[NLRS] Where to put interdigital filter?

[Donn - WA2VOI/0]

'Morning, Scott.

Jon's comments are appropriate, and you might be able to "get away" with a 
simple shorted1/4-wave stub for the receive side.  The short will reflect as an 
open to the receive path for 1296, but everthing else will be grounded out. 
Bandwidth will be a few MHz.  If the interefering signal is already in the 
1296.1 region, no filter will get rid of it, so....

Of the TX side, the filter should go BEFORE the TX amplifier, not after.  Get 
rid of the junk before you waste power amplifing it.

Good luck.  Keep us informed as those XVRTRs look interesting.

73 Donn
WA2VOI/0

----- Original Message ----- 
From: <W0ZQ at aol.com>
To: <nlrs at mailman.qth.net>
Sent: Saturday, April 04, 2009 12:32 PM
Subject: Re: [NLRS] Where to put interdigital filter?


>
>
> In a message dated 4/4/2009 6:49:03 A.M. Central  Daylight Time,
> acepilot at bloomer.net writes: I read somewhere that my idea might  not be good 
> to have the
> interdigital filter in line before the RF amp as it  will increase NF.  So,
> should I place the filter so that it is only in the  TX path (after the RF
> module)?
>
>
> I am not familiar with the GHZ cheap transverter design.  However,  rovers
> tend to like spots that are RF rich and desensing IS a problem on  902 and
> perhaps 1296 if you have a wide open RF front end, if that front end  doesn't 
> have
> a great dynamic range, and/or if the LO is noisy .... all bad  things in the
> presences of strong adjacent signals.  Placing a good  filter ahead of the
> first RF stage helps, perhaps a lot, especially if that RF  stage doesn't have 
> a
> great dynamic range.  You will suffer some decrease in  receiver performance
> because of injection loss of the filter, but it really may  make the 
> difference
> between hearing the other guy and not (due to receiver  blocking).  I would
> suggest a filter design that provides some selectivity  with lower insertion
> loss .... you many not need high selectivity that usually  comes with high
> insertion loss.   In summary, yes, its good to have  some selectivity ahead of 
> the
> first RF gain stage, especially if that gain stage  does not have a wide
> dynamic range.  Perhaps someone else can comment on  the choice of filters 
> ....
> interdigital vs cavity vs simple board mounted  LC.
>
> 73, Jon
> W0ZQ
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