Topics

QSK noise

Jerry Wolczanski
 

40 meter version
S/N 1436

I found the QSK to be noisy when the volume was cranked up (clicks).  Thought it just might be a consequence of having the side-tone injected into the same amplifier chain as the signal.  I found a "sweet spot" where the side-tone and signals seemed in the right proportion....but the gain seemed low.  So....

I added a LM380 audio amplifier to the tail-end of the QCX, putting the supplied audio gain pot on the rear panel and using a 20K pot on the output between the QCX and the LM380.  It was very interesting to "set" the gain at the "sweet spot" with the original gain control...and then have the LM380 provide LOTS of audio gain (I can drive a speaker).

Cool rig - solid 4 Watts output.

I am really really impressed with the opposite side-band rejection.  Also impressive is the sensitivity and quietness of the front-end. 

I put mine in a slope-front cabinet that I picked up at a local hamfest.  I'll upload a photo. 

Got a couple of 40-meter rigs on line, the QCX and the othe,r a 2Watt/10Watt home-brew transmitter in conjunction with a home-brew receiver (a version of the "Binaural Receiver", a design by Rick Campell).  Will be exercising both of them tonight on SKN!



Life is good!
Jerry W
KI4IO
Warrenton, VA

dl5ybz@...
 

Hello Jerry,

thank you for suggestion regarding the QSK  click noise.    Got my QCX  running on 30m band for some days   and had also to find a kind of sweet spot for the side-tone  level  fitting my  my combination of QCX, OP, and uses earphone.  

I ended up with '75'  ..that works for me  using  a simple headset  from Samsung supplied with my mobile phone.       So for me it works without an extra AMP  for level control, the click is still there, but  does not interfere with my CW monitor feeling nor  is to noisy to  kill the great QSK feeling.

I tried to figure out,   what causes the click ..or better the complete noise that happens :) ...  

I'v got at least three  different source:

*   a low frequency ' whupping"  sound  from QSK it self...   this is  a full show stopper for using by  HiFi Headset   that is able to bring in this bass tones... maybe caused by the DC shift of the OP Amp?
*  a click   caused by the QSK .. but happens only after the first dot...  second dot ends with a click.. first not...    ( around Speed 25)
*  Hum  ...  simply  caused by  RF feedback into the audio part... not loud, but part of the game.

Using the relative  high level  side-tone  masked this sounds, because the side-tone  a lot of harmonics .. so the Audio CW bandpass does not really work perfect for side-tone on higher levels.. ( 1400 tone only -20db for me at side-tone level 75...   from the Audio Filter curve you should expect at least - 45.. )     so side-tone is no pure sinus.. and this works to masked the qsk noise :) .. 

I found the nice -45 db harmonic down at a level if sidetone 50 ...    you really hear the difference...  but the QSK noise is terrible ;) ..and semi BK is  really great..      ( if you  bring down the RF noise but using a well desingend antenna ;) .. not the  kind of random wire,  I have to use...)  

The best of all testing...  I learned  that it  is really  tricky to  see the 'click' part  of the audio on your scope to learn more about it's timing  in order to add a extra signal in the right moment for the click banking FET ..   

But this would be a kind of real nerd stuff...   the QSK click  does not really stop me from  having fun with this great  QCX radio...   and because the RX is that great,  it's really worth to get  100% out of it instead of 95 %  even his brings only a little bit more fun  in daily work. ;)

73 de Olaf / DL5YBZ



              
     



 

Jerry Wolczanski
 

Nice work Olaf in sleuthing this out.  I've opted to use the semi-break-in (VOX like!) and the little rig is infinitely nicer to use.
I added an LM380 amp to the tail-end of my QCX and I find myself juggling the two gain controls to find an optimum setting,
where the QSK noise is mitigated to some degree.

I totally agree about the QCX receiver - it is, in a word, superb. 

Jerry
KI4IO
Warrenton, VA

dk4rw@...
 

Jerry,

the QSK noise is caused by the charging current of C22 when Q7 turns on. This can be seen as a small, but fast voltage change at the connection R37/C22 (Ch1 yellow). Ch2 (blue) is the side tone at pin 1 of R36, Ch 3 (pink) is the audio out at the headset output with a headset connected. The scope trigger is derived from the drain voltage of Q4 (Ch4 green).
Replacing C22 (10 uF) with a 0.1uF capacitor reduces the charging current by a factor of 100 – and the thump is gone.

73, Wolfgang DK4RW

John Pagett
 

Wolfgang,

That looks to be a worthwhile modification.

I've added it to the mods to do to mine as I build it.

73
John
G4YTJ


Replacing C22 (10 uF) with a 0.1uF capacitor reduces the charging current by a factor of 100 – and the thump is gone.

DK
 

Wolfgang,

Will this be added to the ongoing list of modifications kept on the QRP labs website?

DK. KD6TK

On Jan 7, 2018, at 8:02 AM, John Pagett <john.pagett@...> wrote:

Wolfgang,

That looks to be a worthwhile modification.

I've added it to the mods to do to mine as I build it.

73
John
G4YTJ



Replacing C22 (10 uF) with a 0.1uF capacitor reduces the charging current by a factor of 100 – and the thump is gone.


@CurtisM
 

We can imagine yes, when Hans notices. Wolfgang thanks for sharing your work with oscilloscope measurements.  it will help many who wish to improve a fine radio.

Curt

Jerry Wolczanski
 

Nice fix Wolfgang - I replaced C22 with a .1uFd capacitor and that did it.  Works nice...so nice I removed my
rather inelegant "fix"...an outboard LM380. 

Thanks!
Jerry
KI4IO

RN4HGS
 

correct me if I misunderstand.
hpf cutoff frequency at -3 dB level = 1/(2pi*R37*C22)
and 0.27uF(600Hz)  0.33uF(500Hz) will be best choice than 0.1uF(1600Hz)?

Oleg RN4HGS

dk4rw@...
 

Oleg,

the frequency response of the AF amplifier chain is modified to some extend by a small value for C22. But due to the excellent AF filter, low noise AF OP Amps and a lot of AF gain, I did not find this to be a problem. For my ears, the receiver still sounds superb.
You may, of course, use larger values for C22. Wheather the QSK noise is then still acceptable I did not check.

73, Wolfgang DK4RW

Ulf DK5TX
 

Hello Wolfgang,

any recommendations on the type of capacitor? C22 is an electrolytic type, but 0.1 µF is hard to find as such. At least in my parts box. Having 100 nF ceramic capacitors though ...

73
Ulf DK5TX

Sven Ladegast
 

Hello Ulf,

Normally a polypropylene type is used for audio purposes. Ceramic capacitors work too but the low ESR of these capacitors can incorporate any new "switching noise". If dU/dt is too high, you can hear it as the popping sound. dU/dt becomes also high if the capacity is too high (in case of the electrolytic cap).

So slow capacitors with low leakage and useful low capacity are the right choice. For example Wima MKP 0.1 µF.

73!

Sven, DJ2AT

Am 19.03.2018 07:44, schrieb Ulf DK5TX:

Hello Wolfgang,
any recommendations on the type of capacitor? C22 is an electrolytic
type, but 0.1 µF is hard to find as such. At least in my parts box.
Having 100 nF ceramic capacitors though ...
73
Ulf DK5TX
--
Sven Ladegast
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98693 Ilmenau

Tel: 03677-4690104
Fax: 03677-4690105
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Alan G3XAQ
 

Ulf/Sven,

I used a common multilayer X7R ceramic 100nF (0.1uF) capacitor and it made the QSK thumping go away. So maybe in this application the type is not too important?

73, Alan G3XAQ

Ulf DK5TX
 

Hi Sven & Alan,

tnx for your comments. I'll try the X7R first, as this is in my stock. If this doesn't work for any reasons, I'll try to get the MKP.

73
Ulf DK5TX

dk4rw@...
 

Ulf,

the capaitor ESR is irrelevant in this application. R37 (1k) is conntected in series with C22. Hence even an (unlikely high) ESR of a few 10 Ohm would not not matter.
Also the capacity is uncritical. I received feedback that a 1uF capacitor already reduces the QSK noise considerably. So any value between 0.1uF and 1uF should be fine.

73, Wolfgang DK4RW