“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 14J, 14K, 14L
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 intermediate frequency filter (IF filter) was constructed.
In order to make appropriate measurements, the input and output of the filter has to be 200Ω. Therefore the instructions have you solder appropriate resistors at each end of this subcircuit.
14J
This is the Vpp (142mV) measured through the IF filter at the center frequency (4.913761 MHz). The center frequency was identified by finding the frequency where the highest voltage passing through the filter was measured. The input frequency was 500mV.
dB = 20log10(V2/V1) = 20log10(142/500) = -10.93 dB
There is -10.93 dB loss through the IF filter at its center frequency of 4.913761 MHz.
This is compared to the LTSpice simulation of this filter, which was reported here. The simulation looked much more like a high pass filter (see linked blog post) than a band pass filter. Regardless, the simulation showed about -12.845 dB at f0 , corresponding to a 17.41% error* or a 28mV difference**. Note, in the trend of AI dumbing us all down, I felt a little lazy and let AI do these asterisked calculations. These formulas are simply the percentage of error formula* and rewriting of the voltage gain in dB** formula.
14K
Looks a lot more like a bandpass filter in real life!
These measured results show a typical (and expected) representation of a bandpass filter.
The USB frequency is 4.915001 MHz (which is 4.913761 MHz + 1240 Hz). At this frequency, 12.6 mVpp is outputted (with the generator inputting 500 mV). This means that the rejection loss of the USB frequency is -31.97 dB. See the formula below.
dB = 20log10(V2/V1) = 20log10(12.6/500) =-31.971 dB
14L
