[Elecraft] MMTTY FSK

[Rick Prather]

Chen,

Very interesting.  Thanks for your link.

Have you looked at the waveform with the AFSK TX Filter turned on?

Rick
K6LE

On 6/12/2010, at 9:09 , Kok Chen wrote:

> On Jun 12, 2010, at 6/12    5:09 AM, Richard Ferch wrote:
> 
>> Brett Howard wrote:
>>> I do see FSK as a method that is slightly less prone
>>> to TX distortions.
>> 
>> I have done some testing with a second
>> receiver, and my K3's RTTY signal seems to be cleaner in AFSK than FSK
>> (lower odd-order sidebands).
> 
> That is indeed correct.  AFSK can potentially produce cleaner signals  
> than FSK, as long the transmit IMD is good.  Take a look at the last  
> two figures on this page:
> 
> http://homepage.mac.com/chen/Technical/K3/Digital/digital.html
> 
> cocoaModem applies a filter to reduce the keying sidebands.  Except  
> for having two tones, the result is similar to what Alex VE3NEA had  
> described in his CW keying experiments article in QEX a couple of  
> years ago.
> 
> The difference between FSK and AFSK can be even greater than what I'd  
> measured if the K3 transmitter were cleaner.  Waveshaping the keying  
> signal created albeit small, temporal overlap between the two RTTY  
> tones, whereas an unwaveshaped FSK signal has no overlap and can be  
> used even with class C amplifiers.
> 
> You can indeed see the low level IMD spikes from the in the last  
> screen capture.
> 
> 73
> Chen, W7AY
>