[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [linrad] RE:I-Q and HF SSB/AM/FM questions, bulk buy for 200Mhz SAW oscs


Thanks for the ever-so-quick reply.

However I am really rather a newbie when it comes to DSP - my RF experience so 
far has been dealing entirely with "Meatspace" implementations, ie, simple 
DCRXs using diode mixers and an AF stage, not even any diplexing. I know, 
shoddy, but cheap inductors only seem to have come along only now with the 
SMD revolution, and cores and toroids are so hard to get in the UK.

I will intersperse your replies with my further questions. I'm afraid you will 
have to assume no knowledge of DSP, FFTs, etc, but I'll try.

> This is cheating;)
> Linrad will have modes for FM and SSB but I have not written the code yet.
> When you enter SSB mode you will get the same software as in weak cw.
> The idea is that you should select different parameters to make Linrad
> run nicely for SSB. You should select the same fft size for fft2 as for
> fft1, a smaller first mixer resampling ratio to allow more output
> bandwidth, deselect AGC and finally you might set a higher output sampling
> speed for more output bandwidth.

OK, I guessed that from the "not implemented yet" in your guide. However I 
seem to have got slightly different parameters when I started in SSB than CW, 
but that could be because I couldn't remember whether I chose weak mode or 
normal CW.

> For AM you can use coherent processing but FM does not work right now.

Could I not just use it in SSB/CW mode and use AFC to match the carrier 
signal? The beat frequency of a carrier-LO (ie zero frequency in my case) 
doesn't bother me too much, but it would be nice to be able to lock it in for 
listening to AM Broadcast (My Ham licence lapsed a long time ago due to never 
bothering to learn Morse, and I was only interested in HF - and as far as I 
know the rules are still the same) as I am an avid SWL.

> There is no AGC in weak cw mode so you will not have the AGC that should
> be in place in "normal" modes. There is an RF clipper at the output
> however so SSB works pretty well without AGC.

Is there a problem with using normal mode for SSB? I presume I just have to 
increase the bandwidth of the first FFT... unless that is just the frequency 
bucket size?

> You just click on the SSB signal to get it in the loudspeaker.
> I have planned for an automatic BFO setting in SSB mode and some
> other fancy stuff but everything takes time....

Ah, I see. So the AFC will only help control the output frequency of a single 
tone I assume... No matter for SSB, and I could probably cook something up 
soldered to my soundcard for AM that would sample the low beat from a 
mismatched carrier in AM and tweak my reference oscillator a little to tune 
it back.

> What does 5ps mean in terms of phase noise? How many dBc/Hz at 20kHz?
> Just curious.

I really have no idea how this would be calculated - I'm really just an 
experimenter looking to build an effective but low-cost computer-controlled 
GCR. All I know is that in the SDR-1000 design that pretty good phase noise 
can be achieved due to the AD9854's 12 bit DAC and other improvements, 
improved over the AD9850/1's 10 bits. The author implies that a really low 
jitter reference will be required to take full advantage of this.

The datasheets are at:


The jitter, by comparison, is extremely low at these kind of frequencies, from 
all the alternatives I looked at. The only better alternative was 
Valpey-Fisher (1ps RMS jitter), and they quoted me UKP 48.00 each, min.order 
10. Way beyond my means.

> 73
> Leif  /  SM5BSZ


Alex (can't even remember my callsign, shame on me).

Gak, I shouldn't be distributing my work sig on a publically archived list - 
more junk mail ahoy!