Mark, You have made some reasonable suggestions. I don't think I can do the surface mount resistor but I might find some equivalent mod possible. I have looked at the spectrum around the TX and RX frequencies but have not looked at IF and combinations.
I have to work for a few days now, so there will be a pause in my attempt to solve the problem. Thanks, JohnT AF4PD -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Mark Harrison Sent: Wednesday, September 03, 2008 8:40 PM To: [email protected] Subject: RE: [Repeater-Builder] Re: Wits End -- Desense (actual Cable-Q contributions) Hi John, I'm not familiar with that particular radio, but would it be possible to disconnect the antenna feed at the Rx PCB and place a 50 ohm surface mount resistor in it's place? That may allow you to differentiate between shielding problems in the receive antenna cabling and other possible issues of control wires and receiver board shielding. I guess it's a bit hard then to feed in a signal on the receive frequency, but you may still be able to detect a change in unsquelched noise when the Tx operates. On another note, I presume you've probed around on the receive frequency as well as the Tx frequency, and everything in between, including I.F. frequencies? Once I had a repeater that was being upset by a 5 volt three terminal regulator chip that burst into oscillation at 50MHz when the supply dropped slightly during transmit. Another repeater had a similar problem with sidebands appearing a couple of MHz either side of the transmitter. It turned out to be a another voltage regulator oscillating at about 1MHz (a discrete component circuit this time). Surprisingly a tiny bit of this got past all the bypass capacitors and found it's way into the PA pre-driver where it mixed and produced the sidebands. Although the sidebands were more than 40dB below the fundamental, even after the diplexer they presented a significant signal on the receive frequency at the receiver input. Of course the sidebands drifted in frequency across the receive frequency, changing with temperature just to make diagnosis all that more interesting! Good Luck and 73, Mark VK3BYY Melbourne, Australia ________________________________ From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com [mailto:Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com] On Behalf Of John Transue Sent: Thursday, 4 September 2008 9:25 AM To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com Cc: [EMAIL PROTECTED] <mailto:repeater%40viennawireless.org> less.org Subject: RE: [Repeater-Builder] Re: Wits End -- Desense (actual Cable-Q contributions) Skipp, Thanks for the suggestion. I have tentatively concluded that the "desense" problem is not classic desense caused by too much RF from the TX getting into the RX. I have used a spectrum analyzer and a "sniffer" probe to locate the RF. But the only RF I can find is at the TX frequency. I don't see any at the RX frequency. The dynamic range of the spectrum analyzer appears to be at least 40 or 50 dB. With the duplexer adding another 79 or so dB and the receiver having selectivity, I can't see how the RF level from the TX can be a problem. Nevertheless, when the repeater transmits, the receiver doesn't hear as well as otherwise. I'm thinking that the COR board might have a problem that is somehow feeding into the receiver. Have you ever heard of such a case? The problem seems to be independent of the external cables and feedline and antenna. I have experienced it with dummy load, with antenna, without the duplexer, with various lengths of cables, etc. I'd like to have some RG214 for test purposes, and I'd like to have some additional RG400. Getting cable is a two-hour round trip for me, so I can't do that for a few days. I hope to get to the cable store (The RF Connection) soon. Thanks for all the help, Skipp. With you an others from Repeater Builders holding my hand, this problem might actually get solved. JohnT __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com
