[Elecraft] JT65A output power variations.

[Joe Subich, W4TV]

> However, RTTY and many FSK or MFSK-type modes (including JT65A, 
> I believe) feature a single constant-amplitude carrier that is 
> stepped between tones. When there is only a single tone being 
> transmitted at any time, compression does not cause intermodulation; 
> it causes harmonic distortion (odd-order harmonics), which can be 
> eliminated by filtering (the transmitter's crystal filter). The 
> net effect after both compression and filtering should be a 
> constant amplitude signal. I believe this is why JT65A users 
> can safely use compression.

I would not use compression with ANY FSK mode.  Compression will 
change the system gain which will effect the wave shaping (e.g., 
raised cosine, Blackman, etc.) applied to the data transitions.  
Improper wave shaping can result in increased bandwidth (digital 
key clicks) or even level spikes on the leading/trailing edge 
of the data transitions. 

73, 

   ... Joe, W4TV 
   




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Richard Ferch
> Sent: Wednesday, November 11, 2009 1:58 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] JT65A output power variations.
> 
> 
> Julian, G4ILO wrote:
> 
> > Surely using a digimode set up for SSB with ALC operating 
> and 20dB of 
> > compression is going to cause distortion and splatter? Admittedly, 
> > JT65A isn't PSK31 but even so I would have thought IMD 
> products would 
> > occur,
> 
> Not all digital modes are equal. PSK31 depends on amplitude 
> shaping to 
> preserve its narrow bandwidth. It must be operated with no 
> compression 
> and with the requested power kept low enough to avoid clipping of 
> instantaneous peaks. Some other digital modes such as MT63 
> are similar.
> 
> Compression in these modes produces IMD which cannot easily 
> be filtered, 
> since it results in unwanted output at frequencies very close to the 
> intended frequency. The recommended way to operate these 
> modes on the K3 
> is with no compression, and with 5 bars showing on the K3's 
> ALC meter, 
> which puts the K3's ALC in an area where it acts to keep the 
> time-averaged power at the desired level without clipping 
> instantaneous 
> peaks and causing IMD.
> 
> However, RTTY and many FSK or MFSK-type modes (including JT65A, I 
> believe) feature a single constant-amplitude carrier that is stepped 
> between tones. When there is only a single tone being 
> transmitted at any 
> time, compression does not cause intermodulation; it causes harmonic 
> distortion (odd-order harmonics), which can be eliminated by 
> filtering 
> (the transmitter's crystal filter). The net effect after both 
> compression and filtering should be a constant amplitude signal. I 
> believe this is why JT65A users can safely use compression.
> 
> 73,
> Rich VE3KI
>