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


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