EMRFD Message Archive 1726
Message Date From Subject 1726 2008-06-19 01:57:24 Stan Link coupled output question REF Solid State Design page 25
In the calculations there are two items which are assumed. One the
load Q which was chose to be 6, I understand that one. The other value
assumed (chosen) was the number of turns on the resonant coil. The
value of 15 turns was chosen which resulted in an impedance of 99 ohms.
I understand all the calculations and other items for the circuit.
My question is the 15 turns. Was the value of 100 ohms chosen and
that resulted in the 15 turns. Or for some reason the 15 turns was
chosen (frequency was 14 MHz). Anyway I am looking for what the rule
of thumb might be on which ever was chosen the turns or impedance.
The RL for the circuit (1/2 watt at 12 vdc) is 144 ohms.
Comment: I do not see this link coupled output circuit used today, but
still feel it is the better design for a small qrp transmitter.
Thanks, Stan ak0b1733 2008-06-20 10:23:33 Wes Hayward Re: Link coupled output question REF Solid State Design page 25
I doubt that there was any specific reason for picking the 15 turn
winding. We obviously can't go back and ask Doug. I think it was
his piece, abstracted from a QST piece. If I did this one, it may
just have been that I looked in the junk box, found a 15 turn toroid,
and ran through numbers to indicate that "yea, that will work."
A link coupled tuned circuit like this will offer poor harmonic
suppression when compared with a low pass type matching network such
as pi or L network. It is usually a reasonable assumption that
impedances transform as the square of the number of turns, but it is
still just an approximation, especially with powdered ir1734 2008-06-21 21:02:04 Kenneth Stringham Re: Link coupled output question REF Solid State Design page 25
Knowing your impedance then you must select your loaded Q. If you want a loaded Q of 6, then multiply the Output Imedance by 6. This value should be used for the XL value and you would then determine how many turns it would take to get the inductance that would yield this value. Finally, you would figure the impedance ratio between the transmitter output load and the antenna of 50. The turns ratio would SQR(Impedance Ratio) and thus the number of turns.
Ken - AE1X1748 2008-06-25 12:03:47 Glen Leinweber Re: Link coupled output question REF Solid State Design page 25
Kenneth Stringham clearly states right at the top that
Vcc^2/(2*Po) is an approximation. Good job. This
equation is an effort to put a linear spin onto a very
non-linear part of a power amplifier.
I think it is not stressed enough that our often-used
and powerful network math doesn't apply here. We make
the mistake of assuming that the filter between power amp
and antenna operates as it would driven from a linear
class-A amplifier having output Z of Vcc^2/(2*Po).
A similar caution applies to any non-linear circuit like
a mixer and many oscillators.
We should not be content to build power amps as we
have for many decades, and assume the result is optimum.
That Class-C circuits can be pushed toward Class-E
efficiency is evidence that there's room for improvement
over conventional methods, where an explicit, optimum
solution is lacking.
There is ample opportunity for experimental methods
espoused in EMRFD to make real advances in this area.
-Glen