[hpsdr] Fwd: Horton LO - further thoughts (shielding)

Sat Jun 17 17:33:54 PDT 2006

I agree. Digital logic is designed with several hundred millivolts of noise
margin. HPSDR will work much better if the analog portions, including LOs,
are shielded.

73,

John
KD6OZH

----- Original Message ----- 
From: "KD5NWA" <kd5nwa at cox.net>
To: <hpsdr at hpsdr.org>
Sent: Sunday, June 18, 2006 00:24 UTC
Subject: Re: [hpsdr] Fwd: Horton LO - further thoughts

> ***** High Performance Software Defined Radio Discussion List *****
>
> I also was in shock when I first saw the inside of a SDR-1000, by the
> amount of hardware, lack of shielding as you mentioned and use of
> what looks like 20 gauge wire to distribute the power. Some of these
> things I look forward to change and improve in the future.
>
> I worked in the Bio Medical field in the past and one of the first
> rules is massive ground, power distribution, and shielding  to help
> minimize electrical noise, and avoidance of ground loops like the
> plague that it is.
>
> I'm not sure of the part number but is one of the AD98XX family
> because of the low frequency clock. I've just recently bought one and
> I been having problems with it.
>
>
> At 04:57 PM 6/17/2006, you wrote:
> >Cecil
> >
> >What DDS does the SDR1000 have? I know it has 200MHz ref oscillator.
> >The AD995x series DDS could go to at least triple that which would
> >help with spurs.
> >
> >Yes, I have read about other causes of spurs - but I also read that
> >the largest contribution was truncation error.
> >
> >I wasn't aware that the SDR1000 actively calculates the location of
> >truncation spurs and adjusts its DDS frequency to minimise them and
> >its effective DSP IF to compensate. Are you sure? Can I read about it
> >someplace?
> >
> >Also, does the SDR1000 have a bandpass filter after the DDS? Bill
> >Carver (whose callsign I temporarily have forgotten) has been
> >experimenting recently with bandpass filters after the DDS,
> >restricting the range of the DDS to 10 or 20 VHF MHz which he divides
> >down to the required HF bands using a variable integral division
> >ratio. He found a dramatic reduction in spurious reponses.
> >
> >I was also alarmed when I saw a photo on the inside of the SDR1000 and
> >noted no visible signs of screening. I feel intuitively that screening
> >is very important for the final reduction of these low level spurs,
> >though I could be wrong. I understand that screening is a very
> >expensive proposition for a commercial product.
> >
> >My first thought when I looked at the HPSDR proposals was also that
> >there is not sufficient provision for screening. Once in the digital
> >domain, no problem. But our digital techniques aren't yet so good that
> >we can directly digitise the whole HF spectrum at the antenna, and
> >process entirely digitally thereafter. We still need an analogue front
> >end, which is going to need to be screened carefully as far as I can
> >see, together with any circuit blocks it is dependent on, such as the
> >LO.
> >
> >I may be completely wrong and I'm still reading a lot and learning -
> >but it seems to me that the choice between PLL synth and DDS is that a
> >PLL has phase noise but not spurs; a DDS has spurs but not phase noise
> >(yes, "not" is probably an exagerration in both cases!). So it's a
> >matter of compromise, and trying to minimise the damage of the
> >unwanted problem in each case. I just wonder if it might be easier to
> >minimise the effect of DDS spurs, then to minimise the PLL phase
> >noise.
> >
> >73 Hans G0UPL
> >http://www.hanssummers.com
> >
> >
> >On 6/17/06, KD5NWA <kd5nwa at cox.net> wrote:
> >>That is precisely what the SDR-1000 does, but as I sadly found, there
> >>are spurs at all frequencies, it's just are a little better. You
> >>still end up with spurs but maybe not as many. You are assuming that
> >>spurs occur only because of truncation error, they occur also because
> >>of defects in the D/A chip, glitches in the power due to the chips
> >>consumption,  and board layout picking up noise, those spurs are
> >>everywhere and not so predictable.
> >>
> >>At 03:10 PM 6/17/2006, you wrote:
> >> >***** High Performance Software Defined Radio Discussion List *****
> >> >
> >> >I think that Jim raises some very interesting points.
> >> >
> >> >I suspect that it will be very hard, if not impossible, to beat the
> >> >low phase noise of a decent DDS with a quality crystal reference
> >> >oscillator.
> >> >
> >> >The trouble with DDS is as we know, the spurs problem. Even with a
> >> >high speed 14-bit DDS such as the AD995x series the spurs can still be
> >> >a problem if the rest of the receiver is sensitive enough to notice
> >> >(which I think we can assume it is!).
> >> >
> >> >Wideband spurs can be removed by a bandpass filter at the DDS output.
> >> >This would require either a tuneable bandpass filter (e.g. by switched
> >> >capacitors), or keeping the oscillator range narrow - which could be
> >> >accomplished by using a VHF oscillator and a variable division ratio
> >> >to get down to the desired HF frequency.
> >> >
> >> >The bandpass filter can't remove close in spurs which also exist and
> >> >are just as troublesome. However, the largest source of spurs is the
> >> >phase truncation in the DDS and thise location of these spurs is
> >> >entirely predictable.
> >> >
> >> >Wouldn't it therefore be possible to say Ok, we want to receive on
> >> >frequency X. But if we set the DDS there, we predict we'll get a spur.
> >> >So we move the DDS to somewhere within the notional 10kHz of where it
> >> >truly should be. And correct for the < 10kHz displacement digitally
> >> >within the DSP (as is being proposed anyway).
> >> >
> >> >Wouldn't this be a good way to have the best possible phase noise
> >> >using a DDS, and yet to avoid the spurs problem of DDS? I wonder if
> >> >this would be a better method than the currently discussed VCO/PLL
> >> >type synthesisers?
> >> >
> >> >73 Hans G0UPL
> >> >http://www.hanssummers.com
> >> >
> >> >
> >> > > I'll admit right off that I'm new to ham radio but have done a lot
of
> >> > > reading in the last year. Although I worked 30 years as an EE
before
> >> > > retiring I never worked in RF so there's been a lot of fun
learning.
> >> > >
> >> > > The one thing I keep reading is that good phase noise
> >> performance in a good
> >> > > design is limited by the Q of the resonator. At HF nothing
> >> comes close to
> >> > > the Q of a crystal. For frequencies up to 500Mhz I'm guessing
> >> that it will
> >> > > be very hard to beat the phase noise of a frequency multiplied
100Mhz
> >> > > oscillator.
> >> > >
> >> > > Second best is probably a well designed and accoustically
> >> isolated resonant
> >> > > cavity obviously at a much higher frequency considering practical
sizes.
> >> > >
> >> > > Although the DDS does have SFDR limitations at least the phase
noise
> >> > > approaches that of a crystal oscillator.
> >> > >
> >> > > I see mention of fine tuning resolution of the DDS as unnecessary
yet it
> >> > > comes for free so I can't see it as a limitation. Small
> >> frequency changes
> >> > > can be useful for minimizing spurs and can be made invisible
> >> by software.
> >> > > Nothing says you have to use the fine stepping of the DDS for
> >> tuning. The
> >> > > SFDR is still a limitation.
> >> > >
> >> > > I guess what I'm saying is that without a good Q at the heart of
the
> >> > > oscillator I doubt we'll be satisified with phase noise results.
> >> > >
> >> > > And achieving that maximum base Q may well require different
> >> means as the
> >> > > frequency goes beyond HF.
> >> > >
> >> > > jim ab3cv
> >> > >
> >> > >
> >> > >
> >> >_______________________________________________
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> >>
> >>
> >>Cecil Bayona
> >>KD5NWA
> >>www.qrpradio.com
> >>
> >>"Windows the worlds most successful software virus"
> >>
> >
> >
> >Cecil Bayona
> >KD5NWA
> >www.qrpradio.com
> >
> >"Windows the worlds most successful software virus"
>
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