"Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Hi Rob
Many thanks for taking the time to explain the mechanics of the mixing process to me. I appreciate it. You are correct that the signals on 13920 and 13970, which I receive with the R-4C tuned to 14020 have their pitch affected by rotation of the PBT. You are also right about my S-meter not being calibrated, so my measurements certainly lack precision. This R-4C has both an 8 kHz Inrad GUF-1 and a Sherwood CF-600/6 switch selectable in the first I/F. Switching between these filters makes no more than 2-3 db indicated difference to the strength of the images and is similar to the difference observed in the strength of the genuine 14020 signal. I would have expected the stopband of either of these filters to have been adequate to prevent a 50 kHz image problem, so I'm scratching my head, as you might imagine. I was in the UK a week ago and checked a friend's R-4C to see if there was evidence of a similar problem. My tests were limited but when listening to the xtal calibrator on 14025 and rotating the PBT nothing but a single carrier was heard, unlike on mine where I can hear multiple carriers weaker than the primary calibrator pip. I feel there must be a fault causing this effect and that same fault is likely also responsible for the troublesome nature of the image signals. Of course, I could easily be wrong. Regrettably, my test equipment is limited here. I'll keep looking. If anything else occurs to you, please let me know. Thanks again. Bob, 5B4AGN ----- Original Message ----- From: "Robert Schenck" <[EMAIL PROTECTED]> To: ""Bob Henderson"" <[EMAIL PROTECTED]>; ""Drakelist e-mail reflector"" <[email protected]> Sent: Friday, July 08, 2005 2:52 AM Subject: RE: [drakelist] R-4C image problem > HI Bob, > > I believe you have a pseudo image situation. when set to receive 14.020 MHz, the combination of the PTO and band crystal oscillator produce an injection frequency of 19.665 MHz. The first mixer combines this with your 14.020 MHz to produce a first IF signal at 5645 KHz. After passing through the first IF filter, the signal goes to the second mixer where it is combined with the BFO at around 50 KHz to produce 5695 KHz which is the nominal frequency of the SSB and CW filters. The third mixer converst the IF signal down to 50 KHz wher it is combined with the BFO inthe product detector. This is the normal sequence. Adjusting the BFO also called the passband tuning, effectively positions the narrow second IF filter as desired for USB, LSB or CW. there is no pitch change when adjuting the BFO as the offset is cancelled by the use of the same BFO signal for the second mixer and the product detector. > > A signal at 13.970 MHz will also mix with the injection in the frst mixer and produce a product at 5695 KHz, the frequency of the second IF filter. Since the second mixer is not balanced, the input signal passes right through to the second IF filter and on to the thifd mixer and then the product detector. This signals pitch will be affected by the setting of the BFO as it is only actually combined with it once. This signal will be down by the stop band level of the first IF four pole filter, roughly 35 to 40 dB at best. This is because the first IF is at 5645 KHz abd the signal is at 5695 KHz. Your S meter is no doubt not calibrated. Mine isn't accurate either. > > The signal at 13.920 MHz combines with the injection frequency to produce a product at 5745 KHz. This will mix with the BFO in the second mixer and produce a product at 5695 KHz and be passed on to the product detcor, as the others. However, the BFO injection is opposite in this case and the BFO will effect the pitch but at twice the rate per BFO knob roattion compared to the 13.97 MHz signal. in other words, the BFO pitch change would seem to be faster with this spur. Again, this signal is also out of the first IF passband and in the stop band attenuation. This signal is far enough away that the preselector will peak it by rotating counter clockwise slightly. > > I performed similar test with my R4C, which has a difference of having a second first IF filter in cascade with the orignal for nearly double the stop band attenuation. With my reciver the same BFO pitch effect was noticed, but the signal level was down by over 70 dB as measured using an HP-8640 signal generator. > > Besides scrounging up second, first IF filter to add to te signal path, you would have to replace the 6BA6 unbalanced mixer with a balanced mixer to try and reduce the 13.970 MHz spur. The 13.920 MHz spur is a true image signal and could only be reduced by the first method, that is the added first IF filter. Ideally, an eight to ten pole first IF filter that has sufficient bandwidth for your needs (CW/SSB/AM) would be something to consider. There is a replacement called the GUF-1 filter. I found them very expensive compared to getting a spare first IF filter from DRake when they still sold spare parts on the web. > > ONe could consider trying to use what is normally a second IF 8 pole AM filter at 6 or better, 8 KHz. bandwidth. The AM filter has a center frequency of 5645 KHz. The second IF filters are 50 Ohm impedances, while the first IF is 500 to 1000 Ohms. I have not experimented with this, but you would need impedance matching at both ends, perhaps by using small torroidal transformers. A 3.5:1 turns ratio on the input and 1:3.5 on the output might work. no idea wht material to use of the actual number of turns. > > Hope this helps. > > Rob - K2CU > > > > >"Bob Henderson" <[EMAIL PROTECTED]> wrote: > > >Dear reflectees > > > >I am wrestling with a rather disconcerting problem on an R-4C I am refurbing and I'm wondering if anyone else might have seen this and can help me nail it. > > > >I find I am receiving spurii of significant strength. So far I have only witnessed this on 20/40m but I suspect the problem may affect other bands. > > > >On 20m I hear multiple AM signals around the bottom of the band. These peak by rotating the preselector a few degrees counter clockwise. These signals are most troublesome as we approach dusk, which I attribute to the MUF falling to around 14 MHz. > > > >I have a similar problem on 40m but the spurii can be peaked by rotating the preselector knob a few degrees clockwise. Predictably these spurii are most troublesome after dark. > > > >The slight rotation of preselector control suggests these spurii are from signals slightly LF of 20m and HF of 40m. The fact that the antennas feeding the R-4C pass through a multiband Dunestar b/p filter, seems to further support this hypothesis, as the Dunestars would severely attenuate signals further away in frequency. > > > >Today, I set up a TS570 running 5W at 14020 kHz into a dummy load on my workbench. I attached a short pickup wire to the R-4C, peaked the preselector and found the signal on 14020 indicated S9+40 on the R-4C S-meter. I reduced the frequency of the TS570 (which is transmit enabled 1.7-30 MHz) and when I got to 13970 I heard a carrier on the R-4C (still tuned to 14020) with an S-meter reading of S9+15 or 25db down. When I reduced the TS570 frequency further to 13920 another carrier appeared on the R-4C, this time at S7 or about 50db down. > > > >I am trying to figure out what is causing this but without much success so far. One further thing I notice, which may well be related, is that if I tune in the calibrator signal to zero beat, I can hear another weaker carrier. I have found I can change the pitch of this carrier by rotating tha passband tuning control. At the 12 o'clock position the additional carrier disappears as it becomes zero beat with the calibrator. > > > >The problem would appear to be caused by harmonics of the 50 kHz BFO getting into places they didn't ought to. > > > >Does anyone have any ideas on what might be the cause of this problem? > > > >Many thanks. > > > >Bob, 5B4AGN > > > > __________________________________________________________________ > Switch to Netscape Internet Service. > As low as $9.95 a month -- Sign up today at http://isp.netscape.com/register > > Netscape. Just the Net You Need. > > New! Netscape Toolbar for Internet Explorer > Search from anywhere on the Web and block those annoying pop-ups. > Download now at http://channels.netscape.com/ns/search/install.jsp > ---------------------------------------------------------------------- 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/ ----------------------------------------------------------------------
