EMRFD Message Archive 11067

Message Date From Subject
11067 2015-05-06 12:25:08 jliving2001 CW Waveshaping
I would be interested in the groups thoughts on CW wave shaping. I would like to design a CW QRP transmitter with a keying circuit that  provides a raised cosine output.  What would I need to consider to design a optimized CW transmitter without necessarily using upconverting receiver architecture? .

Joe W3GW

11068 2015-05-06 13:14:19 Ephemeral Re: CW Waveshaping
You could think of it as an AM transmitter 100% modulated by a raised cosine
keying signal.

>I would be interested in the groups thoughts on CW wave shaping. I would
>like to design a CW QRP transmitter with a keying circuit that provides a
>raised cosine output. What would I >need to consider to design a
> >optimized CW transmitter without necessarily using upconverting receiver
>architecture? .

>Joe W3GW
11069 2015-05-06 13:30:16 Dana Myers Re: CW Waveshaping
11071 2015-05-06 14:21:06 k1rf_digital_stev... Re: CW Waveshaping
Hi Joe. The best article I have come across on the subject is in QEX May/June 2006 "CW Shaping in DSP software" by Alex Shovkoplyas, VE3NEA.  The waveshape is based on a Blackman-Harris window.  Apparently, many smooth looking waveforms look very similar but small differences can have large effects.  This one provides better bandwidth reduction than most with good intelligibillity.

I uploaded a spreadsheet file containing a 128 point waveshape lookup table based on this article.  The file is named K1RF CW waveshaping lookup table.

"Digital Steve", K1RF
11072 2015-05-07 05:34:30 joe living Re: CW Waveshaping

Thanks for the responses.

I am a student of the website put up by Tom, W8JI. He has a great discussion of the poor CW waveforms put out by some not so cheap rigs. You  can hear the clicks from these rigs during any major contest. 

I will review the article mentioned on DSP wave shaping

So, what about keying a mixer that would put out a partly shaped CW signal at an IF of, say, 9 mhz,  and then running this low level signal through a crystal filter with  a 250 or 500 hz bandwidth . But then we would have to use another mixer to generate a signal at a ham band frequency. Would I loose the shaping using this second mixer? I suspect I would need linear amplifiers to preserve the wave shaping. 

Joe W3GW

11073 2015-05-07 09:02:00 k1rf_digital_stev... Re: CW Waveshaping
There are many waves to skin the cat.  Here's an idea: An interesting function I came across is using a diode ring mixer as a current-controlled attenuator.  See page 23 of the following nice tutorial on mixers by Iulian Rosu, YO3DAC: http://www.qsl.net/va3iul/RF%20Mixers/RF_Mixers.pdf This technique will provide about 45 dB of attenuation range. To implement an analog function to drive the mixer, you can use an op-amp based  low-pass filter driven by a clean CW input signal from a CMOS gate that swings between 0V and its full power supply voltage. Or you can implement a comparator to drive the lowpass filter as part of a quad op amp.  The op amp needs to be able to drive  about 10mA but should be current limited to no more than 40 mA. An example of an analog CW waveshaping circuit is shown in Most ARRL handbooks. There is one in my 2010 handbook, Fig 13.36 — This schematic diagram shows a CW waveshaping and keying circuit suitable for use with an SSB/CW transmitter. It would need mods to get the right voltage levels to the diode ring mixer.

The mixer should be fed with the RF frequency of your choice but needs lowpass filtering at its output. 

"Digital Steve", K1RF
11074 2015-05-09 11:00:20 wn5ywn5y Re: CW Waveshaping
The waveshaping challenge I am facing is using a DDS VFO that has a key function that quickly removes the offset frequency for a superhet receiver and outputs the frequency needed for transmit.

Right now the solution is to key the DDS VFO and the end of the VFO amplifier section (which drives an amp and final amp for the transmitter) with an SPDT relay. Critical CW ops have noticed that the keying is hard, which is to be expected with a relay switching solution.

I have always wanted to do the switching electronically so that the CW wave can be shaped according to accepted standards. More specifically, take the edges off the wave form to eliminate the hard keying.

The idea of K1RF looks promising (using a diode mixer) and I wonder if it would shape the CW signal or if there are other solutions to be considered. The diode mixer complicates the solution because of it producing harmonics of the signal which requires a lowpass filter. One of the advantages of a switching DDS VFO is the elimination of a mixer and subsequent filter for the transmitter side.

Would be interested in any ideas.

David White, WN5Y
11075 2015-05-09 12:35:44 K5ESS Re: CW Waveshaping

Don’t know what DDS you’re using but if it’s an AD985x here is a possible solution.  The AD9850 can be AM modulated.  See: http://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf  . 





11077 2015-05-10 11:13:53 wn5ywn5y Re: CW Waveshaping
I am using the China AD9850  DDS module. What a simple solution to try!  I have all the parts and will try it out.
Thanks for the info.

The ultimate solution I would like to find is to replace the relay at the end of the VFO amplifiers with a waveshaping/switching circuit. But if the AM modulation works well, I will keep the noisy relay....

11078 2015-05-10 13:32:46 Tayloe, Dan (Noki... Re: CW Waveshaping
I have shaped the waveform two different ways:  Shape the bias to FET finals (RC rise time off of a keying logic level) or use a very low resistance FET in the ground returns of the finals and then shape the bias to it.

I really like using 3 BS170s as a 4 or 5 w class E final.  They work fine for me even on 10m.

- Dan, N7VE

Sent from my Windows Phone

11079 2015-05-10 20:48:19 K5ESS Re: CW Waveshaping

Let me know how it works out.  I’m working on designing my own CW transmitter and also have several of the AD9850 modules.




11080 2015-05-11 05:30:32 k1rf_digital_stev... Re: CW Waveshaping
Also see the following EDN app note:


You can drive the RSET pin of several DDS chips from a DAC controlled by an external processor chip.  This could, in conjunction with a lookup table, provide very fine control of waveform shaping. It is more complex than the other technique, however.

"Digital Steve", K1RF
11081 2015-05-11 07:49:34 Tayloe, Dan (Noki... Re: CW Waveshaping

If this is a CW signal, it might be challenging to do the shaping from the DDS end.  I assume that the shaped signal will have to go through several stages of amplification before hitting the antenna. This seems like it would work if all the stages up to the final are linear.  However, depending on your design, these stages may not be very linear. 


-          Dan, N7VE


11082 2015-05-11 08:55:07 Dana Myers Re: CW Waveshaping