Roger D Johnson wrote:
> Tony King - W4ZT wrote:
>> I don't know if any of you have looked into this particular product but
>> at first glance it appears that this is one solution for the tuned input
>> problem many of us face and at the right price too:
>>
>> <http://www.ldgelectronics.com/manuals/AT-100AMP%20Manual.pdf>
>>
>> 73, Tony W4ZT
>
> The purpose of the tuned input is to provide a "flywheel" effect to
> smooth out the variations of amplifier input impedance over the
> operating cycle. The Q of the circuit determines how much "flywheel"
> effect we have. The lowest recommended value I've seen is 2. Solid
> state transceivers want even more...perhaps 3 to 5.
>
> The LDG tuner is an L network in which the Q is dependent on the ratio
> of impedances to be matched. For normal input impedances, the resulting
> Q will be very low. It's hard to write the formula in text format but
> in words it's as follows: divide the higher impedance by the lower,
> subtract 1 and then take the square root. For example to match an
> impedance of 100 ohms to 50 ohms, the resulting Q is only 1!
>
> With a 3 terminal network you can choose the Q independently of the
> matching and would seem to be the way to go.
>
The recommendation for an input tuned circuit Q of 2-4 came from a 1961
article by Eimac authors in QST. That recommendation was specifically
based on a pi tank, and it was also pointed out that the output
capacitor provides a direct shunt path from cathode to ground for
circulating harmonic currents. But when an L-network is being used in
impedance step-down mode, it doesn't even have an output C.
Now it may be that the designers know all about these things, have
checked their effects on amplifier IMD performance, and have found some
reason to ignore the Eimac recommendations. If that is the case, it
would be good to hear why.
73 from
Ian GM3SEK
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