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Author Topic: Any one actually want to talk about SDR and future implementations?  (Read 10318 times)
WB6RQN
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« Reply #30 on: June 13, 2011, 08:16:42 PM »

Perhaps a misunderstanding on my part. I understood your reply to my comment as saying that radios with knobs and touchscreens can only be software "controlled" and not "defined". I apparently did not understand your reply correctly. Anyhow, I think we are now on the same page. My comment was meant to relate to the theme of the thread "SDR and future implementations" in that I see future implementations including the larger manufacturers using SDR via touchscreen implementations.

The purpose of my comment was to make it very clear that the user interface; e.g. knobs, touchscreen, mouse, whatever; has nothing whatsoever to do with whether the radio is software defined. There seems to be this massive misconception that the presence or absence of knobs has something to do with a radio's SDR-ness. Nothing could be further from the truth. We need to get this separation clear in people's heads. The presence of an outboard computer with pictures of controls does NOT necessarily constitute SDR. Likewise, the presence of knobs and switches controlling the radio does not necessarily denote the absence of SDR.

73 de Brian, WB6RQN/J79BPL
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WB6RQN
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« Reply #31 on: June 13, 2011, 08:30:08 PM »

Question: Does SDR necessarily imply wideband input to the ADC, or is an equally valid implementation to only digitize after cutting the IF bandwidth way down? After the IF amplifier/AGC? Actually at baseband?
Each of these trades off some possible software capabilities for (probably) better BDR.

As I said earlier, I think you may be mistaken WRT BDR. If you have a fast enough ADC you can set the clipping level as high as you want in order to prevent clipping under any conditions, and then filter and decimate in order to regain dynamic range in narrow-band operation. I believe that, in whatever testing you care to do, you will find that the performance will be equal to an analog radio running with the same bandwidth. The only caveat is that you must have sufficient wide-band noise power to ensure that there are random transitions on the LSB of the ADC after you set the clipping level high, otherwise you won't get the desired increase in dynamic range.

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Now I have to admit to having PA3AKEs awesome receiver board and quartz set on order but I am giving serious thought to going DSP for the detector or possibly doing the DSP at baseband, in any case it will be after some brutal IF filtering, so I probably cannot get much of a waterfall out of it as the IF may have been butchered down to a mere 250Hz wide by the time the DSP gets a look.

You may want to consider what I have just said and do some research before you jump into the narrow first IF. Once you give away the bits you can never get them back again. Keep all the bits you can until just before you are ready to convert to baseband or until you give the CODEC only what it needs.

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Does a DSP based demodulator behind a conventional front end still count as SDR?

It probably means that the radio is going to be limited to narrow-band modes and therefore less capable of being redefined.

My earlier post regarding information theory suggests that there is less difference between SDR and analog that we might like to admit. Or, to paraphrase my namesake, Pink Floyd:

  "There is no digital side of the radio, really.
   As a matter of fact, it's all digital."

;-)

73 de Brian, WB6RQN/J79BPL
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M0HCN
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« Reply #32 on: June 14, 2011, 12:17:57 PM »

While we are talking about BDR with respect to signals lying within the IF passband there is on a theoretical level very little difference between a wide and narrow band IF strip, but where an SDR effectively ONLY has the BDR level that applies for signals inside the IF passband, a narrow band rig often gets MUCH better as soon as you can place the interference outside the first IF bandwidth.

A narrow band IF means that it is usually possible to tune such that a strong signal a few hundred Hz away lies outside the IF passband with the narrow filters engaged, which you cannot do with a wideband IF. 
The effect is that in a narrow band RX the performance limitation is usually the first mixer IIP3, while in a wideband design it is likely to be the IF gain stage IIP3, and associated AGC and converter ranges.

The key point is that with a narrow band IF the front end selectivity will allow a weak signal to be worked relatively close to a strong one without both signals falling inside the IF passband and thus without the later stages needing that much dynamic range, a wideband IF does not have that option and must be sufficiently linear to resolve two signals that may be 100dB or so different in strength when both fit inside the IF passband.

ADCs are interesting beasts as it seems to be that the product of bandwidth and word length is largely (for any given generation of parts) constant, but the issue is only partly the ADC, it is also the gain stage that in all probability precedes it.  I would note that there are few (if any) high speed 24 bit converters that really manage a thermal noise level better then 20 or so bits below full scale, so the dithering is unlikely to be an issue.

It is true that from an information theory perspective I am throwing away bits, but they are bits which (as far as my wanted signal is concerned) contain only noise anyway, so why do I care, if wideband is your thing then a different tradeoff may be appropriate.


Regards, Dan.
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WB6RQN
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« Reply #33 on: June 14, 2011, 10:02:27 PM »

ADCs are interesting beasts as it seems to be that the product of bandwidth and word length is largely (for any given generation of parts) constant, but the issue is only partly the ADC, it is also the gain stage that in all probability precedes it.  I would note that there are few (if any) high speed 24 bit converters that really manage a thermal noise level better then 20 or so bits below full scale, so the dithering is unlikely to be an issue.

I agree WRT any gain stage preceding the ADC to be a limiting factor. Having spent a significant part of my life trying to produce 100dB dynamic range amplifiers in the audio spectrum, I can attest to just how difficult that can be. I also agree WRT the true effective bit-depth of the 24-bit ADCs.

But I guess I don't think of the 24-bit ADCs used to digitize audio as being all that state-of-the-art. The 100+MHz 16-bit ADCs begin to look very much more SotA to me.

Quote
It is true that from an information theory perspective I am throwing away bits, but they are bits which (as far as my wanted signal is concerned) contain only noise anyway, so why do I care, if wideband is your thing then a different tradeoff may be appropriate.

How many channels do you expect to decode at the same time? I hope to decode a LOT of channels at the same time. And I believe that, if you can avoid ADC saturation, you can probably achieve a very credible narrow-band performance on several/many channels concurrently. So I certainly don't want to throw away the bits too soon.

73 de Brian, WB6RQN/J79BPL
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M0HCN
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« Reply #34 on: June 15, 2011, 05:38:06 AM »

Thats a pretty big if Brian!

Depending on what you mean by wideband, with a 100Mhz ADC you could have DC to ~40Mhz or so, assuming you can handle the data rate (might need a FPGA or something), but that includes everything from big MW and LW Broadcast sites (not to say the 40M broadcast band that can put a mW into a 40M dipole on its own!), right down to some PSK31 from half way around the planet running 5W....

Not to mention 40Mhz worth of atmospheric and thermal noise!

The problem becomes exponentially easier as the rx bandwidth reduces and as you say that 100Mhz ADC could be decimated to add word length at the lower rate, provided its differential nonlinearity is good enough to avoid IP2 or IP3 becoming the limit.

ADC aperture jitter will of course add the the sampling clock phase noise problem, but I dont know how well that has been resolved in modern parts.

I think that from an implementation perspective wideband and weak signal work are pretty directly opposed simply because of the reality of the available components and their linearity. 

I must however have a close look at the specs on something like a flex 5K just to confirm this for a commercial state of the art rig.

Regards, Dan.
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KE5JPP
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« Reply #35 on: June 15, 2011, 06:45:54 AM »

Thats a pretty big if Brian!

Depending on what you mean by wideband, with a 100Mhz ADC you could have DC to ~40Mhz or so, assuming you can handle the data rate (might need a FPGA or something), but that includes everything from big MW and LW Broadcast sites (not to say the 40M broadcast band that can put a mW into a 40M dipole on its own!), right down to some PSK31 from half way around the planet running 5W....

Not to mention 40Mhz worth of atmospheric and thermal noise!

The problem becomes exponentially easier as the rx bandwidth reduces and as you say that 100Mhz ADC could be decimated to add word length at the lower rate, provided its differential nonlinearity is good enough to avoid IP2 or IP3 becoming the limit.

ADC aperture jitter will of course add the the sampling clock phase noise problem, but I dont know how well that has been resolved in modern parts.

I think that from an implementation perspective wideband and weak signal work are pretty directly opposed simply because of the reality of the available components and their linearity. 

I must however have a close look at the specs on something like a flex 5K just to confirm this for a commercial state of the art rig.

Regards, Dan.

Yet, there are already direct sampling receivers that handle all these situations just fine.  For example, my Perseus uses a 80 MSPS ADC and covers 10 kHz - 30 MHz and it is in the top 3 of the Sherwood receiver test charts (exceeding the Flex5K which is a QSD, not direct sampling receiver).  My QS1R has even better performance and uses a 125 MSPS ADC and covers 10 kHz - 55 MHz.  It overloads at +8 dBm without an attenuator.  Both use FPGAs and USB 2.0.  You can see up to 1600 kHz on the Perseus and 2000 kHz on the QS1R.

Gene
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WB6RQN
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« Reply #36 on: June 15, 2011, 11:19:06 AM »

Beat me to it Gene. Aren't existence proofs nice?

These issues are especially interesting to me as I need to ensure they are solved for another project (not specifically amateur radio) I am currently working on.

73 de Brian, WB6RQN/J79BPL
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M0HCN
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« Reply #37 on: June 16, 2011, 11:11:41 AM »

Those radios prove that it is possible to build a good radio that way, which is not really in doubt.

My contention is that IF narrowband is all you need then it is possible to build a better radio by specializing the hardware for the purpose then it is by passing everything through to the AD.
The tradeoff of course is that you cannot then do wideband things.

The QS1R has a MDS (in 500Hz bandwidth) of -111dBm on 20M, P3AKEs frontend hits -135dBm under the same conditions, with IMD3DR or 123db.
That 14dB can either be taken as extra sensitivity, or via the attenuators as a higher IIP3 by switching in the attenuator, put that on line and the wideband IIP3 hits something like +65dBm.
At that performance level the LO phase noise becomes an interesting design exercise (and one that is still causing head scratching).

Now the QS1R does all kinds of cool things that a RX with a narrow filter never will, so as I say you need both.

Regards, Dan.
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KE5JPP
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« Reply #38 on: June 17, 2011, 03:11:23 AM »

Those radios prove that it is possible to build a good radio that way, which is not really in doubt.

My contention is that IF narrowband is all you need then it is possible to build a better radio by specializing the hardware for the purpose then it is by passing everything through to the AD.
The tradeoff of course is that you cannot then do wideband things.

The QS1R has a MDS (in 500Hz bandwidth) of -111dBm on 20M, P3AKEs frontend hits -135dBm under the same conditions, with IMD3DR or 123db.
That 14dB can either be taken as extra sensitivity, or via the attenuators as a higher IIP3 by switching in the attenuator, put that on line and the wideband IIP3 hits something like +65dBm.
At that performance level the LO phase noise becomes an interesting design exercise (and one that is still causing head scratching).

Now the QS1R does all kinds of cool things that a RX with a narrow filter never will, so as I say you need both.

Regards, Dan.


You may want both but I am not sure you need both.  Under very extreme conditions, that most guys will never have the requirement for, the additional IMD3DR might be useful.  But let's talk about practical implementations.  P3AKE's design is not really available as a finished product anyhow as far as I can tell.  I can use CW Skimmer Server to skim 7 different ham bands at the same time with the QS1R.  That is definitely something narrow band SDRs with QSDs never will be able to accomplish.

Gene
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KE5JPP
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« Reply #39 on: June 17, 2011, 08:33:41 AM »

The ease of use is more important to me than the ultimate specifications determined in a lab somewhere.  You may have the best lab performing radio with huge IMD3DR numbers or great MDS numbers, but if it has a clunker user interface then all those great lab numbers don't mean much in normal real world operation.  What I am more interested in is how does the radio perform outside of the lab in the real world?  Is the user interface something you can live with from day to day?  Are there features of the particular radio that are really useful and add to my enjoyment of operating it on the air?  (I would answer HECK YES when it comes to the wide band spectrum display available with direct sampling SDRs!) I am guessing that there is a very small subset of operators that actually need such extreme performance as you mentioned under difficult circumstances.  Most guys are not going to notice a difference of 10-20 dB in the numbers that you mentioned.  With the sun supposedly going dormant over the balance of most of the current Ham radio operator's life span, the MDS is not really so important because natural and man made noise is going to be much higher on the bands below 17 meters anyhow.  What good is a -132 dBm MDS when the band noise on 40 meters is -70 to -90 dBm?

Gene
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KQ1Q
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« Reply #40 on: June 27, 2011, 11:12:37 AM »

It appears that new-generation A/Ds will enable direct sampling SDRs into the high UHF range. The National Semi ADC12D1800 is a 12-bit 3.6 giga sample/sec part which gives a Nyquist bandwidth of 1.8Ghz. In theory you could build a totally software-implemented 23 cm radio with this.

Another high-perf ADC is the Maxim MAX5881, which is 12 bits at 4.3 giga samples/sec: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5607

The Intersil ISLA214P50 is a 14-bit ADC at 500 mega samples/sec: http://www.intersil.com/data/fn/fn7571.pdf

The buzzword is "RF synthesis". If you directly sample the RF at sufficient rate and bit depth, the entire remainder of the signal chain can be implemented in software. It's incredible that ADCs and CPUs are now fast enough to fully synthesize all radio functions in software at UHF frequencies.

Since an SDR has fewer parts and greater flexibility than a conventional analog design, it would appear market forces  will eventually drive manufacturers toward this. These SDRs need not have a waterfall display or require an external computer. Indeed, they could look identical to current knob-oriented "box" radios, but with much lower internal parts count,  lower manufacturing costs, and less internal volume.

Of course this won't be limited to amateur products but will impact many areas, including commercial & military communications, TV set-top boxes, etc.

Here's a fascinating video from National Semi which demonstrates the potential. At 2:55 into the video he discusses some performance numbers, then at 3:50, he demonstrates an SDR cable TV set-top box. It digitizes the entire cable TV bandwidth, implementing a TV tuner in a field-programmable logic array: http://www.youtube.com/watch?v=dLRJtUT3JgY&feature=related

It's true current ham SDRs imperfectly implement the potential. However longer term it would appear that market and technical factors will force an increasing move to SDRs by all manufacturers. Some of these may retain a conventional "knob radio" appearance, whereas others will likely experiment with more display-oriented form factors.

Joe, KK4CLP
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W6RMK
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« Reply #41 on: June 27, 2011, 10:43:11 PM »

It appears that new-generation A/Ds will enable direct sampling SDRs into the high UHF range. The National Semi ADC12D1800 is a 12-bit 3.6 giga sample/sec part which gives a Nyquist bandwidth of 1.8Ghz. In theory you could build a totally software-implemented 23 cm radio with this.


Yes, these parts are impressive... but.. that part draws 4.4 W (nominal), but that's ok, because you'll probably need a fair amount of very clean clock drive (1Vpp into 100ohms.. around +7dBm) which will burn another couple of watts. 

And it's ENOB of 9.4bits means its instantaneous dynamic range is only 50-60dB.  You may digitize the entire HF, VHF, and UHF bands, but the TV stations and SW broadcasters will dominate your input power. (and the enob drops to 8.x bits at 1GHz input)

The cable TV tuner is a sort of ideal application:
1) nobody cares how much power a set-top box draws.. certainly not the cable company, they don't pay the customer's electric bill
2) All of the signals are about the same amplitude, so you don't need huge dynamic range
3) the signals are phase modulated (QPSK,8PSK), so they're of constant amplitude
4) reducing parts count is a big deal, especially when you need to build a 3 channel tuner (2 recording , 1 playing live): a conventional superhet tuner would require 3 times as many parts.


So, a very nifty part for some applications, but not for a HF transceiver, at least not just yet.

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KQ1Q
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« Reply #42 on: June 28, 2011, 01:54:48 PM »

...So, a very nifty part for some applications, but not for a HF transceiver, at least not just yet...
I agree those ultra-fast 12-bit ADCs aren't ideal for HF, I just mentioned them to illustrate how fast they are.

However there are 500Mhz 14-bit ADCs, and even 250Mhz 16-bit ADCs with over 100db dynamic range: http://www.analog.com/en/press-release/9_27_10_ADI_Announces_Industrys_Fatest_16bit_ADC_a/press.html

Those are fast and wide enough to handle HF or even VHF. Even if they're somewhat costly and consume significant power, in an SDR they are replacing most of the conventional signal path components. The power consumption, cost and manufacturing budgets of all those replaced components help offset the cost of the single ADC.

In an outboard SDR which relies on a PC, the PC would supply the computer power.

In stand-alone SDR, you'd need significant internal processing, far beyond a conventional radio. That would add cost and power consumption. But computing gets cheaper every year. Just two examples: there's an impressive AF spectrum analyzer for the iPhone: http://itunes.apple.com/us/app/signalscope/id284781777?mt=8 also an iPhone SDR app: http://itunes.apple.com/us/app/isdr/id375603584?mt=8

Those are limited products, but a few years ago those would have required dedicated instruments. Now even a cell phone has sufficient CPU horsepower.

So even if a stand-alone SDR required the CPU, memory and storage of a current PC, eventually that will be available in a single-chip embedded controller.

There's already one small company which makes a direct sampling, stand-alone SDR tranceiver called the  ADT-200A: http://www.adat.ch/index_e.html

It's about $6k, which is expensive but less than the highest-end conventional tranceivers. It's obviously an early product.

However I'm reminded of early digital oscilloscopes, which were expensive and limited. It took a few years, but nowadays they are the standard. My guess is the move to SDRs will be similar.

Joe, KK4CLP
« Last Edit: June 28, 2011, 01:58:02 PM by KK4CLP » Logged
W6RMK
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« Reply #43 on: June 28, 2011, 06:12:25 PM »

Since I make my living developing and encouraging the use of SDRs, I have to agree with you.  For the broadband HF application, the instantaneous dynamic range issue is a real tough one (even with 16 bit samplers and oversampling and decimation). 

As you say, the cost of the computational horsepower (both in watts and dollars) is steadily decreasing, and that *will* be the way it's done.  The ADC is going to be a sticky part for a long time: it's a physics issue, unfortunately, unlike for the digital processing, where Moore's law kinds of processes provide dramatic improvements in a relatively short time.

I think that what will happen is that the sum of "cost of horsepower"+"cost of suitable ADC" will eventually move significantly lower than "cost of analog RF chain", and even with limited ADCs, we'll see SDRs take over.  All the new radios use SDR techniques for all the baseband processing. 

The obstacles in front of letting the user change the processing are a combination of regulatory and market. Small market, today, and significant regulatory hurdles.
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