That circuit looks very wrong for a diode detector, and the resonant frequency of 2.53 nF and 10 mH is 31.6 kHz, far from the MW band.
Is there any possibility that it's just a font formatting error during printing? Perhaps L1 should read 10µH (10 micro henries) instead of 10mH (10 milli henries). The resonant frequency would then be closer to 1MHz (1000.6 kHz) instead of only 31.6kHz
The lab in question does NOT give a schematic - its up to the student to figure it out. It refers to the 3L.1.1 tank circuit of a 10mH inductor in parallel to a 10nF capacitor, fed by a 100K resistor from a signal generator, but says:
- to detect the carrier, use an LC circuit like the one you built at the start of this lab, but showing the following differences:
- the resonant frequency should be around 1 MHz;
- you need no upper resistor in the "divider:" the antenna can drive the LC directly.
- The value of the resistor to ground is not critical; try 10k;
- the low-pass filter's job is to kill the carrier, keep the audio. Fortunately, these two frequencies are very far apart; so, you have a lot of freedom in placing f3dB. The form of the low-pass you design may strike you as odd (though this depends on the way you choose to do the task: the "odd" configuration, described in §3N.8.4, Fig. 3N.36 uses the rectifier's resistor to ground as the R in the RC low-pass). Just make sure to put this in time-domain terms: that RC is very long relative to the period of the 1 MHz "carrier," but short relative to the "signal" or "audio" period.
Fig. 3N.36 suggests a leaky peak detector, i.e. a diode feeding a parallel RC.
The student is thus expected to choose parts from the parts kit for a 1MHz resonant frequency. Unfortunately there are only two inductors in the kit to choose from: 10mH and 100uH both radial unshielded ferrite bobbin construction, so the student is expected to realize that the 2.53pF required to resonate an ideal 10mH inductor at approx. 1MHz is unrealistic, or at least note that the smallest capacitor in the kit is 10pF, and thus choose the 100uH inductor which will need a much more achievable 253pF, given the parts available (5 each 10pF, 68pF, 100pF, 470pF).
Unfortunately as the circuit is only tunable in discrete steps (unless you get cute and try two of the 100uH inductors in series and vary their coupling by varying the angle and distance between them), the book misses the key step of actually checking there is a strong local AM radio station transmitting near 1MHz! Also it doesn't mention that direct coupling the antenna will detune the tank circuit due to the antenna capacitance, so less capacitance will be required than expected.
The addition of a ferrite rod and a spool of magnet wire, and even a variable capacitor to the kit would have been essential if the course went into RF design in any detail but as this was the only radio experiment, it would be hard to justify the several bucks extra cost.