**EMRFD Message Archive 1726**

MessageDateFromSubject1726 2008-06-19 01:57:24 Stan Link coupled output question REF Solid State Design page 25 Ref Orginial Solid State Design book Page 25 figure 16

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 Hi Stan, and group,

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 I agree with Wes. Point number 1, the output impedance of a transmitter stage set by the usual Vcc^2/2*Pout. This too is an approximation.

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 Can't resist ranting a bit:

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