"Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- I understand it's bad etiquette to respond to your own posts but this is more of an addendum than a response, as such :-)
Before I go further, I should point out I have a powerful feeling that if I knew what I was doing, I might possibly be on the verge of achieving something useful! The reality however, is that I obviously don't know what I'm doing and consequently, although I can see the trees clearly, it's hard to discern the wood :-) Unable to avoid tinkering, I have set about reducing the input to the 1st mixer from the RF stage. Having decided to do this, I first took the step of improving the DR of the RF stage by reducing the value of the screen feed resistor from 18k to 6.8k increasing Vscr from around 55V to 95V. I then set about reducing the stage gain. I seperated g1 of the RF stage from AGC1 and instead applied a variable bias voltage. I currently have this set to -7.5V. With this arrangement, all sign of IMD3 products on 40/20m have entirely disappeared. I have adjusted the s-meter to compensate for reduced RF gain and everything appears hunky dory, though I don't believe it really is, even for a moment. Essentially, an S9+40dB signal at the input now yields about 250mV p.p. at the o/p of the RF amp. IMD3 products appear to be >80dB down. With about 2.1V p.p pre-mix injection this is an injection ratio at the input of the 1st mixer of around 8:1. A less enquiring mind might consider this a fix but I'm not convinced! Have I achieved anything more than a front-end attenuator would achieve? What should the pre-mix injection level really be? With strong BC signals on 40m, what is an expected p.p o/p from the 6BA6 RF stage? If I could only lay my hands on the appropriate addition of Soap, I'm sure these, among other, questions would be answered :-) I'm fumbling around in the dark here. Does anyone have a torch? 73 Bob, 5B4AGN ----- Original Message ----- From: "Bob Henderson" <[EMAIL PROTECTED]> To: "drake mailing list" <drakelist@www.zerobeat.net> Sent: Friday, August 12, 2005 8:37 AM Subject: [drakelist] R4C IMD problem (was image problem) > > "Bob Henderson" <[EMAIL PROTECTED]> made an utterance to the drakelist gang > ---------------------------------------------------------------------- > "Oh no he's back!", I hear you say. > > I finally got some time this morning to put the R4C back on the bench. I > have modified my test setup to use direct injection of signals via > attenuators. My signal generators are a TS570 and a TS870. Both are > producing 10W output into dummy loads from which signals are bled off > through 100k resistors to a common 50 ohm resistor. The output across this > second 50 ohm resistor is fed to a 2 way coax switch allowing me to inject > the resulting two tone signal into either the R4C or a TS930. The TS570 was > running on 14100 kHz and the TS870 on 14170 kHz. > > I injected the two-tone signal into the TS930 and it produced signals on > 14100 & 14170 of around S9+50dB on the 930 s-meter. I checked for a third > order product on 2*14100 - 14170 = 14030 and it was audible but making no > discernible s-meter deflection. Somewhere around 90dB down. Next, I > injected the same two-tone signal into the R-4C. Signals at 14100 & 14170 > indicated S9+45 on the R-4C and at 14030 indicated S9. Apparently, only > around 45dB down. > > I then connected the 930 via *10 scope probe to the output of the 5645 > roofing filter and tuned the 930 to that frequency. I tuned the R-4C to > 14100 & 14170 and found 1st mixer output on 5645 at around S9+40. I then > tuned the R-4C to 14030 and the 930 s-meter indicated S8 or around 45dB > down. > > At this point I believe I have reconfirmed the following: > > 1. The TS930 has adequate IMD3 performance to serve as a spectrum analyser > in this test. > 2. My makeshift combiner/attenuator for the signals from the 570 & 870 > introduces no discernable IMD of its own. > 3. The R-4C IMD problem is caused somewhere in the 6BA6 RF stage and 6EJ7 > mixer stage. > > Next, I connected the output of the R-4C rf stage into the TS930 via my *10 > scope probe. I checked for signals on 14100 & 14170. These were > end-stopping the 930 s-meter so I added some attenuation to bring them down > to S9+50. I checked 14030 and found the IMD was at S7 or around 65dB down. > This is not good but not as bad as only 45dB down. > > I increased the screen voltage on the RF amplifier 6BA6 and found I could > increase the gap between the two-tone signal strengths and the IMD to better > than 80db. At this point I thought I was onto a winner. I reconnected my > *10 scope probe to the 5695 roofing filter output and checked for the > relative strength of the IMD to two-tone signals at that frequency. > Disappointingly the gap had narrowed to <40 dB. > > My suspicions at this point are: > > 1. The IMD performance of the 6BA6 RF amp can be improved by an increase in > screen voltage. > 2. Increasing the screen voltage of the 6BA6 also increases its gain which > further overloads the 6EJ7 mixer causing its IMD performance to deteriorate > more than the RF amps performance improves. > 3. The 6EJ7 mixer is primarily responsible for the IMD problem, probably due > to inappropriate levels of input signal. > > Next, I measured the pre-mix injection level at the junction of C31 and T4. > This was a good clean sinewave of 2.1V p.p. I then switched off the pre-mix > chain by selecting an accesory xtal which wasn't present and measured the > amplitude of my two-tone signal output from the 6BA6 RF amp. Measured at > the junction of C29, C197 and C30 this was found to be 1.2V p.p. > > I am very surprised by this roughly 2:1 ratio and not surprised at the > resulting IMD output. I feel sure the ratio of pre-mix to received signals > should be much greater. > > Next, I monitored the two-tone signals emerging from the 6BA6 RF amp and > reduced their level with the RF gain control until it fell to 0.35V or 350mV > p.p. At this level the ratio of pre-mix to received signals has increased > from 2:1 to 6:1. > > I reconnected my *10 scope probe on the 930 input to the output of the 5645 > roofing filter. Signals at 14100 & 14170 were S9+40. THe IMD signal on > 14030 was audible but not moving the S-meter. That's 80dB down! > > So where do I think I've got to? > > 1. I can improve IMD performance of the RF stage by increasing screen > voltage on the 6BA6 RF amp. That's for later though, as the big problem is > apparently with the 6EJ7 first mixer. > > 2. There is a problem with the relative amplitude of the inputs to the 6EJ7, > possibly also with their absolute levels. > > I have two questions: > > 1. Is a pre-mix injection level of 2.1V p.p. measured at the junction of T4 > and C31 what I should expect? > 2. Is an output of 1.2V p.p. from the 6BA6 the expected level from signals > which read S9+45 on the S-meter? > > Many thanks for your indulgence, if you've taken the time to read the above. > Any help will be appreciated. > > 73 > > Bob, 5B4AGN > > > > > > ---------------------------------------------------------------------- > On Behalf of "Bob Henderson" <[EMAIL PROTECTED]> > Submissions: drakelist@www.zerobeat.net > 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/ > ---------------------------------------------------------------------- > ---------------------------------------------------------------------- On Behalf of "Bob Henderson" <[EMAIL PROTECTED]> Submissions: drakelist@www.zerobeat.net 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/ ----------------------------------------------------------------------
