"Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Just in case anyone should be interested, I have today made some progress with my 50 kHz image problem.
As I suspected might be the case, the ability of the PBT control to adjust the amplitude of a signal exiting the roofing filter turned out to be a red herring. The dip in amplitude at the output of the roofing filter ocurrs when the PBT is adjusted such that the signal exiting the filter, when mixed with the BFO, is within the passband of the 2nd I/F filter. When I select the 1.5 kHz filter setting, which has no filter installed, then the PBT no longer affects the amplitude of the roofing filter exit signal. I haven't quite got my head around what causes this effect but as it is a red herring, I won't strain too much over it. Absent a spectrum analyser, I pressed my old TS930 into service, in lieu. I coupled the output of the roofing filter to the antenna jack of the 930. With my carrier from the TS570 on 14020 and the R-4C tuned to that fequency, I found a thumping great carrier on the TS930 at around 5645 kHz. So far, so good. I set the TS570 to 13970 kHz and confirmed that no signals were heard within the passband of the 5645 roofing filter. I tuned the TS930 to around 5695 kHz where a signal was clearly audible and no more than 40 dB down on the wanted carrier on 5645 kHz. This didn't look good but I was pleased to find something obviously wrong. The 8 kHz Inrad GUF-1 filter claims a stopband of some 80 dB and what I was seeing was nowhere near that. This R-4C has the Sherwood CF-600/6 filter installed along with the Sherwood switching pcb to select between GUF-1 and CF-600/6. I stripped out the switching pcb and directly connected the GUF-1. I repeated the above test and found the 5695 kHz signal at the o/p of the GUF-1 (from the TS570 set at 13970 kHz) had now dropped almost to the point of inaudibility. I couldn't say with any certainty it was -80dB down on the wanted signal on 5645 kHz (from the TS570 on 14020 kHz) but it was certainly better than -60 dB. My problem appears to be caused by leakage across the Sherwood filter, switching pcb. I have inspected it for shorts etc but have been unable to find any obvious cause. I will contact Rob Sherwood tomorrow to see what he can tell me. Many thanks to all who offered advice. 73 Bob, 5B4AGN ---------------------------------------------------------------------- On Behalf of "Bob Henderson" <[EMAIL PROTECTED]> Submissions: [email protected] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: http://www.zerobeat.net Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
