“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 9

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 transmit filter is built during this problem.

The transmit filter toroid is wound according to the NorCal 40B directions, which are different from “The Electronics of Radio” directions which were based off of the NorCal 40A. Namely, 30 turns of 28awg enameled wire, for a nominal inductance of 3.5μH.

My toroid ended up pretty close.

A.

Without adding in the variable capacitor, the resonant frequency as indicated by the maximum voltage, is 7.5MHz.

B.

L = A_LN² , where A_L = inductance constant (for T37-2 core); N = number of turns

L = 4 nH/turn² / 30² = 3.6 μH
BINGO!!!

C. The output voltage after the variable capacitor is added to the circuit and adjusted for maximum output is 464 mV.

The half-power bandwidth, found by changing the input voltage from 1Vpp to 1.4Vpp and then finding the frequencies (high and low) that achieve that maximum 1Vpp center frequency (max) voltage.

The lower half-power bandwidth frequency (F_L) is 6.747MHz.

The upper half-power bandwidth frequency (F_L) is 7.342MHz.

Q_P = f_C / (f_U – f_L) = 7MHz / (7.342MHz – 6.747MHz) = 11.76

D.

X_L = 2πfL = 2π(7MHz)(3.67μH) = 161.4Ω

Q_P = R / X_L
R = Q_P * X_L = 11.76 * 161.4 = 1.899 kΩ

E.

This one gave me quite a bit of trouble. The point is to find the output voltage based on the Norton equivalent of the function generator and 5pF capacitor. I am going to pass this one by for now…

F.

The signal generator is turned down to 2.8MHz and the amplitude increased to 10Vpp in order to simulate the transmit mixer’s difference frequency F_(-) .

The voltage rejection factor, R_(-) , equals the resonant frequency voltage, V_Fr , divided by the F_(-) voltage.

R_(-) = V_Fr / V_F(-) = 464mV / 24 mV = 19.3

Of note, the 2.8MHz waveform was carrying an additional sinusoidal wave with a period (τ) of 10.76ns.
The frequency of this secondary waveform is 1/τ = 92.9MHz.

92.9MHz is a local FM broadcast station. I used this as an excuse to turn on my spectrum analyzer.

G.

R_(-) = 20 log (V_Fr / V_F(-) ) dB = 20 log (464mV / 24 mV) = 25.72 dB

I am skipping parts H and I for now. I want to keep this project moving along.

KM1NDY