**Next message:**BUHYOFF@VTVM1.CC.VT.EDU: "CMOS Superkeyers SOLD"**Previous message:**Greg Buhyoff: "CMOS Superkeyer II boards"**Maybe in reply to:**Richard Urmonas: "Solid state QSK"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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

**Next message:**BUHYOFF@VTVM1.CC.VT.EDU: "CMOS Superkeyers SOLD"**Previous message:**Greg Buhyoff: "CMOS Superkeyer II boards"**Maybe in reply to:**Richard Urmonas: "Solid state QSK"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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