"Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Still struggling with the image problem, I have this morning decided to scope the output of the roofing filter whilst injecting signal on the wanted frequency and then 50 kHz image frequency. It feels like a spectrum analyser would be really useful at this point but my test equipment is basic and an old scope is as advanced as things get here.
What I found is that with my S9+40 signal from the TS570 on 14020 and the R-4C tuned to that frequency I see a signal on the output of the first I/F filter of some 140 mV. When I change frequency enough so that the mixing products fall outside of the passband of the filter then I see noise of around mV. Reducing the output frequency of the TS570 to 13970, I see nothing discernable either above or in the noise from this image signal. I guess that might not mean it isn't there, just that my measurement is noise limited. I reset the TS570 to the wanted frequency of 14020 and restored the 140 mV signal to the scope screen. What I found next is puzzling. I rotated the PBT and found that this affected the output signal from the roofing filter significantly. In fact by rotating this control I could adjust the amplitude of this signal in the range 140 mV p-p down to 50 mV p-p. I would have thought the PBT should have no effect on this signal as my TS570 carrier should, by the time it exits the roofing filter, only have been mixed in the 1st mixer with the premix combination of VFO and band xtal oscillator. The first injection point for the BFO is into pin 7 of the 6BE6 second mixer, somewhat downstream. I briefly wondered what kind of stray pickup might cause the BFO to interfere with signals exiting the roofing filter but then decided to interupt the established injection path for thr BFO to see what that did. This was easy. I removed the 6BE6 2nd mixer. The interference caused by the BFO ceased immediately. This at least confirmed that what I was seeing was not caused by BFO signal somehow entering the premix chain. To rule out a fault with the 6BE6, I replaced it. That made no difference. Just now, I am pondering how a BFO signal injected into a grid of the second mixer gets to affect the output of the roofing filter two stages earlier. Can the BFO signal really propagate through the 6BE6 appearing on g1 then feed into the drain of Q1 and appear at its gate. Seems odd to me but I'm no expert. Help! 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/ ----------------------------------------------------------------------
