor adjusted for its highest output voltage.

C.

Once the capacitor adjustment is made so that the LC resonant frequency is at 7MHz, the upper and lower frequencies of the half-power bandwidth are determined.

The generator amplitude is bumped up to 1.41 Vpp, and the frequency is adjusted until the voltage equaled that seen for the resonant frequency of 7MHz, i.e. 37.6 mV with 10X attenuation. The lower half-power bandwidth frequency is 6 MHz.

The upper half-power bandwidth frequency is 12.4 MHz.

D.

Of note, this is a good reference for this question.

Now something of course is amiss. My measured half-power bandwidth, BW, was 6.4 MHz.

E.

F.

The generator is set at 1MHz at 10V to approximate an AM radio station. Note that the 10V is to amplify the small signal, and that the final output voltage would have to be divided by 10. The output voltage, now consider V_AM is 51.2mV with a 10X probe, and divided by 10 it becomes 5.12 mV.

R_AM = V_RF + V_AM

Where: R_AM = AM voltage rejection factor; V_AM = 1 MHz output; V_RF = voltage at resonant frequency (7MHz)

R_AM = 37.6 mV / 5.12 mV = 7.34

G.

On to Problem 9!

KM1NDY