[NLRS] Where to put interdigital filter?

[W0ZQ at aol.com]

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
**************Hurry! April 15th is almost here. File your Federal taxes FREE 
with TaxACT. 
(http://pr.atwola.com/promoclk/100126575x1220239440x1201335902/aol?redir=http:%2F%2Fwww.taxact.com%2F08tax.asp%3Fsc%3D084102950001%26p%3D82)