Direct Conversion Receiver

Vladimir Vassilevsky <nospam@nowhere.com> wrote:

(snip)
> What is the point?  The 100 of single IC analog receivers would beat the 
> 100-channel digital monster by every parameter; except, perhaps, size.

The first one that comes up on Google:

http://www.onsemi.com/pub_link/Collateral/EN8271-D.PDF

seems to have RF input for AM, but IF input for FM. 
Also, a fair number of resistors, capacitors, varactor diodes, go 
along with it.

http://www.datasheetcatalog.org/datasheets2/36/365198_1.pdf

has the LO and mixer on chip, but still needs off chip RF amp.
the paper also has a long description on AGC and intermodulation.
It seems that it reduces the gain when there is interference, but
not so much if there isn't.

-- glen

On 6 Nov., 23:28, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:
> Vladimir Vassilevsky <nos...@nowhere.com> wrote:
>
> (snip, I wrote)
>
> >> For FM, it is a little different. From 88MHz to 108MHz, so 20MHz wide.
> >> Might be better to mix down first, but there might be affordable
> >> ADCs that could do it. You still want to filter out some of the
> >> lower frequencies that might be very high power, though.
> >> Then filter and demodulate the 100 FM signals. Well, you probably
> >> don't have all 100 anywhere, so you can get by with fewer filters
> >> and demodulators, but all in the digital side.
> >> Then, still in the digital domain, convert to MP3 and write the
> >> data stream out, maybe to a disk drive.
> > What is the point? &#4294967295;The 100 of single IC analog receivers would beat the
> > 100-channel digital monster by every parameter; except, perhaps, size.
>
> Well, size is sometimes important.
>
> But OK, 100 analog receiver ICs, 100 ADCs,
>
> no, maybe 100 analog receivers, 100 to 1 analog MUX, 1 ADC, then
> digital stream into 100 way MP3 encoder.
>
> I haven't looked lately, what is available as single IC receivers?
> All the way from FM RF input to stereo audio output?
>
> Is there a data sheet PDF available for one?
>
> But otherwise, it is size and the challenge of doing all
> the processing digitally, and in one IC.
>
> -- glen

https://www.sparkfun.com/products/10342
https://www.sparkfun.com/products/10663

-Lasse

glen herrmannsfeldt <gah@ugcs.caltech.edu> writes:
> [...]
> Well, size is sometimes important.

LOL!
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

Nice document about software defined radios and direct-conversion receivers:

"A Software-Defined Radio for the Masses, Part 1 - ARRL"
http://www.arrl.org/files/file/Technology/tis/info/pdf/020708qex013.pdf

Especially Tayloe-detector is very simple and straightforward mixer/detector concept (Fig. 10 and Fig. 14).

"mnentwig" <24789@dsprelated> writes:

Hi Markus,

>>> anti-aliasing requirements
>
> that would be the least of my concerns, run a fixed-frequency NCO to shift
> the whole band near 0 Hz still at the high rate, then lowpass -/ decimate
> in multiple stages (that said, I've never actually implemented this, and
> there may be better ways)

Well, my idea wouldn't require an NCO or a mixer at all. By careful
choice of the high sample rate, one could simply perform a complex,
one-sided bandpass filter (which results in a complex output) at the
high sample rate followed by decimation.

Yes (Dale), this would require a complex filter in general, but remember
the "effective" (i.e., computational) length of the filter is also
decimated by the decimation factor if you use polyphase filtering. One
may be able to do get away with doing this filter in stages as well,
depending on the frequency planning. Don't know - haven't thought
that all the way through yet. 

Either way, an FPGA should be able to do the job. 

> no, I think getting the RF signal through the ADC is the hard part, unless
> the radio requirements are very easy.
> The main difficulty in receiver design is that I have to be prepared for
> any likely combination of unknown interferers. 

I was also planning on doing the preselector in stages based on
oversampling the desired RF bandwidth by a factor of two. (Isn't modern
technology wonderful that we can do this type of thing these days?!)
First stage would be the analog that would have the desired stopband
attenuation at 2x the highest frequency. That should afford a) a more
reasonable design, or b) higher stopband attenuation depending on your
goals. The second stage would be "digital preselector" after the ADC,
complex, one-sided bandpass filtering the desired RF bandwidth further.
The decimation step would purposely bandpass alias the desired RF band
to baseband.

I think. I'm bad at doing this stuff in my head - need to draw some
diagrams. Thanks for the feedback, Markus.
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote:

> I had wondered in this group some time ago about a radio receiver
> that would receive all stations at once.
>
> For AM, the band ks 540kHz to 1600kHz, so the whole thing is
> less than 2MHz. It should easily be possible to filter and sample
> to get the whole band into an ADC, then digitally filter and
> demodulate the 100 or so possible signals.
>
> For FM, it is a little different. From 88MHz to 108MHz, so 20MHz wide.
> Might be better to mix down first, but there might be affordable
> ADCs that could do it. You still want to filter out some of the
> lower frequencies that might be very high power, though.
> Then filter and demodulate the 100 FM signals. Well, you probably
> don't have all 100 anywhere, so you can get by with fewer filters
> and demodulators, but all in the digital side.
>
> Then, still in the digital domain, convert to MP3 and write the
> data stream out, maybe to a disk drive.

I have DSP board with 2 MHz ADC (SFDR ~ 90dB). It receives AM stations 
without any problems (actually, picking up AM stations as interference is a 
problem for intended application of this board). It could probably process 
3-4 stations at the same time; but 100 channels would be too much for MIPS 
available.

Processing of 100 channels would be simple: common windowed FFT, then  AM 
detectors, then mp3 coders. The FFT would be 256 or 512 points, 16 bit 
input, and it has to run with the rate of 11.025 or 16kHz.
That makes for computing load at the order of 1000 MIPS, which is feasible.

FM is more difficult as bruteforce approach would require ~400ksps per 
channel. I am not sure if smarter algorithms would reduce sample rate 
required, especially if the goal is receive stereo, RDS and secondary audio 
channels.

Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com

Vladimir Vassilevsky <nospam@nowhere.com> wrote:

(snip on multiple channel AM and FM digital radio receiver)

> I have DSP board with 2 MHz ADC (SFDR ~ 90dB). It receives AM stations 
> without any problems (actually, picking up AM stations as interference is a 
> problem for intended application of this board). It could probably process 
> 3-4 stations at the same time; but 100 channels would be too much for MIPS 
> available.
 
> Processing of 100 channels would be simple: common windowed FFT, then  AM 
> detectors, then mp3 coders. The FFT would be 256 or 512 points, 16 bit 
> input, and it has to run with the rate of 11.025 or 16kHz.
> That makes for computing load at the order of 1000 MIPS, which is feasible.
 
> FM is more difficult as bruteforce approach would require ~400ksps per 
> channel. I am not sure if smarter algorithms would reduce sample rate 
> required, especially if the goal is receive stereo, RDS and secondary audio 
> channels.

Yes, it might be necessary to cheat, and figure out which ones actually
have a signal then only demodulate them. 

As I understand it, for cable TV they can use all the channels as
the signal levels are pretty constant between channels. For broadcast
TV and radio, they have to space them out enough that one can receive
a weaker signal with nearby strong stations. 

So, in one area you might have 20 or 30 FM stations, not 100.

I would hope for stereo, but maybe not RDS or SCA.

I haven't heard about SCA for years. 
(If I remember it, 67kHz FM subcarrier at about 10% modulation.)

-- glen

<kalvin.news@gmail.com> wrote in message 
news:fdc57980-fc10-460a-94be-ae756da584c2@googlegroups.com...
> Nice document about software defined radios and direct-conversion 
> receivers:
>
> "A Software-Defined Radio for the Masses, Part 1 - ARRL"
> http://www.arrl.org/files/file/Technology/tis/info/pdf/020708qex013.pdf
>
> Especially Tayloe-detector is very simple and straightforward 
> mixer/detector concept (Fig. 10 and Fig. 14).

It is a strange idea trying to receive anything with crappy direct 
conversion receiver positioned close to PC.

VLV

On Thu, 8 Nov 2012 08:54:52 -0600, "Vladimir Vassilevsky"
<nospam@nowhere.com> wrote:

>
><kalvin.news@gmail.com> wrote in message 
>news:fdc57980-fc10-460a-94be-ae756da584c2@googlegroups.com...
>> Nice document about software defined radios and direct-conversion 
>> receivers:
>>
>> "A Software-Defined Radio for the Masses, Part 1 - ARRL"
>> http://www.arrl.org/files/file/Technology/tis/info/pdf/020708qex013.pdf
>>
>> Especially Tayloe-detector is very simple and straightforward 
>> mixer/detector concept (Fig. 10 and Fig. 14).
>
>It is a strange idea trying to receive anything with crappy direct 
>conversion receiver positioned close to PC.
>
>VLV

So use a good one, instead.   802.11 cards are universally direct
conversion, and they work fine in PCs and laptops and lots of other
similar equipment.

It just has to be engineered competently.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

"Eric Jacobsen" <eric.jacobsen@ieee.org> wrote in message 
news:509bdd0a.313794014@www.eternal-september.org...
> On Thu, 8 Nov 2012 08:54:52 -0600, "Vladimir Vassilevsky"
> <nospam@nowhere.com> wrote:
>
>>
>><kalvin.news@gmail.com> wrote in message
>>news:fdc57980-fc10-460a-94be-ae756da584c2@googlegroups.com...
>>> Nice document about software defined radios and direct-conversion
>>> receivers:
>>>
>>> "A Software-Defined Radio for the Masses, Part 1 - ARRL"
>>> http://www.arrl.org/files/file/Technology/tis/info/pdf/020708qex013.pdf
>>>
>>> Especially Tayloe-detector is very simple and straightforward
>>> mixer/detector concept (Fig. 10 and Fig. 14).
>>
>>It is a strange idea trying to receive anything with crappy direct
>>conversion receiver positioned close to PC.
>>
> So use a good one, instead.

Indeed. It took us a long time to find a 802.11 solution that would be good, 
reliable and performing to the standard.

>802.11 cards are universally direct
> conversion, and they work fine in PCs and laptops and lots of other
> similar equipment.

That's why there are so many poorly performing cards that hiccup at tabletop 
distances.

> It just has to be engineered competently.

So, where is it? Could you please engineer one for me.

Vladimir Vassilevsky
DSP and Mixed Signal Consultant
www.abvolt.com