tag:blogger.com,1999:blog-7590176649168185428.post8943311101485799136..comments2024-03-27T17:14:31.032-04:00Comments on SolderSmoke Daily News: A New Look at BITX Carrier Suppression; N6QW IN EMRFD! Bill Mearahttp://www.blogger.com/profile/07662500663603350847noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-7590176649168185428.post-41558187886733531762014-08-12T15:16:35.345-04:002014-08-12T15:16:35.345-04:00HI,
My target number has been not less than 40db ...HI,<br /><br />My target number has been not less than 40db and more like 50,<br /><br />Layout is important as leakage past the filter or more correctly around the filter is problematic. The BITX layouts I've seen put the Carrier o/c near the IF enough that it can blow by the filter.<br /><br />The balanced mod should hit 30db alone and some will do as high as 60 (mc1496) but practical is in the 45 to 50 realm. The filter should add about 10db to that, This means that the balanced mod should have both a resistive balance and capacitive balance. the key is balanced not adjusted to minimum. <br /><br />As to method many work. Since I build and often adjust before completion (milliwatt maybe) that is like a first light adjustment to insure the balanced mod is functional and balanced. I generally do it with a diode probe. adjust for balance and insure its a minima. To Do that I often offset the carrier os to the middle of the filter bandwidth or even jumper out the filter so that I have gain before the probe to see the really weak signal. I can then reset the carrier to the right point for the filter.<br /><br />Its tricks. Much of it learned while building my first examples over 40 years ago.<br /><br />Allison/KB1GMX<br />Allison/kb1gmxhttps://www.blogger.com/profile/16361028147602484427noreply@blogger.comtag:blogger.com,1999:blog-7590176649168185428.post-17673337867695885622014-06-25T15:07:38.969-04:002014-06-25T15:07:38.969-04:00I think one problem in the old days was heat, no m...I think one problem in the old days was heat, no matter how you balanced things, they'd come unbalanced when the tubes heated up, or the components aged.<br /><br />It wasn't uncommon to see a resistive balance, and a trimmer capacitor in the old circuits. Now, there seems to be less fussing, not sure if the circuits are better, or the components better matched, or what.<br /><br />There was one commercial phasing rig that had a notch filter at the carrier frequency, maybe a reflection of the balanced modulators of the day, but also no filter to knock down the carrier.<br /><br />With everyone buying lots of crystals on the same frequency in order to make ladder filters, maybe one of the surplus crystals would work as a notch filter at the output of the balanced modulator, maybe not much needed beyond the crustal, it would just be there to add that tiny bit more of carrier suppression. <br /><br />Reminds me of a 1974<br />in Ham Radio about building an SSB IF strip with TTL logic ICs, an intriguing idea but I suspect nobody went further with it. But they just used a gate as a mixer, so no ability to balance out the carrier. Instead,it relied on a crystal at the output to notch out the carrier.<br /><br /> Michael VE2BVW<br />Michael Blacknoreply@blogger.comtag:blogger.com,1999:blog-7590176649168185428.post-38291723281549238582014-06-23T16:15:38.086-04:002014-06-23T16:15:38.086-04:00whoops, the "diode trick" was posted/men...whoops, the "diode trick" was posted/mentioned by Arv and Leonard over on the Yahoo group -- not my idea (just trying to give proper credit!).<br /><br />-- Bert WF7IAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-7590176649168185428.post-72161353105164383352014-06-23T16:14:00.138-04:002014-06-23T16:14:00.138-04:00Hi again Bill.
Leonard, KC0WOX, was helping me ov...Hi again Bill.<br /><br />Leonard, KC0WOX, was helping me over on the Bitx Yahoo group. I'll re-post the link he posted there:<br />http://golddredgervideo.com/kc0wox/bitxver3/version3balmod.wmv<br /><br />He walks through his testing of the bitx "version 3" balanced modulator(a different kit than the one I built) and with it hooked up to a spectrum analyzer he reports 50-60 dB suppression (20 W out on the SSB signal), which seems quite good to me and better than what I obtained. You can keep sliding the BFO down in freq to improve this but after awhile the speech gets pretty ugly, sounds like a serious speech processor to me.<br /><br />I really appreciated that Leonard took the time to post such detailed vids of his testing, it helped me verify how mine was working in comparison with someone else.<br /><br />Another trick which I found fun to play with and easy to do is to find diodes with closer matched characteristics for the balanced modulator. At first I thought the variable cap and resistor ought to nullify slight differences in turn-on voltages with diodes and that didn't matter. But in my own testing I found a slight improvement in the carrier null I was able to achieve by just picking different diodes. <br /><br />Unfortunately I didn't do very systematic testing of this but I'd say maybe another 5 or even 10 dB might be possible, if such things matter (and I'd appreciate others to weigh in on "how much carrier should we suppress"). For my kit board, having less carrier suppression seemed to instigate oscillation problems in the PA taking place at lower output powers (which I guess makes sense). But maybe with a different layout and better isolation, that doesn't really matter.<br /><br />73,<br />Bert WF7IAnonymousnoreply@blogger.com