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

[Tom Azlin N4ZPT]

Hi Mike,

there is another comparison page out there told from the QS1R view.

http://www.srl-llc.com/QS1R-vs-perseus.pdf

in addition to http://microtelecom.it/perseus/tests/Perseus-vs-QS1R.pdf

Perhaps both read together are useful. And the side bar in the ARRL
SDR-IQ review on measuring performance in sampling receivers.

BTW, the Elecraft IF frequency is not 10.7 MHz. It is 8.215 MHz.
Filters from either Elecraft or Inrad are available. I use the SDR-IQ as
a panadapter slaved to the K3 via the LP-Bridge software. Do not have
the IF output mod that raises the gain on that path but it works nicely.
And if I put a second sound card in my computer I would have two more
demodulators to use.

73, Tom n4zpt

On 1/25/2010 6:51 AM, Mike O'Dell wrote:
> 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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