> 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
