Re: Solid state QSK

---------

From: Thang Le (tl@hprnd.rose.hp.com)
Date: Tue May 17 1994 - 13:34:58 EDT


I like to offer my observation regarding Pi-filters as mentioned by
Richard Urmonas (VK3DRU) (a T/R switching method):

>
> TX-----||-------------------PIN DIODE-----------||------OUTPUT FILTER
> | |
> BIAS----Resistor--` |
> |
> Ind. L |
> RX----||-----------------UUUUU--------------`
> | | |
> P - Cap. C2 - Cap. C1
> I - -
> N | |
> | | |
> ___ ___ ___
> //// //// ////

Where the Pi-filter (L/C1/C2) acts as a 1/4 wave section (inversing the
parallel of the Rx PIN diode and the receive input impedance).

As long as L and C1 are chosen to resonnate at the frequency of interest,
a short provided by the Rx PIN diode will ground the input to the receive
section at the same time present a resonnant LC tank circuit to the
Tx circuit.

The Pi-filter is kind of interesting:

  if C1 == C2 == C
 and L * C * (omega)**2 == 1 (L and C are resonnant at omega)

any load impedance Zout will be reflected at the input of the Pi-filter as

                             Z = L / (C * Zout)

In the above application, L and C can be chosen such that the receive
input impedance is matched with the output filter impedance.

Another interesting special case of the Pi-filter is the back-to-back
Pi-filters with identical components: C-L-C in series with C-L-C. Any
load impedance is preserved regardless of L and C as long as they
resonnate at the frequency of interest:

          ----- L --------- L -------
             | | | |
   Z <= C C C C <= Zout
             | | | |
          ---------------------------

      Z = Zout

--
Best regards,

Thang Le, AA6SV tl@hprnd.rose.hp.com


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