It comes down to "where are the IM products really being generated".
I think what Skipp is trying to say is that if the preamp generates spurious products from overload that fall outside of the center frequency, that filtering behind the preamp will help keep those products out of the receiver. While this may be true, keep in mind that any IM products that are generated by the preamp are going to be at fairly low levels because of the inefficiency of the mixing action in the preamp. A mixer is what it becomes when you get into the non linear range of the preamp. But as with any mixer its product amplitude is going to be way down from the signals that cause the mixing. The off frequency products generated by a non linear preamp are not the real problem. It is the on frequency products that get thru. They are going to be relatively weak also but because they are on frequency everything in the chain is going to amplify them. Filtering at the output of the preamp will do nothing to reduce any mixing action in the preamp as that is dependent on input level. Measuring 3rd order products is done at the output of the device so with filters at the output it is going to look like the filters are helping reduce these off frequency products but that is not how you measure IM performance of a device. They are usually referenced to on frequency levels. Also keep in mind when reading IM specs for an amplifier that some manufacturers reference to the input and some reference to the output of the amplifier. Referencing to the output makes the spec look better by the amount of gain that the amp has. Adding filters to the output of the device can help reduce the IM tendencies in the following receiver however by keeping off frequency signals out of the receiver. It is the total amount of power that reaches a device that causes overload. But any off frequency IM products that may be generated in the preamp will be much weaker than direct off frequency signals So the addition of filters after the preamp may seem like they help the preamp but they are really helping the receiver from generating IM in its first active stage. Remember that when you add a preamp you destroy the IM performance of the receiver by the amount of gain in the preamp. 73 Gary K4FMX > > Letting the preamp generate "poop" & then filtering the > > off-channel garbage you've already generated in that > > preamp is a poor solution; > > Ah, now were getting close... now assume the preamplifier > generates really bad unwanted products in extremely overloaded > conditions that don't occur most of the time. When the > buckshot flies for relatively modest times... the trailing > internal/external filters (regardless of location) would help > a good receiver better deal with the event. > > > the in-band garbage generated in the preamp goes right on > > through. > > And now we ask how much F-center and close adjacent in band > garbage actually gets to the receiver front-end and how well > do the receiver(s) handle this event? With the right hardware > layout a lot better than you might assume at first glance. >
