I'm using strictly band pass filters on both transmit and receive, with at least one filter on the output of each transmitter (after the isolator), so something is being done to knock down any off-channel outputs from the transmitters and isolators.. the V8000 specifies -60db or better for spurious output, add on another 30db or so for the band pass filter in the combiner, 30db or so for the circulator isolation, plus path losses before anything spurious that is on a receiver frequency can make it to the receiver front end. so better than -120db. given a +47dbm signal, that's no better than -80dbm, more likely -90dbm at the receiver - good enough or do I need more??
Re: selectivity and rejection: I'm looking at the Icom V-8000, which specs 75db for all of spurious/image/rejection. I have no data on dynamic range or 3rd order intercept. (haven't seen data like that on any of the radios I've been considering for this system) Is there any way to estimate the missing info from spurious/image/rejection data?? Is there anything meaningful that can be determined from the info I have available?? No trade secrets now!! Spill all the dirty details :-) _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Tuesday, October 23, 2007 12:05 PM To: [email protected] Subject: RE: RE: RE: RE: [Repeater-Builder] Re: duplexer isolation and reciever noise bud > Total isolation = 97 to 127db (as good or better than most > stock duplexer setups) That's for carrier attenuation only. Aside from the passive losses in the system, you've done nothing to attenuate transmitter noise, which is going to be a bigger enemy in your close-spaced frequency assignments. > However, the receiver specifies 75db spurious/image/intermod > rejection which I take to mean that any off channel signal > -48dbm (-123dbm sensitivity + 75db rejection) or less should > be completely ignored by the receiver No, you're thinking dynamic range (blocking) and/or selectivity, but quoting the spec for spurious and image rejection. The spurious spec is generally a function of the "purity" of the receiver's LO. If the LO has spurs, new mix products will be generated in the mixer, and some of those undesired mix products can end up falling at the IF center frequency. In other words, you're creating spurs within the receiver itself. The image spec is largely a function of front-end filtering. The tighter the front-end filtering, the further attenuated image frequencies will be before reaching the mixer. The intermod spec is mostly a function of dynamic range of the mixer (or RF gain stage if one precedes it), typically quanified by the third-order intercept point. > That's what I need confirmed - if the receivers stated > sensitivity + spurious/image/intermod rejection = maximum > adjacent channel signal before de-sense kicks in. No. > Is there an RF Engineer in the house ?? Several. --- Jeff
