Snip,


Going back to some of the earlier filter flatness discussion, well toss
that idea into the mix when you think in terms of TRF. Not only do you
want to achieve a specific width but you want it to be flat. My 3/25 kc
TRF scenario isn't flat at all. Its a big peak that just broadens out.
When we say a 'bandwidth'number we are relating to something specific
like 3 or 6 db down from the peak. Its still a peak in this context.
So whats happening at 20 db down? You guessed it, that 25kc number is
150 kc wide. I dunno how you could control the width AND the flatness
AND the skirts.

I'm a fairly recent convert to crystal radios. For the sake of
discussion there's little difference in xtal technology vs trf
technology in that both are non-superhet. I get absolutely glorious
quality audio from my xtal set when fed thru an amp. With 6 or 8 knobs
on the front panel and top notch components I can find a dead spot
between semi-locals on 680 and 690. With a local station on 1370 it
takes traps and VERY hi-q stuff to ferret out semi-locals on 1240, 1290
and 1480. Its as if it were a totally different radio from one end of
the band to the other and this has been the plague of TRF circuits since
day one. If I didn't have the local 1370 I could safely say, hey Jon,
this is the ticket, but there's scarce few of us who don't have a strong
undesired local station to bollox up the works.

Go superhet, my man.

-Bill M

The only way to gain enough RF bw at any F on the BCB to ensure there is no
sideband cutting
which would restrict the AF bw, you have to use two LC circuits and couple
one to the other,
and I used a 39k resistor.
At the low end of the BCB, the coils are tuned about 10kHz apart, and at the
top end,
they are tuned to the same F.

If you only have two LC circuits, and the bw is 25 kHz for each at 1,500
kHz,
then the Q is 60 only .
Using two LCs with a Q like that in cascade, the bw will be reduced to 19
kHz, or
thereabouts, and the selectivity away from the pass band will be twice that
of a single circuit.

But another powerful station at 50 kHz away will be heard,
although it won't be loud.

Once you are 50 kHz away from say 1,000 kHz, the rate of attenuation is
at 6 dB /octave only for the one tuned circuit.
So a station at 500 kHz of equal strength is only -6 dB below the 1,000 kHz
station.

You need multiple tuned circuits to give decent selectivity, and here the
superhet is king.


But it is possible to series three double LC twin gang stagger tuned stages.

This gives 6 tuned circuits. The final Q has to be 60 to allow full audio
bw,
The initial Q therefore has to be much much lower, maybe 15 only at 1,500
kHz,
allowing 100 kHz of bw.
Such a tuned circuit has a blunt nose, and no advantage can be had as with
with the double tuned IF transformer's flat topped steep sided bandpass
characteristic

The only reason for RF input selectivity with relatively low Q
tuned front ends in AM BCB sets is to make sure the mixer does not
get overloaded by too much signal from a poweful unwanted station
which would then try to cross modulate the mixer tube.

The purpose of the RF front end is spelled out in RDH4.

For those struggling on Q issues, go find out, I'm sick of repeating text
books.

Patrick Turner.