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scratch built am radio...LC frequency not making sense

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rf-loop:
Sorry, I have not read carefully... start of topic there was:
"1N914 diodes so I decided to try and build a simple am radio with one of them"

so I have this in mind when I write...

Your diode type is not problem number one ;) but also it is not best.. (if it is shottky 1N60... FV 0,3V @ 1mA  (sure there do not go 1mA... in this case need think how diode work with extremely low current. (before I give link where can find really huge amount of theoretical AND practical information about crystal radio..  also about high Q resonant circuits and also what is effect of loading this resonant circuit. (oscilloscope probe btw destroy nearly all... if want measure HF LC resdonant circuits it really need also tools... not common scope probe. (yes it can use but it need understant what is LC+Probe circuit together.

Maybe your system loaded Q go very low. (detectorr load resonant circuit, antenna also maybe make heavy load. For HF (MW) frequencies need guite lot of wire for coil.. so it is better use litz (RF resistance is more low and loss is more low so Q go more high.) Also capacitor need be high Q. Many small "pocket radio style" tuning capacitors are not.  One old solution is also (specially if can not use good capacitor or if need do "small" dimensions. (High Q 100...400pF capacitors are big... tens of years ago I use this permeability tuning. Fixed high Q capacitor and.... tuning with inductance. (you know... ferrite rod can move inside coil... yes it works also. (permeability tuning is not bad as we know also it is used in some professional receivers)

From old times I remember one 20cm ferrite rod and 80 turns coil... it was fun to listen some utility stations. (yes many of these use this time AM modulation)


It do not cost much of if you try forward biasing this diode...  (this is classic "trick" from real life in time when there was no other radios)
example some small DCV through 10Mohm resistor to diode anode and also cut DC road to L with small capacitor. (of course best is do it with inductor for cut RF "road" but experimental test can do without...

Zero999:

--- Quote from: rf-loop on August 14, 2010, 07:59:54 am ---For scope probe. Do NOT load parallel LC resonant circuit with scope probe.  It destroy Q value and it add also its capacitance to circuit and change it resonant freq. (also if you adjust your probe low freq risetime, it do not delete (eliminate) this capacitance! )
--- End quote ---
Doesn't using a compensated 'scope probe minimise the probe's capacitance?

jahonen:

--- Quote from: Hero999 on September 04, 2010, 04:16:16 pm ---
--- Quote from: rf-loop on August 14, 2010, 07:59:54 am ---For scope probe. Do NOT load parallel LC resonant circuit with scope probe.  It destroy Q value and it add also its capacitance to circuit and change it resonant freq. (also if you adjust your probe low freq risetime, it do not delete (eliminate) this capacitance! )
--- End quote ---
Doesn't using a compensated 'scope probe minimise the probe's capacitance?

--- End quote ---

No, it just equalizes the frequency response seen by the scope. It can't get rid of ~10-15 pF load capacitance which typical 1:10 probe has.

Regards,
Janne

Zero999:
All right I didn't know about that.

Another question: does the capacitance still depend on the cable or is it solely dependant on the 10:1 probe?

If the capacitance of the probe is fixed, is it given on the datasheet (I've not seen one before)? If so then it will be predictable so can be included in the calculations.

Another option is to measure the capacitance of the probe so it can be subtracted from the calculated value.

eternal_noob:
Hmmm... How can I tell this without sounding cruel.
First of all.. You should really take a second look at what rf-loop is trying to tell you. Forget that ringing thing on the scope.
Second. The most common mistake first-time builders of crystal receivers is to load down the tuning circuit. Crystal receivers requires taking great care in impedance matching in both front and rear of the tuning/detector circuit. You can't expect ANY good results at all if you do not. At best, you will have, as you already have achieved; one station in the middle of the dial.

Second. Crystal receiver building has nearly become a science in itself. Take a look at the impressive work that the experts in this field do. Here we are talking about coils with a Q well over 1000. Multiple front-end tuning circuits ahead of the detector. Variable capacitors with silvered vanes and modified insulators (even ceramics isn't good enough here) fort best Q possible. Math, talk and tinkering that I for one do not understand a ting of (you should take a look at their discussions).
What the result is, you ask? It is 200+ stations spread over the whole tuning range on  the Medium Wave band, with a reasonable good selectivity.

Third. I will guess you are still loading that tuning circuit to much. This time with the detector. Try to tap it waaaay down on the coil.

Fourth. If you really want to achieve some results with a simple receiver, try a regenerative one instead and get rid of some of the Q problem. Use any JFET from the junk box. Audio and switching JFET's works good.

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