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Front End Protection Diodes?
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Just put two diodes back to back at the input of the receiver. On Thu, Jul 27, 2017 at 11:37 AM, Al Duncan VE3RRD <ve3rrd@...> wrote: There is a mod to protect Q13 during receive with 2 diodes. See for mod info. |
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Good luck On Thu, Jul 27, 2017 at 8:18 PM, Mr Sheesh via Groups.Io <rd232d@...> wrote:
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Back to back? I think not! That gives avalanche/breakdown voltage, and that can be up to kV depending on device. Try Head to tail - and across the input. +/-0.6V max, turn-on (forward-bias) voltage of silicon junction. 0.3V or so for germanium. If you're worried about capacitance, point-contact. This, of course is for RX ONLY! Tx into that will blow either the diodes or the Final! Not quite sure how it fits in the BITX scheme, probably in amongst the antenna relay circuitry.
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1n4148's seem to work fine.
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1n914's likely do as well, the Vishay datasheets look about the same. Could be Vishay ships the exact same part. The 1n914 is older, other manufacturers might spec a lower max forward current or something.? "Back to back" and "front to tail" are both a ambiguous. I suppose you could look at one of the drawings/photos previously posted. ? ?? Jerry On Thu, Jul 27, 2017 at 01:44 pm, Mr Sheesh wrote: I had been wondering if we were talking PIN diodes or what, 4148s |
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Dale Brooks KG7SSB
I have been trying to reduce the 2nd and 3rd harmonics leaving the BITX40v3 for days and I just added the 100pf capacitor across L7 LPF coil and I will say it has solved all of my problems with higher than ideal harmonics. It is mentioned in the WAX articles along with the opposing diodes on the base of Q13. `My results on the spectrum analyzer was dramatic reducing all spurious signals well below 50 dbm. I have added the diodes as well as the100pf cap and recommend it for all BITX users.? On Thu, Jul 27, 2017 at 2:10 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote: 1n4148's seem to work fine. |
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That's the term we have been using for ages. It means that one diode is placed with the cathode connected to one side of the circuit and the other connected the other way round so that they clip both sides of an AC voltage. (A DC voltage will also be clipped by the forward biased diode) Taking the input of a receiver, one diode would have its cathode connected to the antenna input with the anode connected to earth. The other diode would have its anode connected to the antenna input and its cathode connected to earth. Guess that different terms are used in different parts of the world. Regards. On Thu, Jul 27, 2017 at 9:53 PM, Dexter N Muir <dexy@...> wrote: Back to back? I think not! That gives avalanche/breakdown voltage, and that can be up to kV depending on device. Try Head to tail - and across the input. +/-0.6V max, turn-on (forward-bias) voltage of silicon junction. 0.3V or so for germanium. If you're worried about capacitance, point-contact. This, of course is for RX ONLY! Tx into that will blow either the diodes or the Final! Not quite sure how it fits in the BITX scheme, probably in amongst the antenna relay circuitry. |
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Dale Brooks KG7SSB
If you are installing 1n4148's across the antenna and ground you are just limiting the peaks, coming in on the antenna, to approx. .560 volts plus and minus.? If you use a germanium diode set you will limit the signals to .3 volts But you may get into trouble with strong signals limiting at this small voltage deviation causing distortion when the signal is clipping. Just a guess have not tested.? On Thu, Jul 27, 2017 at 10:58 PM, Lawrence Galea <9h1avlaw@...> wrote:
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Dale Brooks KG7SSB
The BITX40 vs3 is what I'm running and I purchased it about 2 or so months ago. I'm not sure what the older models require but I will say it really helped my rig to add the capacitor (100pf) to the L7 coil.? My second order harmonic (14Mhz) was only 12dbm lower than the primary frequency. To be in compliance with the FCC you should be 40dbm lower than the primary frequency or in this case 7Mhz band. To make matters worse the 3rd order component (21Mhz) was even more disturbing. Now with the addition of the capacitor across L7 I'm down -50dbm from the primary frequency including all spurious signals and well within FCC compliance. The person who started this thread had an attachment with full instructions showing where to place the diodes and the cap. Great hobby have fun! Dale Kg7ssb On Fri, Jul 28, 2017 at 9:55 AM, David <dokrent@...> wrote: I am a bit confused. I just recently bought the kit and from this thread I can't tell if you are referring to current or older units, or both. Seeking clarity. |
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Dale, are you using a 5MHz vfo? or have you changed to 19MHz and generate USB at 12MHz.
I note you have been using a spectrum analyser. Do you see any close in spurious outputs like in Tom Hurst's "BitX40 Spectrum Output" on page 2 of the files section? For me, the 19Mhz is not a solution because the xtal filter is so bad on that side. 73 Brian. |
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Michael Davis
The diodes are not in any build Bitx as far as I know They need to be added. Also a fuse and the 100pf cap. It does make it confusing for new members to catch up with all that has gone on since late 2016. It might make sense to list all the most important hacks that make this radio more reliable and more versatile. As an SSB guy, the cw hacks don't interest me. With that said, what is important to some may not be interesting or necessary to others. So maybe 2 lists. The SSB list and the CW list, with links...published occasionally for new owners. Not sure how this can be accomplished.
Sent from Mike's iPad WA1MAD |
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Dale Brooks KG7SSB
Brian, I'm just using the 5 mhz in its original format. I'm not familiar with the article by?Tom Hurst's I'll look it up but real quick I'm just seeing the harmonics at 14, 21, 28 the rest of the spurious signals are way down in the -100dbm range and not a concern. All said by adding the capacitance to L7 it's a clean signal.? Hope this helps and I'll have to find the file you are mentioning. ?73's Dale On Fri, Jul 28, 2017 at 9:08 PM, Michael Davis <maddmd818@...> wrote: The diodes are not in any build Bitx as far as I know They need to be added. Also a fuse and the 100pf cap. It does make it confusing for new members to catch up with all that has gone on since late 2016. It might make sense to list all the most important hacks that make this radio more reliable and more versatile. As an SSB guy, the cw hacks don't interest me. With that said, what is important to some may not be interesting or necessary to others. So maybe 2 lists. The SSB list and the CW list, with links...published occasionally for new owners. Not sure how this can be accomplished. |
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Here's one such list: ?/g/BITX20/message/28827
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I suppose I could edit it some and dump it into the files section, or maybe into the wiki. If you want a list that only has stuff in it that you care about, then go write it. Some scheme for current limiting should be added, perhaps separate fuses for the IRF510 and everything else. Also a voltage regulator to keep things to 12.0 volts for all but the IRF510, and reverse voltage protection.? ?? Jerry, KE7ER On Fri, Jul 28, 2017 at 09:08 pm, Michael Davis wrote:
The diodes are not in any build Bitx as far as I know They need to be added. Also a fuse and the 100pf cap. It does make it confusing for new members to catch up with all that has gone on since late 2016. It might make sense to list all the most important hacks that make this radio more reliable and more versatile. As an SSB guy, the cw hacks don't interest me. With that said, what is important to some may not be interesting or necessary to others. So maybe 2 lists. The SSB list and the CW list, with links...published occasionally for new owners. Not sure how this can be accomplished. |
AA9GG
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