“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 14A, 14B, 14C, 14D, 14E, 14F, 14G, 14H, 14I
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.]
The next part of this radio to build is the intermediate frequency, or IF, filter. The design is a four crystal and five capacitor Cohn filter. Each crystal and capacitor should have respective resonant frequencies and capacitances that are identical to eachother.
A.
The first problem in Chapter 14 shows how to find the resonant frequency of a single crystal with an oscilloscope and function generator. I will point out that one of the unique characteristics of the Electronics of Radio is that it shows me how to use my equipment in ways I would not have thought of. In this case, channel one of my oscilloscope has a tee-connector. One end of the tee is attached to a function generator with a sine wave oscillating at 4,913,500 Hz. The other end of the tee has a positive and negative clamp that attach to the legs of one of the Cohn filter crystal. The frequency of the function generator is varied until the lowest voltage is seen on the oscilloscope (in this case 4,913,659 Hz), thus identifying the crystal’s resonant frequency. The theory is that at the crystal’s resonant frequency, the crystal’s impedance would become minimal, and thus voltage across the crystal would also drop to its lowest point.
Interestingly, one of the reasons I put off doing this exercise is because I did not think I had a suitable generator as there was no sync output port for triggering the oscilloscope as required by the instructions for this exercise. It turns out the Koolertron 60MHz generator can actually sync the outputs of its two ports, and one of the outputs can be used as a trigger sync. Cool! It worked great.
B.
C.
The measured upper (fu) and lower (fl) frequencies when the oscilloscope shows 150.7 mV are 4.914.767 and 4.913.642 MHz respectively.
Q = f0 / Δf = f0 / (4.914.767 – 4.913.642) = 4.913.659 / 34 = 144.5E3
D.
f0 = 1/[2π√(LC)]
4.913659M[2π√(LC)] = 1
√(LC) = 1 / 30.87343003E6
LC = (1 / 30.87343003E6)^2
LC = 1.04913225E-15
L = C = √1.04913225E-15 = 32.39031105E(-9) H or F
E.
This is the 4-element Cohn IF filter of the NorCal40A (and B) LTSpice simulation. Note that this first simulation uses the “XTAL” components with RLC quantities of 6Ω, 32.39031105nH, and 32.39031105nF respectively. The port impedances are 200Ω. The filter should be centered on 4.913659MHz. And the initial sweep is from 100kHz to 10MHz.
Ummm…this does not look like a bandpass filter…
I went ahead and used the series RLC equivalent circuit for each of the crystals, as this is what the book requires when being used with its proprietary circuit simulation software (called Puff).
S21 looks the same. Clearly this simulation is not expressing what I expected it to.
F.
This simulation is the same, except now the port impedances are changed to 50Ω.
G.
This simulation returns the port impedances to 200Ω and reduces the five capacitors from 270pF to 200pF.
H.
The capacitances are returned to 270pF and the questions asks for the “minimum loss in dB in the pass band.” All I can say for this one is lol. I did reduce the sweep to a 6kHz span centered on the fundamental frequency of 4.913659 MHz.
I.
The next question asks what the rejection is at 1240 Hz above the fundamental frequency, i.e., the upper side band frequency. Well, obviously this simulation is not working properly. Or at least I am not working this simulation properly…