I found a cure for the μBITX T/R click that WORKS! The scheme by VA7AT reduces it down to near nuthin'. I documented it on Thanks Wayne!

This is great Don, thanks! Somebody ought to market a small kit because I think everyone of the uBitxs need this mod.

Joel
N6ALT

开云体育

I thought the sketches were a nice touch. ?

--

Glad you like this method of? using resistor? attenuation? and get away from charging and discharging of capacitors when we power up circuits.? this method I have use last year on a commercial product I was working on to get rid of microphone mechanical switching noises. The 0.1uf? added to the gate gives it a slight delay and just in case RF pickup since we are using this in a transmitter. For a faster switching might try a smaller capacitor eg. 0.01uF.
??? i am busy lately on a new commercial project so I have to set aside the UBITX? hobby. Analogue? is more fun than digital but digital simplifies controls with more complexities .

?? wayne cheng va7at or ve7hcw 72

Hi Don
? I have revise the circuit to ensure that the mike thumping or tapping will not leak thru to the speaker. re-posted on the uBITX group in Facebook .

? wayne ve7hcw/va7at???? on Hf I use va7at. 72

From a visual inspection of Wayne’s original VA7AT “9-component” Pop Fix circuit (awesome document by Don ND6T on his website), I see the potential issue with why a?loud noise to the microphone could ‘sneak’ into the uBITX audio output in RX mode. The Source and Drain of the n-FET Q1 (To C50, Audio receive) are swapped in the design. ?There’s a body diode from source (pin 3) to drain (pin 2).? As drawn, there is still an attenuated signal path from the microphone to the receive audio amplifier though this body diode. Swapping the Source and Drain will eliminate this path, with no negative consequences that I can see.? I am curious if this would fix this “glitch”.

I am not at a point where I can try this out. ?But since I spied a possible simple circuit enhancement, I thought it better to share my thoughts and be wrong, then to be correct and not share.? That wouldn’t be of any use.

Best Regards,
Gary
AG5TX

Be careful of that as you need the gate to source relationship for switching.
The trick is to keep the diode "elevated" so its reverse biased with enough
voltage to prevent its conducting.

A 4053 analog switch would work there too.


Allison

A hint,

Most of the mics I use the audio line is switched so all the tapping and thumping never gets to the radio.
Prevents a lot of issues, like audio on the CW.

Allison

If there is no voltage applied to Q6 during transmit how much
microphone signal is going to be conducted through Q6?

The mic signal will see the base-emitter junction of Q6 but no collector
current can flow because of no voltage on the collector. So there
shouldn't be any attenuated mic signals at T7 and C222 to be conducted
by Q1 of the pop circuit.

tim ab0wr





On Sat, 07 Apr 2018 15:59:09 -0700
"Gary Anderson" <gary.ag5tx@...> wrote:

From a visual inspection of Wayne’s original VA7AT “9-component” Pop
Fix circuit (awesome document by Don ND6T on his website), I see the
potential issue with why a?loud noise to the microphone could
‘sneak’ into the uBITX audio output in RX mode. The Source and Drain
of the n-FET Q1 (To C50, Audio receive) are swapped in the
design.?There’s a body diode from source (pin 3) to drain (pin
2).?As drawn, there is still an attenuated signal path from the
microphone to the receive audio amplifier though this body diode.
Swapping the Source and Drain will eliminate this path, with no
negative consequences that I can see.?I am curious if this would fix
this “glitch”.

I am not at a point where I can try this out.?But since I spied a
possible simple circuit enhancement, I thought it better to share my
thoughts and be wrong, then to be correct and not share.?That
wouldn’t be of any use.

Best Regards,
Gary
AG5TX

Most communication mics are wired that way. The PTT switch doesn't just
switch the PTT leads but the mic leads also.

tim ab0wr

On Sat, 07 Apr 2018 20:22:07 -0700
"ajparent1" <kb1gmx@...> wrote:

A hint,

Most of the mics I use the audio line is switched so all the tapping
and thumping never gets to the radio. Prevents a lot of issues, like
audio on the CW.

Allison

Tim wrote:

> If there is no voltage applied to Q6 during transmit how much
> microphone signal is going to be conducted through Q6??

There's definitely power to Q6 during transmit.
I assume you mean during receive.
But with the VA7AT pop fix, Q6 is always powered.

?
From near the bottom of? ?
under section? Fifteen Component Fix:

"This circuit rewires the preamplifiers to be permanently powered and inserts serial gating in each of the inputs, controlled directly from the TX and RX power busses."

So Q6 is always powered up.


Jerry

Ahhhh!

I didn't use that pop fix. I should have read closer!

The message was about receive mode and mic leakage.

Thanks for the correction.

tim ab0wr

On Sat, 07 Apr 2018 21:15:36 -0700
"Jerry Gaffke via Groups.Io" <jgaffke@...> wrote:

Tim wrote:

If there is no voltage applied to Q6 during transmit how much
microphone signal is going to be conducted through Q6??

There's definitely power to Q6 during transmit.
I assume you mean during receive.
But with the VA7AT pop fix, Q6 is always powered.

?

From near the bottom of? ?

under section? Fifteen Component Fix:

"This circuit rewires the preamplifiers to be permanently powered and
inserts serial gating in each of the inputs, controlled directly from
the TX and RX power busses."

So Q6 is always powered up.

Jerry

On Sat, Apr 7, 2018 at 08:58 pm, Tim Gorman wrote:

If there is no voltage applied to Q6 during transmit how much
microphone signal is going to be conducted through Q6?

I am new to the BITX and that was my first post.? I thought it would follow the thread on the Pop Fix, but in hindsight, I should have given more background.? I was looking at the differences between the VA7AT “9-component” verses “15-component” pop fix. On the 15-compent fix the microphone signal is shunted to ground through a n-FET in RX.? To allow this shunt, a 220 Ohm resistor is added between Q6 and the shunt.? I was thinking that the added series resistor isn’t the way I want to go, given the reports of already low microphone audio. So, what was the issue with the 9-component fix?? Then I read through Don’s incredible write up on the 9-component fix.? At the end, he mentions the issue with loud sounds from the microphone going through the speaker during RX.? What is in the circuit would allow that?? Why is the 9-component fix not robust enough?

I am very rusty on my circuit design and analysis. In RX mode, Don’s Q1 (To C50, Audio receive) is hard on, so the body diode doesn’t come into play.? What was I thinking yesterday? Embarrassed today, and owning up to it.

It looks like the control signals RX and TX on the uBITX are driven to 12V, but not actively driven to 0V.? This may be a clue.

I concur with the comments, hints, and heads-up that there are other methods to remove the microphone signal during RX and CW-TX. I was just focused on the uBITX with factory given wire-up with the addition of the 2 proposed pop fixes. The directed comments did get me thinking more.? Thank You.

Also Thank You for being kind, patient and gentle with me as I am both humbly relearning what I used to know and learning new things along the way.

I bought the uBITX, to join in on the fun, tune up forgotten skills, learn and think.? Maybe one day, contribute.? Yesterday wasn’t that day.

Regards,
Gary
AG5TX

Gary,

I think we all bought the ubitx to have fun with it and to
learn/re-learn lots of different things.

I make all kinds of comments that aren't fully thought through and ask
all kinds of newbie questions. So far no one has gotten upset about it.
I doubt they will with you either.

The one thing your questions do is make us all think as well. That's
always a good thing. It *is* a contribution.

tim ab0wr



On Sun, 08 Apr 2018 11:05:49 -0700
"Gary Anderson" <gary.ag5tx@...> wrote:

I am new to the BITX and that was my first post.?I thought it would
follow the thread on the Pop Fix, but in hindsight, I should have
given more background.?I was looking at the differences between the
VA7AT “9-component” verses “15-component” pop fix. On the 15-compent
fix the microphone signal is shunted to ground through a n-FET in
RX.?To allow this shunt, a 220 Ohm resistor is added between Q6 and
the shunt.?I was thinking that the added series resistor isn’t the
way I want to go, given the reports of already low microphone audio.
So, what was the issue with the 9-component fix??Then I read through
Don’s incredible write up on the 9-component fix.?At the end, he
mentions the issue with loud sounds from the microphone going through
the speaker during RX.?What is in the circuit would allow that??Why
is the 9-component fix not robust enough?

I am very rusty on my circuit design and analysis. In RX mode, Don’s
Q1 (To C50, Audio receive) is hard on, so the body diode doesn’t come
into play.?What was I thinking yesterday? Embarrassed today, and
owning up to it.

It looks like the control signals RX and TX on the uBITX are driven
to 12V, but not actively driven to 0V.?This may be a clue.

I concur with the comments, hints, and heads-up that there are other
methods to remove the microphone signal during RX and CW-TX. I was
just focused on the uBITX with factory given wire-up with the
addition of the 2 proposed pop fixes. The directed comments did get
me thinking more.?Thank You.

Also Thank You for being kind, patient and gentle with me as I am
both humbly relearning what I used to know and learning new things
along the way.

I bought the uBITX, to join in on the fun, tune up forgotten skills,
learn and think.?Maybe one day, contribute.?Yesterday wasn’t that day.

Regards,
Gary
AG5TX

Gary,

I think you may be on the right track with that first post.
In it, you suggest the intrinsic source-to-drain diode in the supposedly off nFET tied to C63
of the mike amp might be conducting enough to disturb the audio during receive.
For this diode to conduct, the drain will have to be negative with respect to the source.

Since C63 blocks DC, and since the D5 conducts if the modulator sees more than a fraction?
of a volt either side of ground, that AC signal from the mike amp through the nFET will be near ground.
The AC coupled mike amp output will be driving the drain of that new nFET below ground
at times, and thus the nFET could be conducting a little bit during receive.
Reversing source and drain there is worth trying, though you might then see conduction on
positive peaks of the AC from the mike amp.??

Could be that adding the third nFET is the best solution here.
It is all a puzzle, these things are often way more complicated than they first appear.

There are paths to ground for the 12v TX rail at R80,81 and R85,86, that TX rail should fall
to 0 volts during receive.? No need to actively drive the nFET gates low.
But worth checking.? Use a DC voltmeter to check the gate voltage during receive.?
The 2n7002 has a minimum gate threshold voltage of 1.0 volts, anything less than that
and there should be no conduction through the nFET.

Here's Wayne's 9 part pop fix as documented by Don:??
and the new improved 15 part pop fix:from Wayne:??
?
Jerry, KE7ER
?

If you need more mike gain, could decrease the value of R63 as suggested by Raj in post 40742

Jerry, I need to call you out on that one! Things are ALWAYS more complicated than they first appear! :-)

Gentlemen, the reason for the microphone feed through is actually due to the peak negative excursions on the source. If you exceed the gate biasing then it will turn on. That bar of silicon is only a resistor while it is turned off but the thing that turns it on is the difference in potential between source and gate. Drive the source too far negative and it conducts just as if that gate were driven positive.
The microphone pass-through is a non-problem as far as I am concerned. My microphones all disconnect when the PTT button is released. Only if you have it wired so that it remains "hot" can you experience this when you drop the microphone hard. For those amplifying the microphone I would advise them to not use that much gain (it would sound horrible [I've done that] ) and to de-power that amplifier when not transmitting. Just a suggestion. It's always best to just try things. Otherwise we would never learn and certainly not happen onto something really fun!? 73,Don

I built up one of these anti thump circuits but have yet to actually try it in the rig.? My pcb is as below, about 26 x 10mm in size.?? Only difference to ND6T's version is I placed the 10uF's on the PCB also. ie remove from uBITX board and fit to this board. Otherwise just follow ND6T's web page for installation. Its all smd, 0805 parts.