[aprssig] SAW filters/di(du)plexer

Jim Lux jimlux at earthlink.net
Wed Sep 7 12:02:08 CDT 2005

> >-----Original Message-----
> >From: aprssig-bounces at lists.tapr.org [mailto:aprssig-bounces at lists.tapr.org]
> >On Behalf Of Jim Lux
> >Sent: Tuesday, September 06, 2005 8:18 PM
> >To: aprssig at lists.tapr.org
> >Subject: [aprssig] diplexer/duplexers for APRS
> >
> >It occurs to me that since APRS is a single frequency kind of operation,
> >you might be able to get a SAW filter made that just passes 25kHz around
> >144.39, and another that rejects the 25kHz around that same frequency
> >(actually, it might be the same filter).
> >
> >
> >
> >
>SAW filters are low power devices suitable only for receiving.  You
>can't place them in the external antenna line where they would be
>exposed to transmit power.

Indeed... I was just thinking out loud about possible approaches that are 
different than the traditional series of cavity resonators.

>    Inserting these into the D700 would require
>extensive internal hacking of the D700 to insert these devices in low
>level receive-only front end circuitry inside the radio.   In turn, this
>would be a major undertaking since the D700 doesn't have the traditional
>metal cans interconnected by mini-coax with RCA connectors for it's
>receiver frontend, mixer, IF block, etc.   EVERYTHING RF, IF, audio and
>digital, except for the TX PA, are on a single PC board. This means
>hacking PC traces with an Xacto knife and tacking on coax jumpers
>running to external filters.

You're right... it would be a nightmare for a retrofit.  Furthermore, you'd 
never get the isolation you'd need, just sticking them down to a board (at 
least, not without a lot of cut and try or development time.. Cellphones 
get 100dB+ isolation but they also spend millions of dollars of engineer 
time to get there.

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