[hpsdr] Fwd: Horton LO - further thoughts

Sat Jun 17 17:24:49 PDT 2006

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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