QS1-R vs SDR-14 vs Perseus -- Direct Sampling HF Receivers

[Mike O'Dell]

that piece was indeed interesting reading as well.
Nico is clearly a very sharp dude. when a bunch
of people started spouting about phase noise, etc,
he published a screen shot from an R&S source analyzer
showing that indeed, a rock-solid sample clock with
digital down-conversion has as much phase noise as,
well, a rock. it took a while to convince people that
no, there is no DDS or PLLs in the picture to make
phase noise.

the discussion on the K3 list swirled around for
quite some time but managed to remain far more
civil than i would have imagined given the vehemence
with which some of the doubters expressed their opinions
we saw a similar kind of thing when the group was
doing spread spectrum: people are so used to thinking
"narrowband" and "signal-by-signal" they cannot
let go of it. There is an interesting gray area,
though, typified by the K3, of a single-conversion
superhet with a reasonably high&wide IF - 10.7MHz in the
case of the K3 - followed by a good, fast ADC+DSP
to do everything from there wideband. is that an SDR?
or rather where does it land on a scale of 0 to 1,
with 0 being a classic Conrad-Johnson triple-conversion superhet
and 1 being a Hypres intercept receiver with a superconducting
front-end running at 40Gigasamples/second with a -145dB noise floor?

there was a rather intense discussion
about how an SDR could not possibly hear a signal
at -115dB in the presence of really loud local stations.
Phil finally just emailed a screen shot of 4 or 5 carriers
at 0dB with a tiny little spike down at -115 clear as day
under the display cursor.

as i said, the Perseus is advertised as a high-performance
SWL receiver, and there's just no arguing with that.
they really are optimized for different purposes, although
the "feasible regions" overlap a very large amount.

Phil does have a front-end board in the works which has
a switchable preamp (*not* in the path all the time with
a switchable attenuator - the preamp is bypassed) with
a set of "roofing" filters (aka "preselector"?) to help
deal with really hot local signals that drive the ADC
into clipping (which is at +9dBm - not a small signal).
that way he can just squelch the argument about front-end
filters, preamps, etc.

as for the cost, the big improvement would be to get it
fabbed by someone with some buying leverage. he's buying
expensive parts at very high prices because of no quantity.
and since he's decided to not take any prepayments, all
his working capital is coming out of his hide. very clean
from one point of view, but very, very painful from several
points of view.

	-mo



On 1/24/10 3:42 PM, Tom Azlin N4ZPT wrote:
> Hi Mike,
>
> Yes, interesting differences. Did you see
> http://microtelecom.it/perseus/tests/Perseus-vs-QS1R.pdf
>
> I looked at the Perseus at Dayton last year but in the end chose to buy
> a QS1R as a building block for learning. And if the other boards do come
> along then it may be a nice full receiver as you say. To complement my
> HPSDR. In the meantime a nice back end especially if I can use my GPS
> stabilized oscillator as an input to the QS1R and downconverters.
>
> 73, Tom n4zpt
>
>
>
> On 1/24/2010 2:10 PM, Mike O'Dell wrote:
>> thinking hard about how far i wish to put my foot
>> into the SDR pot, I've been doing a lot of reading,
>> trying to distinguish data from opinion from venom.
>> i'm recording them here to elicit commentary from
>> the knowledgeable chaps around the table.
>>
>> if one is willing to spend a grand on an SDR receiver,
>> that puts one in the realm of the "direct sampling" receivers.
>>
>> after comparing these three at some length (reading a lot
>> of extremely animated conversations, esp on the E-K3 list,
>> of all places), the Perseus appears to have the edge if
>> what you want is a full-up *receiver in a tin*. The SDR-14
>> seems also to be "dedicated receiver" but a bit more
>> bare-bones (or purist, depending on your religion).
>>
>> However, if one is looking for a receiver which can be the basis
>> of a *platform*, then the QS1-R appears to have the edge
>> as it was clearly designed as such, and new companion
>> boards are coming along.
>>
>> It should be noted that the Mercury receiver card and the
>> RX section of the Hermes one-board xceiver from the "OpenHPSDR"
>> project are essentially identical to the QS-1R.
>>
>> As for sampling performance and BW (sans undersampling),
>> the QS1-R samples at 125MS/s and goes to 55MHz (hello 6M!)
>> the Perseus samples at 80MS/s and goes to 30MHz (limited by preselector)
>> and the SDR-14 samples at 67MS/s and goes to 33MHz
>>
>> the QS1-R and the Perseus have a much
>> newer A/D part than the SDR-14.
>> That's one of the huge paradoxes in the
>> electronics biz - the guy who starts 6 months
>> after you has a big advantage because he usually
>> has better parts to pick from. The cellular radio
>> base station business is driving the evolution
>> of digital RF hardware really hard. The LTE/WiMAX
>> MIMO air interfaces assume massive digital
>> processing.
>>
>> the other possible advantage the QS-1R and Perseus
>> have over the SDR-14 is that all of the DDC
>> processing (Quadrature NCO + Mixers + Digital Filters)
>> is done in an FPGA while the SDR-14 uses
>> a fine, off-the-shelf DDC from ADI. The QS-1R
>> has a very large FPGA while the Perseus seems
>> to have been cost-optimized (not a bad thing!).
>>
>> the net result, though, is that in line with
>> being a platform, the QS-1R has lots of room for
>> further expansion of the digital processing.
>> (one option being explored is a true multi-channel
>> receiver).
>>
>> In all 3 cases, the raw A/D samples
>> are available, as are the I/Q signals from the
>> DDC.
>>
>> so my take is that if one were to take the plunge
>> for a "big bite" direct-sampling receiver intended
>> to be the basis for AMRAD-style geekery-hackery, the
>> Quicksilver QS-1R is the best of 3 quality alternatives.
>>
>> again, this is my digestion of the current state of things
>> and the whole point of posting this is to have my
>> understanding improved by facts known to others.
>>
>>      cheers,
>>      -mo
>>
>>
>
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-- 
"Of course it's hard!
If it was easy, we'd be buying it from somebody else!"