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Critic of circuit before I build it.
I wrote:
The thing to watch is the Q factor, the detector circuit can be seen asa resistor parallel with the ferrite rod and tuning cap. ---------- |( | | |( === [ ] Rp |( | | --------- The higher Rp, the higher the Q factor, the more signal, the narrower the bandwidth B = f / Q http://en.wikipedia.org/wiki/RLC_circuit scroll down to 'parallel RLC circuit'. PS so a high Q gives a narrow bandwidth, better 'selectivity'. That may be needed if you receive 2 stations close together in frequency. Especially in the evening on the AM broadcast band you may get very strong signals from far away stations at the same or right next to the frequency of the one you are listening to, you then hear both at the same time, or this gives a few kHz interference. But, too high a Q (hard to obtain [1]) will attenuate the high frequency sidebands of the station you are listening to, reducing high frequency sound components. [1] RF feedback can be used in a RF stage to increase Q, Am stations are spaced 9 kHz or 10kHz apart depending on location, so all sorts of interference from far away stations exists. This site is worth reading: http://en.wikipedia.org/wiki/AM_broadcasting RR and IF stages with tuned circuits or bandfilters of any kind (mechanical, crystal, piezo, LC, etc) can help create an accurate bandpass curve. I think you should perhaps get hold of one of the RSGB radio books, and start from the http://www.rsgbshop.org/acatalog/Onl...___CDs_29.html |
#2
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Critic of circuit before I build it.
On 4/3/2015 2:02 AM, Jan Panteltje wrote:
I wrote: The thing to watch is the Q factor, the detector circuit can be seen asa resistor parallel with the ferrite rod and tuning cap. ---------- |( | | |( === [ ] Rp |( | | --------- The higher Rp, the higher the Q factor, the more signal, the narrower the bandwidth B = f / Q http://en.wikipedia.org/wiki/RLC_circuit scroll down to 'parallel RLC circuit'. PS so a high Q gives a narrow bandwidth, better 'selectivity'. That may be needed if you receive 2 stations close together in frequency. Especially in the evening on the AM broadcast band you may get very strong signals from far away stations at the same or right next to the frequency of the one you are listening to, you then hear both at the same time, or this gives a few kHz interference. But, too high a Q (hard to obtain [1]) will attenuate the high frequency sidebands of the station you are listening to, reducing high frequency sound components. Yes, I'm aware of how Q affects selectivity, I can only hope for too high of a Q, easy enough to spoil. I've been following a crystal radio group for a few years, they recently got hold of a low loss ferrite and getting Q's in the 1000 to 1200 range over the broadcast band. Using Litz wire. The material I'm using is some in my stock, and not real good Q at AM band frequencies. What I have is twelve-1/2 potcores stacked as a rod. A coil is wrapped on that and resonated with a cap. With a radio near the core You get an increase in the signal as you tune to resonance. The rod is 67.5mm Dia. x 335mm long, or 2.66 in dia. x 13.2 in long. - [1] RF feedback can be used in a RF stage to increase Q, I just yesterday, tried a Hartley oscillator on part of the winding in an attempt to raise Q and get more selectivity. It didn't oscillate, but I only spent a few minutes before supper, I'll be trying again. Am stations are spaced 9 kHz or 10kHz apart depending on location, so all sorts of interference from far away stations exists. This site is worth reading: http://en.wikipedia.org/wiki/AM_broadcasting RR and IF stages with tuned circuits or bandfilters of any kind (mechanical, crystal, piezo, LC, etc) can help create an accurate bandpass curve. I think you should perhaps get hold of one of the RSGB radio books, and start from the http://www.rsgbshop.org/acatalog/Onl...___CDs_29.html Oh that one! I have the ARRL Handbook. :-) Thanks, Mikek --- This email has been checked for viruses by Avast antivirus software. http://www.avast.com |
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