Crystal Radio SPICE Simulation
by Ralf Siemieniec (Brösel)
This was originally posted at: http://theradioboard.com/rb/viewtopic.php?t=1932
Hi, Some time ago, we had a little discussion about simulating a crystal radio in spice in the german language forum (http://theradioboard.com/rb/viewtopic.php?t=1857). Now I want to share some examples about what can be done using the circuit simulator Spice (I used an elder version of PSpice, but using the freely available LTspice should be a good option as well). It might not be perfect into the last detail, but it hopefully shows what it can be good for.
First lets start with a simple, single-tuned circuit:
The tuned circuit consists of C1 and L1, R3 adds some reality to the circuit and simulates the losses. R2 stands for the transformer impedance. The antenna is simulated by a simple equivalent circuit consisting of series connected capacitor, inductor and resistor. The voltage source V3 applies an AC voltage of 10mV. In the simulations, a frequency sweep is done for several values of C1 (500p, 400p, 300p, 210p, 150p, 100p, 80p, 60p, 50p, 45p) and the rf voltage across the tuned circuit is shown.
The next two figures show the result in the usual range of the BCB. Due to the large capacitance of Cantenna, one can not tune the whole band. Looking at the range up to 15MHz, one can see that this radio not only tunes a part of the BCB, but also is susceptible to the shortwave band although the signal is weaker:
Things are changing while reducing Cantenna from 200pF to 20pF. Now one cane tune almost the full BCB, but still shortwave ghosts will be heard, this time from the 31m band. Furthermore note that the signal strength reduces at the low end of the BCB which has not been the case for the larger value of Cantenna. Thus, it is from advantage to add a series capacitor, preferably a variable type - the antenna now can be matched. I know, this is not a really new deal
Now the same thing for an inductively coupled antenna:
In this case, the radio tunes the complete range of the BCB also in case of a large Cantenna, but selectivity isn't that good at the high end. Signals at the lower end are rather small, but at least no spurious shortwave ghosts can be seen - the second resonances are now found around 2MHz.
A reduction of Cantenna improves selectivity, but sensitivity is obviously lost. Instead, shortwave ghosts can be found again. It is probably no good idea to add a series capacitor this way. The better way would be to increase L2 and add a series connected variable capacitor, making a double-tuned circuit. Ofcourse, now you need to tune two circuits...
Finally, a kind of "workaround" to realize a fairly well-behaving single-tuned circuit. In fact it is a combination of the capacitively and inductively antenna coupling. I also added the small capacitor C4, simulating the capacitance between the two coils:
Looking at the signal levels for the different values of C1, the signal strength is now more or less constant. There are obviously no shortwave ghosts in this configuration, since the second resonances are found around 1.9MHz. This single-tuned radio should work fine, avoiding some of the usual shortcomings.
I built a little radio to verify the simulations are correct. I made the coupling of L2 to L1 variable which was a good idea. Such I can adjust the coupling and thus balance influences I can not include that easy into simulation. What I can say is, that I got a pretty nice performing, single-tuned radio which is remarkably immune towards shortwave ghosts - just done with a simple, single-ganged variable capacitor.
Once again - happy simulating! Anyway keep in mind that Spice elements are rather ideal elements.
Brösel
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a.) Why is the L2 coil folded back on itself?
To make this circuit work as expected, the coil needs to be connected in the right way. If one does connect L2 the wrong way, the circuit does not work that good anymore.
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What is the purpose of R1 and C3? (I assume C4 is the capacitance between L1 & L2.)
R1 is justed needed due to numerical reasons. If you have a closer look, you see that the upper point of R1 connects to floating point only connected to capacitors. In such cases PSpice gives an error message, one solution is to place a very large resistor against ground - in this case 100 Gigaohm which should not change the normal behavior of the circuit.
C3 together with the correctly connected L2 does the trick to ensure a more or less constant signal strength across the tuned circuit. It is actually not my own idea, in fact this way of couplings were done at old tube radios. I found the basics about the different antenna coupling schemes in an old book from Helmut Pitsch, a Telefunken development engineer, published in 1959.
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What are units for R1?
Gigaohm
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Why did you have to increase L1's inductance and decrease the coupling?
L1 has to be increased to counterbalance the series connection of the tuning capacitor C1 and the antenna coupling capacitor C3. A loose coupling does improve the selectivity of the circuit.
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I can almost imagine how to measure antenna L, C, & R for a loop antenna, but how would one measure those parameters for an arbitrary wire antenna?
I did not measure these values for my antenna. To do so, Ben Tongue wrote an article on how to determine the most important antenna capacitance. The equivalent circuit for a longwire antenna was also given in the book of Mr. Pitsch, assuming that the length of the antenna is shorter than a quarter of the wavelength. I found this circuit very useful.
In difference to Dave, I almost always had problems with shortwave ghosts in single-tuned circuits. The worst case happened when using a mystery circuit. I am sure, such problems depend on the antenna/ground configuration used. Again the equivalent circuit of the antenna helps understanding it, even if one does not know the exact values.
The link to the description of the diode model in Spice is good to explain some of the basic problems related to make Spice models. Anyway this standard diode model has its disadvantages due to its simplicity (yes, this model is really a simple one!!!), but it is more than sufficient in the range of small signals. We had a little discussion in the german language forum when I introduced my model for the FO-215 diode: http://theradioboard.com/rb/viewtopic.php?t=1914 I already had to parallel a large resistor to the standard diode model to improve the reverse current behavior.
Brösel
Btw., this is the "real" circuit of the radio I built. Looks more simple as in the simulation, doesn't it?
