Message-ID: <50745EC1.1090402@iup.uni-heidelberg.de>
Date: Tue, 09 Oct 2012 19:28:33 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; de; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
References: <50731B6C.2060506@iup.uni-heidelberg.de> <201210091526.52793.smtp01@email.it>
In-Reply-To: <201210091526.52793.smtp01@email.it>
Subject: Re: LF: active antenna output termination?
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Thank you Claudio,

I will consider that when the probe is down from the roof next. There 
are a few dB in reserve, i'm still far away from the RX noise.
But now it looks much better! Still some IM but only a little. Taking 
into account that AFN is arround, that's a positive result.
If there would be oscillations on the output of the probe, then i would 
see them across the 50 Ohm resistor in the shack, at full amplitude, right?

73, Stefan/DK7FC


Am 09.10.2012 15:26, schrieb Claudio Pozzi:
>
> The emitter follower transistor have a very low output impedance, no problems
> for receiving purpose but...   if you use a long coaxial cable you connect a
> capacitance of about 100 pF/meter between output and ground. The circuit can
> become unstable and oscillate, I found that a 27 or 33 ohm resistor in series
> to the output help to kill the oscillation and the signal loss is a little
> less than 6 dB (voltage, 3dB power).
>
> I's a good practice to put a few ohm resistor in series at the low impedance
> output devices (OP-AMP, emitter follower, 74HCxxx buffers etc) driving a
> capacitive load to prevent oscillation.
>
> 73
>
> Claudio, ik2pii
>
>