Larry, you may be right. But there are no commercial 8877 amplifiers using
that scheme and don't recall seeing any published homebrew amps using it
either.
The 54 ohms and 42 pF are not in a series configuration. It is 54R shunted
by 42 pF. The series equivalent will be different for every band because
the 42pF will have a different xc for every band. If the parallel xc is
different from band to band the series equivalent circuit is going to have a
different R and xc from band to band. Conventional design uses an input
network for each band to deal with those band to band variations.
A Q of 3 or so is thought to be needed to preserve linearity. That would
would require a shunt C of about 18 ohms when R=54. That is like 315 pF at
28MHz and doubling for every halving of frequency. At 28MHz, 42 pF is only
about 135 ohms or a Q of about .4.
I will stop there since I certainly can't say I have been there and done
that with this new design thinking.
73,
Gerald K5GW
In a message dated 3/28/2014 1:48:36 P.M. Central Daylight Time,
xxw0qe@comcast.net writes:
If the load is 50 ohms (using 50 just for example purposes) in parallel
with 42pF a series inductance matches well enough that a given
inductance will be good for more than 1 band. However if you add
another 42pF shunt capacitor from the exciter side of the inductor and
change the inductance to about 91nH the circuit will match from 1.8 to
30MHz with a maximum exciter SWR of 1.15:1 with NO switching required at
all. If you want it even lower a combination low pass and high pass "L"
network can be done (only 1 more component).
73,
Larry, W0QE
On 3/28/2014 11:54 AM, TexasRF@aol.com wrote:
> Hardy, the 54 ohms is a nominal figure that will vary with rf drive
level.
> Certainly close enough for designing input networks. Higher drive levels
> will result in a lower input impedance; lower drive levels will result
in a
> higher input impedance.
>
> The 42 pF is the capacitance between the cathode and grid primarily. So,
> you will need to provide an inductance to cancel that out. The inductance
> will be different for each band of course.
>
> 73,
> Gerald K5GW
>
>
>
>
>
> In a message dated 3/27/2014 7:55:49 A.M. Pacific Daylight Time,
> n7rt@cox.net writes:
>
> I am looking for the input impedance of an 8877. I found one source and
> that was K6DC's (SK) article years ago that said it was 54 ohms with no
> frequency dependency or reactive component. The input C is 42 pF
according to
> the data sheet. So I will assume I can use 54 ohms in parallel with 42
pF.
> Anyone have anything different?
> 73 Hardy N7RT
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|