“The Electronics Of Radio” NorCal 40B Transceiver Build Lab Notes: Problem 20
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.]
I installed all of the instructed parts for this problem (see pictures); these were for the voltage regulator and receiver switch. Note: I also installed capacitor C58 for this problem. This was based on comparing the schematics of the NorCal 40A & NorCal 40B and seemed to be appropriate. J2, the power jack, was also installed for this problem.
The Magnecraft W171DIP-7 relay datasheet can be found here: Datasheet for Schneider Electric-Legacy Relays W107DIP-7 . This is the relay specified in Problem 20.
Instead though I used a Omron G5V-2-H1-DC9 relay. Datasheet here: https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/2369/G5V-2%20H.pdf . Of note, this relay is only rated for 10 Hz (unlike the Magnecraft which has a much higher frequency rating). I have previously removed the plastic casing for another study.
The relay is shown open below.
And the relay shown closed below.
I connected the function generator between pins 1 and 16 of the relay with a 9V 20Hz square wave. The normally open contacts are pins 13(common) and 9. (Or since the relay is a double throw, pins 4 and 8 are also normally open.) Pin 13 will connect to the PCB ground and pin 9 will connect to the center pin of “Key Jack J3”.
I tested the continuity of the relay.
Before connecting the relay to the PCB, I probed pin 13 with the oscilloscope. Before energizing the circuit with the function generator, the oscilloscope showed an approximately 60Hz sine wave. I am not sure where this is coming from, though I suspect it is common mode current noise.
When I turn on the function generator and power up the relay (not the PCB!), this is what I see. I am not sure exactly what it represents.
There are positive spikes at a frequency of 20 Hz (50.2ms periodicity). However, there are also negative spikes at the same periodicity. The negative voltage spikes are confusing, because I set a voltage off-set to avoid them (9Vpp with an offset of +4.5V).
The voltage spikes reach 0.565V. Keep in mind that this is NOT a storage oscilloscope and that at this very low frequency it is very difficult to visualize these traces. I actually took a long(ish) exposure with my camera in order to obtain what looks like a solid trace. While this all does not look correct, it is the only relay I have available for these problems.
I also needed to build a power cable in order to complete this problem. I cannibalized a cord from a long discarded appliance. The barrel connector fit the power jack perfectly.
Finally, I prepared the PCB for the experiments. The book instructs that 1Ω resistors be placed in some of the holes of the Switch 1 (SW1). See the pink arrows.
…a closer view…
I attached an oscilloscope probe to the switch and its ground. This would monitor the input into the transceiver’s power supply (which I energized by attaching the newly constructed power cable to my lab power supply set at 10V 0.1A max.) I attached the pin 13 of the relay to the ground of the PCB and pin 9 of the relay to the anode of diode D11. The function generator was again connected between pins 1 and 16 of the relay with a 9V 20Hz square wave.
When I powered everything on, I could see 10V DC entering into the circuit (top line in the picture below) and 8V DC at the anode of the D11 (line second from top). The bottom line is the reference line at 0V. Turning on and off the relay had no effect. I was not convinced that at 20Hz (past the device’s maximum operating frequency of 10Hz) that the relay was actually making solid contact.
A.
Since the relay in the circuit I built did not seem to be working, I sketched it in LTspice. The arrows at the circuit correspond with the traces shown below. Instead of the relay providing a 20Hz signal, I added a voltage source (V2) with a 20Hz 5Vpp square wave with a +2.5V offset in the simulation. I also eliminated the voltage regulator circuit and just added an 8V DC voltage source (V1).
And the voltages of the base (blue) and collector (green) of the transistor Q1 (below).
When I constructed the simulation circuit, I chose a representative “TX-LOAD” of 1MΩ. This lead to an output of nearly 8V (see above).
I changed the “TX-LOAD” to a low characteristic impedance of 50Ω. Now I have the a Q1 collector voltage of around 0.3V to 1.4V with a waveform following the square wave of V2 (i.e., representing the relay of the initial problem). These simulation results are finally in keeping with my expectations of this problem, although I was NOT able to show this on my oscilloscope.
And the graph representing capacitor C57 is below. It takes 1μs for C57 to charge from its minimum to half-maximum voltage.
Expected would be:
τ=RC
&
0.693τ=1μs where 1μs is the time it takes C57 to charge halfway between the minimum and maximum voltages of the capacitor.
∴ τ=1μ/0.693 = 1.443μs
&
C=47nF
∴ τ=RC
1.443μ=R*47n
R = 1.443μ/47n = 30.7Ω
τ=RC
τ=30.7*47n=1.443μs
0.693τ = 1μs
Okay….that is all I am attempting for this problem.