Laryn Lohman wrote: >Tuning a TFE6030 duplexer. I am aware that the high/low arrangement >of frequencies should normally remain the same. However this unit has >three cavities (helicals?) on what was the TX (higher I believe) side, >and only one cavity on the RX (lower) side. For the ham band the TX >will be the lower frequency. What is the recommendation in this case? >
Not to use it on a repeater where the transmitter is the lower frequency. Why? Read your question below first, then read on. >Also, those three cavities have an additional adjustment on the coax end of >each. What is this for? > To notch out transmitter side-band noise. (undesired energy in which appears directly on the receiver frequency) You see, it generally much more important (or more of a requirement) to eliminate transmitter side-band noise *on* the receiver frequency than it is to eliminate transmitter carrier power from getting into the receiver. This is the reason why there are more sections of filtering on the transmitter side of this duplexer. Let's go into some math and theory. While this text represents a 2 meter duplexer scenario, the basic theory is the same for any band. A duplexer must attenuate the transmitter carrier, this prevents receiver overloading, which in turn would reduce receiver sensitivity. A duplexer must also attenuate any noise or spurious energy from the transmitter on or near the receive frequency. In addition, a duplexer must provide a proper match between transmitter, antenna and receiver. If you are having trouble visualizing these functions, take a look at this link: <http://www.repeater-builder.com/pix/dupfig13.jpg> Transmitter output on 146.94 MHz going from point C to D should not be attenuated. However, the transmitter energy should be greatly attenuated between points B and A. Duplexer section 2 should attenuate any noise or signals that are on or near the receiver input frequency of 146.34 MHz. For good reception, the noise and spurious signal level must be less than -130 dB (0 dBm = 1 milliwatt into 50 ohms). Typical transmitter noise 600 kHz from the carrier frequency is 80 dB below the transmitter power output. For 60 watts of output (+48 dBm), the noise is -32 dBm. The duplexer must make up the difference between - 32 and -130 dBm, or -98 dBm. Now, let's talk about the received signal. First of all, the received signal must go from point B to A with a minimum of attenuation. Section 1 of the duplexer needs to provide enough attenuation of transmitted energy to prevent receiver overload. For an average receiver, the transmitter signal must be less than -30 dBm to meet this requirement. It is the job of duplexer section 1 to make up the difference between the transmitter output of + 48 dBm and the receiver overload point of -30 dBm, or a reduction of about 78 dB. A typical duplexer, that has the same construction for both sides, will generally give several dB of receiver overload headroom, but may *just make* or *not even make* the transmitter side-band requirement for no desense. My recommendation is not to use this Motorola TFE6030 duplexer in a situation where the transmitter is on the side having only one cavity because insufficient transmitter side-band noise reduction will result and your system will have unresolvable desense. It will, however, work fine on a repeater (or full duplex link) that transmits high and receives low. Hope this helps... Kevin Custer Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
