Jim, Many portable and mobile radios use a single IC in the back end that incorporates the second LO, the second mixer, some additional amplification and filtering, and detection. Such ICs often include a Received Signal Strength Indicator (RSSI) output that may be used in the squelch circuit, but it may also be unused. My comment to Terry was to suggest that he connect a sensitive analog meter to this output, which may be suitable for tweaking the notch capacitors for minimum signal while tuning a duplexer. This usually works only on very low power repeaters, since most service monitors cannot produce an output much above +10 dBm.
73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jim Cicirello Sent: Wednesday, September 23, 2009 12:36 PM To: [email protected] Subject: [Repeater-Builder] RE: RSSI VOLTAGE Eric, After reading this article, I have searched everywhere I could to learn about RSSI Meters. I found the Signal Meter by Mark Weiss built around the LF353 JFET Op-Amp. Can you elaborate on this meter, such as where does it hook to the radio being tuned or the radio being used to tune a Duplexer or Filter? One article mentioned the IF Section of the radio. On the schematic for the meter it states RSSI Input and at that point it has a couple of caps for Preemphasis for Audio Use, which I thought may indicate the Discriminator Output of the radio. As you may know, there is not a lot of information out there. I have a SINAD Meter and I would like to know if this can be as user friendly and give me more precise readings. Thanks for your information, help and patience with us who want to learn how to do things a little better with the with equipment we have or in this case can build. 73 JIM KA2AJh From: [email protected] [mailto:[email protected]] On Behalf Of Eric Lemmon Sent: Sunday, September 20, 2009 1:53 PM To: [email protected] Subject: RE: [Repeater-Builder] UHF Duplexers Terry, In lieu of the ideal bench equipment, yes, the IC-2710 is better than a handheld, since its impedances are supposed to be close to 50 ohms at the connector. You are to be commended for your creativity in making do with what you have. Since your duplexer has been moved from its original settings, you can only improve matters. Let's review your process: You generate your 449.625 MHz receive signal with the service monitor, and inject it into the duplexer's antenna connector, with your monitoring radio set to 449.625 MHz and connected to the RX (high pass) connector of the duplexer, and a 50-ohm dummy load on the TX (low pass) connector of the duplexer. The SINAD input of your service monitor is connected to the monitoring radio's speaker, and you adjust the center rods of the high-pass cans to get 12 dB SINAD with the lowest level signal at the RX connector. Next, without changing any of the connections, you set the service monitor to generate 444.625 MHz and program the monitoring radio to receive 444.625 MHz. With your service monitor generating the highest-level signal possible (at least 0 dBm, but higher is better), you adjust the notch capacitors on both high-pass cans to minimize the received signal. You might get better results by monitoring the RSSI voltage at the radio. Do not move the threaded tuning rods at all. SNIP
