Two tuning circuit compared - Page 1
Forum section: electronics / beginners

author: JhonStan
author_rank: Contributor
timestamp: April 01, 2022, 12:54:21 pm
content: Hi everyone, today I was messing around with LtSpice and I got two tuning circuits to compare. Circuit A is the same circuit used in the old crystal radios and is also visible on Wikipedia athttps://simple.wikipedia.org/wiki/Crystal_radio. The problem is that this A circuit on the simulator seems not to work while the tuning B circuit is working perfectly. The tuning values have been chosen at random just to give functional examples. So the question is : Why circuit A on the simulator is not working properly mind circuit B has no problems?

author: Dave
author_rank: Super Contributor
timestamp: April 01, 2022, 01:24:03 pm
content: The impedance of your simulated source is 0R, while the impedance of an antenna is not.

author: CaptDon
author_rank: Super Contributor
timestamp: April 01, 2022, 01:36:35 pm
content: While it is true any pair of inductor/capacitor combinations will be resonant somewhere, your choice of 1pf is poor for use with the inductor you chose. 50 to 100pf would be a sweet spot in the real world. But I don't know if LT Spice would understand that 'sweet spot'. It has to do with the actual Xc/Xl values at resonance. You could pick combinations with Xc/Xl from 10 ohms to 100,000 ohms and this value along with circuit resistances will effect the Q of the circuit giving anything from a barely noticeable wideband peak to a very sharp narrowband peak. As the other poster mentioned your antenna source impedace (signal generator in this case) should be typically set to 50, 300 or 600 ohms as these represent the values of real world off-the-shelf antennas.

author: Benta
author_rank: Super Contributor
timestamp: April 01, 2022, 04:34:48 pm
content: It's two different functions.The first is parallel tuned, meaning it will give a voltage peak at resonance.The second is... I don't know, somehow series resonant, which gives zero at resonance.

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2022, 05:04:32 pm
content: The series-resonant circuit (drawing B) will show some kind of peak (if not over-damped) in thevoltage across the capacitor, which is rectified by the diode.The "zero" impedance of the driving voltage source will try to short out the antenna terminal, but there will be a "high" current through the capacitor.The zero impedance of the voltage source driving the parallel-resonant circuit (drawing A) will completely damp the resonance by shorting out the capacitor.

author: JhonStan
author_rank: Contributor
timestamp: April 01, 2022, 07:46:22 pm
content: Sorry but I'm a newbie, are you telling me that design A which doesn't work on LTspice but works in real life and circuit B which works on LTspice doesn't work in real life? I created circuit B on a base (with different component values) and it works. I used a variable capacitor with a variable capacity that allowed a certain range of frequencies. Then I took a generator and set a frequency that was in the range of the variable capacitor. Rotating the capacitor as the circuit resonated I saw an increase in amplitude on the oscilloscope. In circuit A this I have not seen. I want to do a variable tuning with a range from 10 to 20Mhz and according to the circuit B on LTspice I can get it with a variable cap which would do from 12 to 50p. Even without a diode it seems to work. Thank you!

author: TimFox
author_rank: Super Contributor
timestamp: April 01, 2022, 07:59:02 pm
content: As I wrote: With a voltage-source drive, the series circuit in your drawing B will show a resonant peak in the voltage across the capacitor, which the diode will rectify.On the other hand, a voltage source has zero output impedance and will short out the capacitor in your drawing A. However, I don't know what your practical voltage source is: it probably has a non-zero source resistance (like 50 ohms for an RF generator or something else for an antenna).Ideal voltage sources have zero output impedance, which means that the voltage across the two terminals is independent of the current flowing through it. They exist in Spice simulation, but real voltage sources can have a substantial source resistance.

author: JhonStan
author_rank: Contributor
timestamp: April 02, 2022, 09:06:58 pm
content: Tim, i've used the "IN1" source only for the the simulation like using IN2 . May be seen like a RF virtual signal.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2022, 09:28:39 pm
content: You will not see good resonant behavior (peaking) when driving a parallel-resonant circuit from a "stiff" (ideal) voltage source. In practice, you either drive it from a reasonably-stiff current source, or from a voltage source through a substantial impedance (small coupling capacitor, reasonable output impedance, output from a vacuum pentode, etc.). For a "crystal set" radio, the source is an antenna with a substantial impedance. When driving from a low-impedance (e.g., 50 ohms) generator, you could add a small coupling capacitor from the source to the circuit. To simulate that in Spice, you need to add the actual output impedance of your generator.Circuit B, the series-resonant case, should work in real life from a voltage source with reasonable output impedance. In Spice, the detailed behavior (width of the frequency peak) will change as a function of the total series resistance (your 5 ohms plus the source resistance).