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Ashhar Farhan VU2ESE's BITX40 is an amazing QRP SSB Radio and has set an unbelievable price-performance point. ?And many folks are adding new bells/whistles every day (like Allard and Jack). ??I've been working with a friend of mine, Willy W1LY, and his BITX40. ?Willy had installed the version 1.13 software from Allard and the HW modifications needed for CW. ?We have both noticed some interesting RF behavior when the radio first enters TX, even when on LSB (just PTT, with no audio).
After some investigation we are reasonably certain that the burst of RF is coming directly through the balanced mixer which is unbalanced for about 60 mS due to the microphone amplifier stage being switched on for transmit. ?If one starts with the radio in RX, then C124, the 47uF cap in the mic amplifier is sitting at zero volts. ?When the radio goes into TX, that cap has to charge up via R127 -- which takes a LONG time. ?During that time, Q12 is also powering up and C1
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Arrghh -- sorry, phone call and I hit the wrong button.Continuing ...
When the radio goes into TX, that cap has to charge up via R127 -- which takes a LONG time. ?During that time, Q12 is also powering up and C122 has been sitting at zero volts, so it starts charging as well. ?This is the response of the Microphone Amplifier to the rising edge of the TX signal -- and the net result is that the Balanced Mixer (T4/D15/D16) is knocked out of balance until the amplifier stabilizes. ?The result of that is a spurious RF transmission that looks like the one I have attached as a file. ?The RF rises almost immediately to about 5-8 watts, within 5 ms drops down to about 3 watts, builds back up to 5-8 watts in another 5 ms, then slowly decays over a period of 60 ms decays down to close to zero. ?(All of this observed with the PTT line held).
That's pretty much the equivalent of a DOT at 20 WPM. ?You can hear that TX burst clearly on a monitor receiver.
When sending CW that initial burst may be masked by the first element as long as the speed is slow enough (< 20 WPM).
Since the unintended RF burst is caused by the transient response of the stage being powered up, a more optimal solution would be to keep that stage powered at all times, but shunt the audio to the base of Q12. ?Reducing the size of C124 would help shorten the burst.
73, Bob, WB4SON
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Folks have been reasonably certain of that on Bitx rigs for about 15 years now. ?A simple fix for the kerchunk into the headphones when entering receive was recently posted here, and there have been many others. ?The transmit kerchunk you describe has not been addressed so much. ?Easier to fix now that we have the Raduino, could shut down the Si5351 VFO buffer at CLK2 until that kerchunk is over with. ?Though 60ms is a rather long time, if using VOX that timeout I'm suggesting could easily clip off something significant. ?Let us know when you find a better fix.
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On Wed, May 31, 2017 at 02:07 pm, Bob wrote:
the burst of RF is coming directly through the balanced mixer which is unbalanced for about 60 mS due to the microphone amplifier stage being switched on for transmit.
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Hi,
I think PTT noise is an issue which most everyone if not all have noticed.? There are several solutions. I finally decided on a circuit which uses a FET and transistor to cure the problem, I have done it on both ?of my BITX40 and it cleared the problem.?
Here is the article:Email -- Subject - ?PTT Thump mute
On Sun, Apr 30, 2017 at 6:28 PM, John Pieper?<j.pieper@...>?wrote:
Hope this what you were looking for Joe VE1BWV
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Hi Joe,
That's a good cure for the RX thump, but it doesn't deal with the TX spur that is about 60 mS wide.
73, Bob, WB4SON
On Wed, May 31, 2017 at 6:12 PM, Joe <joeman2116@...> wrote: Hi,
I think PTT noise is an issue which most everyone if not all have noticed.? There are several solutions. I finally decided on a circuit which uses a FET and transistor to cure the problem, I have done it on both ?of my BITX40 and it cleared the problem.?
Here is the article:Email -- Subject - ?PTT Thump mute
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Message 22961 proposed a solution to this. I implemented it thinking it was a solution to the audio pop and so unimplemented it when it didnt solve for problem. Unfortunately i didnt test to see if it solved the carrier burst issue.
Regards?
Simon VK3ELH
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See my solution: /g/BITX20/topic/ptt_pop_hack/4460108?p=,,,20,0,0,0::Relevance,,PTT+pop+raj,20,2,0,4460108 Raj
At 01/06/2017, you wrote: Ashhar Farhan VU2ESE's BITX40 is an amazing QRP SSB Radio and has set an unbelievable price-performance point. And many folks are adding new bells/whistles every day (like Allard and Jack). I've been working with a friend of mine, Willy W1LY, and his BITX40. Willy had installed the version 1.13 software from Allard and the HW modifications needed for CW. We have both noticed some interesting RF behavior when the radio first enters TX, even when on LSB (just PTT, with no audio).
After some investigation we are reasonably certain that the burst of RF is coming directly through the balanced mixer which is unbalanced for about 60 mS due to the microphone amplifier stage being switched on for transmit. If one starts with the radio in RX, then C124, the 47uF cap in the mic amplifier is sitting at zero volts. When the radio goes into TX, that cap has to charge up via R127 -- which takes a LONG time.
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A very nice $0.01 solution.
Except it kills how Allard is doing CW.
He unbalances the carrier by putting 5v into a 10k resistor that goes to the top of C107.
Maybe fix this PTT pop somehow in microphone preamp so we can continue to use Allard's CW method??
Could transmit CW like Farhan is doing it on the uBitx: ?Move the VFO to the transmit freq and unbalance the first ring mixer. ?Those three mixer ports got stirred up considerably on the uBitx, though it is otherwise the same mixer as on the Bitx40. ?Is there a some simple way to do this without a significant hardware hack to the existing Bitx40 boards?
Don Cantrell was injecting RF at the transmit freq from a spare Si5351 output, ?Could key it by writing to an Si5351 register, so could be done with just a resistor. ?But this method does not offer an easy method for key shaping, whereas the mixer unbalance schemes used by Allard and Farhan do. ?With Allard's it's C107 that gives key shaping, on Farhan's uBitx it's C1. ?Though I think the uBitx's C1 may need to be considerably larger (maybe 1uF, though I'm not sure how the BiDi amp affects this) to give appropriate rise and fall times to the keying envelope. ?Allard's original 47k resistor was probably about right for the 0.1uF C107, the move to 10k makes the edges a bit fast, but we could go back to 47k if the BFO is driven from CLK0 and thus can be centered in the crystal filter passband. ?Keying envelopes may not matter much for a one off rig at 5W, but for wide distribution and the inevitable use with QRO amps, I'd want clean CW key shaping.
Jerry, KE7ER
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On Wed, May 31, 2017 at 08:13 pm, Raj vu2zap wrote:
See my solution:
/g/BITX20/topic/ptt_pop_hack/4460108?p=,,,20,0,0,0::Relevance,,PTT+pop+raj,20,2,0,4460108
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Jerry,
We could put an audio isolation transformer between the mic/preamp stages
and the mixer.
The mixer side will need to be ground isolated then Allards CW will work
and probably the carrier leak on SSB will be minimum.
Some thing like these transformers:
I have some isolation transformers, will experiment when I get back
to town next week.
Cheers
Raj
At 01/06/2017, you wrote:
A very nice $0.01 solution.
Except it kills how Allard is doing CW.
He unbalances the carrier by putting 5v into a 10k resistor that goes to
the top of C107.
Maybe fix this PTT pop somehow in microphone preamp so we can continue to
use Allard's CW method?
Could transmit CW like Farhan is doing it on the uBitx:? Move the
VFO to the transmit freq and unbalance the first ring mixer.? Those
three mixer ports got stirred up considerably on the uBitx, though it is
otherwise the same mixer as on the Bitx40.? Is there a some simple
way to do this without a significant hardware hack to the existing Bitx40
boards?
......snip........
Jerry, KE7ER
On Wed, May 31, 2017 at 08:13 pm, Raj vu2zap wrote:
- See my solution:
-
/g/BITX20/topic/ptt_pop_hack/4460108?p=,,,20,0,0,0::Relevance,,PTT+pop+raj,20,2,0,4460108
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Possible. ?Wants $1.39 + $3.15 for shipping, so the transformer is kind of painful.
Especially with a thousand or more rigs spread around the world now.
I'd probably advise that most just live with the pop and use Allard's scheme for CW
Those that aren't interested in CW can use your easily scrounged 0.1uF fix.
?
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On Wed, May 31, 2017 at 09:39 pm, Raj vu2zap wrote:
We could put an audio isolation transformer between the mic/preamp stages and the mixer.
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Same transformers are available from China, 10 for 3$ with free shipping
or so.
Some sellers with 2/5 pcs even cheaper.
Raj
At 01/06/2017, you wrote:
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Possible.? Wants $1.39 +
$3.15 for shipping, so the transformer is kind of painful.
Especially with a thousand or more rigs spread around the world now.
I'd probably advise that most just live with the pop and use Allard's
scheme for CW
Those that aren't interested in CW can use your easily scrounged 0.1uF
fix.
?
On Wed, May 31, 2017 at 09:39 pm, Raj vu2zap wrote:
- We could put an audio isolation transformer between the mic/preamp
stages and the mixer.
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I like Raj's 0.1uF fix, very simple and elegant. ?Oh, and very cheap.
Could add a jumper across it when operating CW.
Also, remove a new jumper that supplies power to the mic amp, not needed for CW.
That fixes the transmit pop on both CW and SSB without too much trouble.
If willing to do a significant hack to avoid messing with jumpers,
rearrange the first mixer for CW injection as on the uBitx and add Raj's 0.1uF modulator cap.
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On Wed, May 31, 2017 at 09:58 pm, Jerry Gaffke wrote:
I'd probably advise that most just live with the pop and use Allard's scheme for CW
Those that aren't interested in CW can use your easily scrounged 0.1uF fix.
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Hi
I just become a proud owner of BITX40. After 2 evenings tinkering,?I got "first smoke" out (as literally, not virtually ,?whew). I found this?RF spur issue, with among some others, but I think this?must be solved.?It is unacceptable.
After two seconds of thinking,?I'm afraid that the audio trafo between the mike amp and mod won't do any good. When the power is applied to the mic amp, this will cause sudden?current flow through the primary, and what does the secondary ?
The spike will be there present there too.
Instead of sticking in this perculiar way of generating CW, one could think alternative methods. Raduino's extra oscillator maybe and for those w/o , injecting sine wawe to mic will work also.
Timo,OH7LMQ
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Just to report back, changing C124, originally a 47 uF cap, to 4.7 uF, reduces the duration of the TX burst on PTT from 50 mS, to less than 25 mS.? In fact, it is now so short, that it has essentially zero impact on any CW speed -- in other words, the burst is covered up by the first dot or dash that is sent.
Real easy fix to swap that out before the board is installed in the case, as it is a thru-hole component.? My thanks to Willy, W1LY, for making this mod on his Bitx40.
73, Bob, WB4SON
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hi
my way of generating cw is a completely separate cw transceiver.
terry gm4dso
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Bob
Raj vu2zap