EMRFD Message Archive 7805

Message Date From Subject
7805 2012-09-21 19:51:59 jwharding3 Mixer Termination w/Balanced Amp
Might I ask the mixer/termination "think tank" gurus to consider the diode mixer termination used in the Harris RF-390 (see new file RF390 1st Mixer).This is an upconversion receiver with a high-level diode mixer followed by a quadrature hybrid, an FET balanced amp and another quadrature hybrid. It has no diplexer. Some questions 1- with the "wide band power splitter" doesn't the mixer output termination appear to be the 51 Ohm resistor R4 (on the input) and R9 for the following IF filter- not shown) over a very wide frequency range. 1a - does this eliminate the need for the diplexer after the mixer 2- with an FET such as a Crystalonics CP643 with roughtly a 50 ohm apparent input impedance- does this offer an advantage over the two xtal filters for a front end design. I am considering this approach at an IF of 5MHA to 9MHZ, without the pin diode attenuator (CR1, CR2). I know the CP643's are expensive - but I keep thinking the second xtal filter would serve better as a series element versus the paralled approach taken in the H Mode mixer designs. Thanks for some thoughts- John