Re: How to improve SNR in Antenna Cirsuit Design

From: Johnson Liuis (gpsabove_at_yahoo.com)
Date: 03/10/05 

Date: Thu, 10 Mar 2005 13:36:20 -0700

Thanks for your quick reply, Paul,

I just made a mistake. The LC network is not a low-pass filter, actually it
is a bandpass/bandstop resonant filter around 1575MHz. I just calculated the
resonant frequency and to my surprise, I found the calculated frequency is
about 1.9GHz instead of 1.575GHz, so it seemed that the previous designer
took the parasitic capacitor and inductor into account. Could you please let
me know if it is reasonable to leave about 0.3GHz room for parasitic
capacitor and inductor?

BTW, the RF signal is first feed into the LC bandpass filter, then to a
Low-noise Amplifier, then to the Pi Attenuator.

If the attenuator is behind the LNA, can I still get some nenefits in
improving SNR?

You mentioned that it is a very bad way to tame the amplifier by adding Pi
attenuator, do you have any better way?

Thanks.

Johnson

"Paul Keinanen" <keinanen@sci.fi> wrote in message
news:ci8131hpobpsvvamr72in1nfvh4eiugape@4ax.com...
> On Thu, 10 Mar 2005 11:44:30 -0700, "Johnson Liuis"
> <gpsabove@yahoo.com> wrote:
>
> >I am looking into an existing GPS mini-size patch antenna design
(1575MHz),
> >which includes an LC low-pass filter,
>
> An LC filter made of lumped components at 1.5 GHz ????
>
> >an amplifier with RFI suppression
> >(BFP640), and a pi attenuator about 3 dB.
>
> Is the attenuator between the antenna and preamplier or after the
> preamplifier ?
>
> If the attenuator is in front of the amplifier, removing it would
> improve the SNR by 3 dB. The only sensible reason for putting a
> resistive attenuator in front of the preamplifier would be that the
> amplifier was unstable, when connected to a highly reactive antenna.
> Inserting a resistive attenuator would create a nearly 50 ohm
> resistive source impedance and the amplifier would be stable. However,
> this would be a very bad way to tame the amplifier.
>
> Paul
>

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