Cheap and nasty sources - was LF: DK7FC WSPR bad quality problem

[Andy Talbot <andy.g4jnt@gmail.com>]

It is unfortunate these cheap and simple low power solutions have come-about at all.   The originators who worked out how to generate a few milliwatts of Rf from very non-purpose designed hardware presumably never anticipated that eh great masses would end up connecting this unsuitable kit to high power amplifiers.    I hope they only ever anticipated their being used for very low power uses.

As for the R-PI, that really does look like a bad way of generating narrow band RF.     I don't know the details of its internal hardare, but strongly suspect that it uses a jittering   Fract-N route to generating arbitrary frequencies.   Again, fine for a few milliwats of test signal, but certainly not for amplification to hundreds of milliwatts.

If you want a decent high power source, use a DDS chip  or a modern Fract-N synthesizer like the LMX2541.  That device has an internal VCO and an output divider that allows it to generate as low as 35MHz - and then any external CMSO divider will allow you to go lower.   And they are CLEAN - especially so when divided down to LF / HF 

Andy  G4JNT

On 27 March 2016 at 18:14, Graham <g8fzk@g8fzk.fsnet.co.uk> wrote:

Andy,

 

This time its not really Stefan's  fault  :-,)

 

This is a  problem , using the  raspberry PI ,  the  Pi processor uses  interrupts, which, cause the  execution  time  of the  timing  loops to  vary  slightly ,

the 'small' time  variations , produce side bands .

 

This was evident  during  the  development of  a  R-PI  routine  for  sending  Opera , as the  R-Pi support documentation is not  particularly  comprehensive, its  'not' possible to  fully address the  i/o  functions and as such there exists  a conflict  with higher order irq   requests .  Producing  seemingly  random widening  of the  spectrum,  depending  on the  irq  deployments.

 

Ie the  pulse train suffers  from  timing  jitter

 

The  alterative of the  U3 beacon  key'er , driving  the  DDS 'oscillator' is  a more  elegant  solution,  but as noted by  Andy [rad-com]  some time  back, the  later  U3 with the  PPL  suffers from  settling  time issues ,  when  commanded with , step  frequency changes.  these sever to produce sidebands , at the  step changes.

 

73-G,

 

From: F6CNI - Andy - JN19QB

Sent: Saturday, March 26, 2016 8:36 PM

To: RSGB LF Group

Subject: LF: DK7FC WSPR bad quality problem

Hello Stefan.

 

I am sorry, but this evening you have a very poor quality WSPR signal on MF with many splatters, I think it is perhaps due to an overload TX AF level.

 

That is not in phase with the normal ham spirit. So, please be kind to stop now your transmission and correct this problem.

 

On the way, can you say honnestly if your power is not over the legal limit ?

 

Sincerly yours and  my best 73 de Andy.

 

P.S.

 

Have a look below at some reports last hour showing the problem :

 

 

2016-03-26 19:00	DK7FC	0.475683	+12	0	JN49ik	1	PI4THT	JO32kf	336	338
2016-03-26 19:00	DK7FC	0.475783	-25	0	JN49ik	1	PI4THT	JO32kf	336	338
2016-03-26 18:50	DK7FC	0.475783	-23	0	JN49ik	1	PE1RKT	JO22fb	418	316
2016-03-26 18:50	DK7FC	0.475683	+14	1	JN49ik	1	PE1RKT	JO22fb	418	316
2016-03-26 18:40	DK7FC	0.475683	+12	0	JN49ik	1	LA8AV	JO59cs	1153	4
2016-03-26 18:40	DK7FC	0.475783	-24	0	JN49ik	1	LA8AV	JO59cs	1153	4
2016-03-26 18:50	DK7FC	0.475783	-28	0	JN49ik	1	LA3JJ	JO59bh	1102	4
2016-03-26 18:50	DK7FC	0.475683	+10	0	JN49ik	1	LA3JJ	JO59bh	1102	4
2016-03-26 18:40	DK7FC	0.475780	-28	1	JN49ik	1	G3KEV	IO94sh	829	314
2016-03-26 18:40	DK7FC	0.475679	+10	0	JN49ik	1	G3KEV	IO94sh	829	314
2016-03-26 19:10	DK7FC	0.475682	+13	0	JN49ik	1	F6CNI	JN19qb	389	266
2016-03-26 19:10	DK7FC	0.475782	-27	1	JN49ik	1	F6CNI	JN19qb	389	266
2016-03-26 18:30	DK7FC	0.475682	+15	0	JN49ik	1	F5WK	JN18hp	452	261
2016-03-26 18:30	DK7FC	0.475782	-22	0	JN49ik	1	F5WK	JN18hp	452	261
2016-03-26 18:00	DK7FC	0.475680	+19	0	JN49ik	1	DL6II	JO30nx	205	327
2016-03-26 18:00	DK7FC	0.475780	-18	0	JN49ik	1	DL6II	JO30nx	205	327
2016-03-26 18:50	DK7FC	0.475682	+17	0	JN49ik	1	DL4RAJ/2	JN68kj	326	109
2016-03-26 18:50	DK7FC	0.475782	-22	0	JN49ik	1	DL4RAJ/2	JN68kj	326	109