missed the dummy load part, circulator probably wont help...
--- In Repeater-Builder@yahoogroups.com, "Eric Lemmon" <[EMAIL PROTECTED]> wrote: > > I think you may have overlooked the fact that desense was observed even when > a dummy load was connected directly to the output of the duplexer. That > pretty much eliminates anything external to the repeater itself, doesn't it? > > 73, Eric Lemmon WB6FLY > > > -----Original Message----- > From: Repeater-Builder@yahoogroups.com > [mailto:[EMAIL PROTECTED] On Behalf Of nj902 > Sent: Friday, January 25, 2008 8:48 PM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Re: Desense Problem on 222 MHz Repeater > > This question has initiated an interesting exchange of ideas. Many > good suggestions have been provided. > > The fact that the desensitization exists when operating duplex on > either of two antennas, or when using split antennas, combined with > the results of the other tests [minimal desensitization on a dummy > load, measured duplexer isolation, etc.] suggests that the initial > concern may be true - that the issue is related to an external mix > between the repeater transmitter and the DTV signal. > > Issues related to IM between narrow band and wide band systems will be > a growing concern as more services convert to digital formats. There > is not a great deal of published guidance regarding the identification > and resolution of such issues. > > The use of a spectrum analyzer at the receiver port of the duplexer is > a good place to continue the investigation. The problem with this > test is that the mix between the repeater transmitter and the DTV will > produce a wide band product. Wide band digital transmissions are > noise like in character and must be observed using techniques that one > uses to analyze noise. [see Agilent AN150, AN150-4, AN150-7, and > AN1303] > > You did not indicate the instrument used to make the tests or the > instrument settings and configuration. The issue is that a spectrum > analyzer may not be able to "see" the problem. The noise figure of > the spectrum analyzer could be on the order of 30 dB or more. On the > other hand, the repeater receiver's noise figure will be less than 10 > dB. > > The noise figure of your analyzer can be easily determined. Modern > analyzers have a noise marker function. If you activate this function > with the analyzer's input terminated with a 50 ohm load, you will get > a result based on the analyzer's internal noise. The readout will be > in the form of dBm per Hz. This metric is used because it is > independent of bandwidth. Regardless of the currently selected > analyzer RBW, the analyzer's processor will compute the noise marker > to yield the same result. > > The difference between this number and the thermal noise floor [kTB] > of -174 dBm/Hz is the analyzer's noise figure. > > Based on the receiver's sensitivity [and hence its noise figure], it > will have an inferred noise floor. Noise which enters the receiver > through the antenna port will add to this noise floor resulting in > degradation of your effective receiver sensitivity. This external > noise will be comprised of site noise, sideband noise from your own > transmitter and any IM between your transmitter and the DTV signal. > > This new noise level can be determined based on the amount of > desensitization you have measured. Based on your measurements, the > new noise level will still be perhaps 10 dB below the ability of the > spectrum analyzer to observe due to its much higher noise figure. > > The ability of the spectrum analyzer to see noise can be enhanced > through the use of a high gain low noise amplifier such as the Agilent > 11909A. The LNA and the spectrum analyzer combine to comprise a > receiving system with a much lower noise figure than the analyzer > alone - lower even than your repeater receiver. This will allow you > to see the noise that is causing the issue. [Unfortunately - you still > have to figure out where the noise originates] > > Some analyzers have a built in preamp. Generally these are optimized > for display flatness and have a lower gain than an external > amplifier. The use of an internal amplifier will improve the > analyzer's noise figure but not to the extent that an external high > gain amp can. This improvement may be sufficient or not depending on > the specific instrument and the noise level you are attempting to > measure. > > The use of the LNA has tradeoffs in the form of reduced dynamic range > and reduced IM performance of the analyzer. Additional selectivity > may be required ahead of the LNA. >
