[ARC5] ARC-5 TX output circuit modeling [long]

[W7QHO at aol.com]

In a message dated 1/2/07 2:50:46 PM, ianmwilson at earthlink.net writes:


> In order to drive a 50 ohm resistive load, we need to do two things:
>   - match 50 ohms to the TX output resistance
>   - resonate out the inductance of the link + roller inductor
> 
> The resistive part of the output impedance is around 4 ohms as far as I can
> guess. We can transform 50 ohms to 4 ohms by adding a parallel capacitance
> of about 1500pF. This has a series equivalent of 4 ohms in series with
> 1629pF.
> 
> In order to get rid of the unwanted inductances in the output circuit - the
> link coil and the roller inductor - we need to add series capacitance. A
> fixed capacitor will do, since we can adjust the roller as required. The
> output circuit then looks like all these in series:
>   - link coil
>   - roller inductor
>   - fixed capacitance
>   - 1629pF
>   - output load (50 ohms)
> 
> A fixed capacitor of 50-100pF will resonate within the range of the roller
> inductor.
> 

Ian,

Don't understand why/where on the 1500pf parallel cap.   In my experience, 30 
- 50pf in series with the roller inductor alone works into 50 ohms just fine 
with the BC-459 (100pf for the 75M BC-696).   Also, don't have figures at hand 
but haven't experienced losses at the levels you're predicting.

Dennis D. W7QHO
Glendale, CA