QUESTION re. the '888 ReceiveR

[David Eckhardt]

I've posted this question dovetailed on other messages in the past and possibly it was missed.  So, here it is"

Is the '888 receiver capable of displaying a full HF spectrum in real time on the support PC?

Additionally, is it capable of using the direct USB-B or -C data without an intervening virtual audio cable?  Most of these virtual audio cables drop packets at random. 

As all are aware, the AirSpy and SDRPlay units max out at 6 MHz of displayed usable spectrum width.  I'd like more for Jupiter and solar observations.   Ideally that would span LF through 60 (or so) in a real time spectrum display.  

Any comments are welcome (other than don't buy from Aliexpress!).

Dave - WØLEV

Virus-free.www.avg.com

[Philip Gladstone]

Yes, it will display the entire 30MHz:

No virtual audio cables involved.

I have a couple of filters installed so that the LF range is suppressed as is stuff much over 30MHz.

Philip

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[Dave Typinski]

I have the same question. If the 888 does support such a wide real-time
spectrum, I'd sure like to know how it sends IQ data in real time at such a wide
bandwidth in sustained operation over a USB connection without dropping samples.
In my experience, one can get fast burst speeds over USB, but doing continuous
24-hour observation runs at 30 MHz BW (60 Msamp/sec data rate for combined I and
Q, or 120 MBps assuming 4-byte combined I and Q samples) isn't tenable even for
USB3. For 60 MHz RF BW, the data rate would be be twice that. If someone found
a way around that for USB, it sure would be nice. Far as I know, the only way
to get that kind of sustained data bandwidth is to use Ethernet (or SATA or
PCI-e, etcetera).
--
Dave

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[Dana Whitlow]

If the '888 only supports 30 MHz BW with a 60 MHz sample

rate, it is not sending both I & Q.

Dana   K8YUM

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[Terry Bullett]

I would like to know about the data rates and dropped samples too.  As W0LEV points out the virtual audio cables are problematic (VBCAble being the worst I have tried, VAC being the best, for a reasonable license fee)  But dropped audio packets are real and destroy the SFDR, phase noise and the entire statistical nature of the data.  

The performance of an Airsspy R2 at 10 MHz under USB is really limited because of the internal downconverter issues:

here are all the warts when looking at 1420 MHz (hydrogen line) into a relatively dark sky.   From Little Thompson Observatory:




These receiver issues and COVID has placed the LTO radio astronomy project on hold since 2019. 

6 MHz of useful bandwidth is no enough to do HI and HII observations, even on 1000 Jy objects (with a 16 ft reflector).

And Forget about Pulsars! 

With a better receiver, I might be convinced to replace the front end with a good UHF Local Oscillator and a image rejection mixer like the Haystack Small Radio Telescope, and push the 60 MHz limit of the RX888. 

73,

Terry
W0ASP

-- 
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"Life is Complex.  It has a Real part and an Imaginary part." 

[Philip Gladstone]

Note that the bandwidth of 30MHz (or 60MHz) is only available at HF. At VHF the bandwidth is (AFAIK) limited by the tuner chip and I think that only gives you 8MHz or so.

The USB device descriptor of the Rx888 includes the information below. This seems to indicate that it needs to run at the 5Gbps rate for full functionality.

Philip

 SuperSpeed USB Device Capability:
    bLength                10
    bDescriptorType        16
    bDevCapabilityType      3
    bmAttributes         0x00
    wSpeedsSupported   0x000e
      Device can operate at Full Speed (12Mbps)
      Device can operate at High Speed (480Mbps)
      Device can operate at SuperSpeed (5Gbps)
    bFunctionalitySupport   3
      Lowest fully-functional device speed is SuperSpeed (5Gbps)
    bU1DevExitLat          10 micro seconds
    bU2DevExitLat        2047 micro seconds

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[John Ackermann N8UR]

I was hoping one of the folks with more detailed knowledge will jump in,
but lacking that...

The RX888 has an LT2208 16 bit ADC. It's a single ADC, not quadrature,
so Dr. Nyquist says the sample rate has to be twice the receiver
bandwidth. The "stock" RX888 runs at 122.88 msps, giving a bandwidth
from near DC to about 60 MHz. There is no FPGA in the receiver, so what
comes down the USB3 link is raw samples. I *believe* that the data is
sent down the USB3 interface with two bytes per sample to support the 16
bit ADC.

That's about 2 gbit/second data rate, which is well below the minimium 5
gpbs that USB3 can handle.

Because of concerns about heat dissipation, most of the RX888s in the
HamSci community have been running with a 64 MHz clock generated by a
GPSDO-controlled clock. The slower sample rate allows coverage to 30
MHz, and the ADC doesn't get quite so hot. (There is now doubt whether
the heat really is a concern, but most folks continue to run this way.)

At 64 msps, the data rate down the USB3 is about 1 gbps.

USB3 can easily handle either of these data rates; the issue is whether
the PC can process that firehose fast enough. The general experience is
that a Raspberry Pi 5 is not quite able to handle 122.88 msps, but can
deal with 64 msps. Something like an i5 PC can handle the full data
rate and do some useful work besides.

A couple more notes --

* The internal antialiasing filter cuts off somewhere above 70 MHz,
which is fine for the 64 MHz bandwidth at 122.88 msps. When running at
64 msps, you need an additional low pass filter at around 32 MHz or you
will get aliases from low-VHF band and 6M transmitters.

* The above is all talking about using the "HF" antenna input which is
directly sampled in the ADC. The RX888 also has a VHF/UHF input that
can be software selected (the hardware is similar to that in the Airspy,
RTLSDR, etc. radios). It covers something like 60 to 1700 MHz. Its
tunable-front end filter is only about 8 MHz wide which limits the
useful bandwidth even though the sample rate would support more. The
tuner is locked to the same clock as the ADC, so the frequency
accuracy/stability at VHF is the same as at HF.

Hope this helps, and also hope that someone with deeper technical
knowledge will correct any errors I've made above.

73,
John
----

On 7/26/24 20:02, 'Terry Bullett' via HamSCI wrote:
> I would like to know about the data rates and dropped samples too. As
> W0LEV points out the virtual audio cables are problematic (VBCAble being
> the worst I have tried, VAC being the best, for a reasonable license
> fee)  But dropped audio packets are real and destroy the SFDR, phase
> noise and the entire statistical nature of the data.
>
> The performance of an Airsspy R2 at 10 MHz under USB is really limited
> because of the internal downconverter issues:
>
> here are all the warts when looking at 1420 MHz (hydrogen line) into a
> relatively dark sky.   From Little Thompson Observatory:
>
>
>
>

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[Phil Erickson]

Hi John,

  That's an excellent summary.

There is now doubt whether
the heat really is a concern, but most folks continue to run this way.

  Can you provide a bit more information on the perhaps revised conclusion downweighting concerns about overheating at full bore rate?

*  The internal antialiasing filter cuts off somewhere above 70 MHz,
which is fine for the 64 MHz bandwidth at 122.88 msps.  When running at
64 msps, you need an additional low pass filter at around 32 MHz or you
will get aliases from low-VHF band and 6M transmitters.

  Has anyone ended up plotting the internal A-A filter response cutoff at 70 MHz using (tedious) injected signals or a triggered sweep or something equivalent, at the full rate of 122.88 Msps?   Wondering about the details of how well they implemented the LP filter.

73

Phil W1PJE

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----
Phil Erickson
phil.e...@gmail.com

[John Ackermann N8UR]

On 7/27/24 10:42, Phil Erickson wrote:

>   Can you provide a bit more information on the perhaps revised
> conclusion downweighting concerns about overheating at full bore rate?

Early on, there were reports of units failing and the assumption was
that it was due to poor thermal management. Clint KA7OEI did some
testing and came up with a set of changes to improve the heatsink
effectiveness:
http://ka7oei.blogspot.com/2023/08/improving-thermal-management-of-rx-888.html

Just slowing the clock from 122.88 to 64 MHz significantly reduced the
heating. Since 99% of our activity was below 30 MHz, that was a no-brainer.

Then, most folks found that the simplest mechanical step -- adding a
thicker/better thermal pad on the bottom of the PCB (contacting the
case) and stick-on heatsinks on the tops of the chips -- made a
significant further improvement without requiring a lot of messy
mechanical work.

The TAPR RX888 accessory kit:
https://tapr.org/product/rx888-clock-kit-and-thermal-pad/
includes a piece of the thermal pad.

But lately others have questioned whether those early issues were in
fact thermal-related as we've found other creative ways to break the
radio :-) that could account for them.

The bottom line is that it's unclear whether the thermal issues are real
or red herrings, but the two fixes mentioned above are simple enough
precautions to be worthwhile in any case.

> *  The internal antialiasing filter cuts off somewhere above 70 MHz,
> which is fine for the 64 MHz bandwidth at 122.88 msps.  When running at
> 64 msps, you need an additional low pass filter at around 32 MHz or you
> will get aliases from low-VHF band and 6M transmitters.
>
>
>   Has anyone ended up plotting the internal A-A filter response cutoff
> at 70 MHz using (tedious) injected signals or a triggered sweep or
> something equivalent, at the full rate of 122.88 Msps?   Wondering about
> the details of how well they implemented the LP filter.

It would be hard to test the filter in-line since by design it's above
the maximum frequency the radio can hear even the highest sample rate.
The schematics available do show the filter values so they could be
analyzed; I don't know if anyone has done that.

We *do* know that with a 64 MHz clock aliasing is a real problem. Phil
Karn hears local FM low-band highway patrol radios, and I heard our 6M
beacon come blasting through at about 13.9 MHz (64 - 50.08 = 13.92).
Putting a simple 30 MHz low pass filter ahead of the antenna fixes the
problem. Paul Elliott's shelf filter board (hopefully also to soon be
available from TAPR) includes a good LPF.

73,
John
----

[Phil Erickson]

Hi John,

It would be hard to test the filter in-line since by design it's above
the maximum frequency the radio can hear even the highest sample rate.
The schematics available do show the filter values so they could be
analyzed; I don't know if anyone has done that.

  A test that can be done for this full sampling case is to add an external LPF at 50 MHz (say) and compare the before/after even in this 1st Nyquist zone aliasing case.  If the input is swept above 70 MHz, you should be able to understand the difference.  I wonder why they selected 70 MHz given that the sampling rate is 122.88 Msps, though; that filter should be < 61 MHz (ideally maybe 90% of 0.5*(122.88 Msps) or about 55 MHz in my experience to add some guard band protection) so this is simply adding ~9 MHz of extra aliased noise to the input.  Perhaps it has something to do with the internal analog architecture that is separating out the VHF and HF streams?

  Or I've made an error above..

73

Phil W1PJE

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[Dave Typinski]

One thing to note is that 5 Gbps is the /maximum/ rate for USB3 Gen 1 (kind of
like how your ISP says you'll get "up to x MBps" but rarely achieve that in real
life). Real world USB3 burst speed is slower -- and the sustained speed is even
slower still.

https://tripplite.eaton.com/products/usb-connectivity-types-standards

For a 30 MHz RF BW, we'd be looking for a sustained data rate of 120 MBps plus
overhead. Even with a USB3 external SATA HDD, large file transfers see maybe
half of that data rate. Some PCs are faster than that, some slower -- the error
bars are large since OS overhead is a big unknown. "Your mileage may vary."

Point is, one should not count on a continuous 120 MBps making it across to the
host PC from the 888. Dropped samples seem likely assuming the average new PC.

Also worthy to note that there are USB3 Gen 2 and USB4 standards that will more
easily support that data rate -- but it's an open question whether the 888 and
one's host PC support any of the newer standards.

In perhaps five or ten years this will be moot. For now, however, the real
world sustained bandwidth of a USB connection is still worth consideration.

Lastly, some applications may not care about dropped samples. If one is simply
using the device as a panadapter, dropped samples are a non-issue. On the other
hand, if one is hunting for pulsars or recording high speed spectral data
containing Jovian S-bursts, then dropped samples are a deal breaker.
--
Dave

[Jonathan]

Dave Typinski,

Are you hunting for pulsars below 30 MHz? 

Jonathan

KC3EEY 

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[David Eckhardt]

Overhead: 

QUOTE from Dave T's latest contribution):  the error bars are large since OS overhead is a big

                                                                             unknown.  "Your mileage may vary.

While Fibre Channel isn't USB, experience speaks to the overhead issue.  Fibre Channel starts out a roughly 1.3 GHz.  After all the overhead continuous data rates are in the vicinity of 140 MHz or Mbps.  Can't we do better???!!!  Maybe this has changed over the 10 years that I've been retired?  Is all this overhead necessary???!!

Dave - WØLEV

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Dave - WØLEV

[John Ackermann N8UR]

On 7/27/24 11:24, Phil Erickson wrote:
  I wonder why they selected
> 70 MHz given that the sampling rate is 122.88 Msps, though; that filter
> should be < 61 MHz (ideally maybe 90% of 0.5*(122.88 Msps) or about 55
> MHz in my experience to add some guard band protection) so this is
> simply adding ~9 MHz of extra aliased noise to the input.  Perhaps it
> has something to do with the internal analog architecture that is
> separating out the VHF and HF streams?
>
>   Or I've made an error above..

I should have been more clear -- that was a SWAG at the filter frequency
and I haven't measured or calculated it. Sorry for the confusion.

John

[Larry Dodd]

Dave
Right. I personally have no interest in pulsars. Ha! Nor little above 30 MHz. I leave that to the big dishes. As long as we are stuck with RSS it’s a limiting factor. When you start recording IQ wide bandwidth with 16 bit SDRs the files are hugh. I don’t do that often certainly not every day. Even with .sps files at 20 MHz BW I have had to resort to limiting it to 6 hour files. Otherwise they won’t save. Luckily you can stitch them together if desired. Such fun.
Larry

> On Jul 27, 2024, at 1:14 PM, Dave Typinski <dav...@typnet.net> wrote:
>
> One thing to note is that 5 Gbps is the /maximum/ rate for USB3 Gen 1 (kind of like how your ISP says you'll get "up to x MBps" but rarely achieve that in real life). Real world USB3 burst speed is slower -- and the sustained speed is even slower still.

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[Dave Typinski]

Tried that ca. 2010 despite the math indicating the array I had available didn't
have the requisite directivity to make it a success.

Turned out the math was right. Who'da thunk? :) Despite abject failure, it
was a /very/ fun project. I learned a lot from it.

Hunting pulsars in the HF band is a /really/ tough row to hoe. Pulsar flux
density decreases below about 100 MHz and the galactic background is hotter than
hades in the HF band. Which means so you need an array with high directivity,
meaning lots of array elements, meaning lots of land in an RFI-free location
upon which to plant 'em. Oh, and lots of money, too. Neither of which I have
in unlimited abundance, so that was the end of that.
--
Dave

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[Terry Bullett]

Phillip,

Yes, thank you for pointing that out.   This is exactly why for 1420 MHz radio astronomy I would have a low phase noise LO, a very high quality image cancelling mixer and a very good band pass filter to bring 1420 down to about 30 MHz.

Something I forgot to mention:  Ifanyone is using linux to collect high speed USB data and have access to the source code, it is not that difficult to give the data collection loop access to the Linux kernel real time (RT) process handler and give it full priority.  I did this with a USB2 device that was running at the limit of USB2 data rates.  I'm sure others here have done that too.

USB dropouts tend to be in large data blocks, making a failure much more obvious than the few dropped samples from an audio stream.

Terry 

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[Terry Bullett]

The way to do this is with a broadband noise source that has a flat frequency response and enough noise power to beat the receiver's Noise Figure.
Most noise sources are specified > 30 MHz and into the multiple GHz range.   
If the receiver's output is not flat and absolutely Gaussian, there is a problem.  

Terry
W0ASP

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[Terry Bullett]

I am thinking about pulsars in the 300-400 MHz spectrum, depending on what local RFI will allow.
the internal down-converter for the Rx-888B might be good enough.  Only a test will tell.

Terry
W0SAP

Terry....@noaa.gov    720-446-9775 (google voice)  978-337-9092 (cell)   
"Life is Complex.  It has a Real part and an Imaginary part." 

[Dave Typinski]

That would be entirely do-able. Much smaller array (physically). If you're not
besotted by RFI, it will be a fun, challenging project.

Pulsars are extremely fascinating beasts. Millisecond pulsars even more so.
Those live at the bleeding edge of what's physically possible in this universe:
a mass 1½ to 3 times that of our entire solar system packed into a ~20 km
diameter ball spinning at 40,000 RPM. Surface gravity ~100 billion gees.
Velocity at the rotational equator of a quarter c. Magnetic field on the order
of a billion tesla.

Here's an amateur who observed pulsars successfully at low-UHF:

https://sites.google.com/view/hawkrao/home

--
Dave

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[Fernando Castellani]

Hi!

Wich software are you using for this beautiful spectrum view?

Thanks!

73

Fernando 

PT2FHC 

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[Black Michael]

https://www.g0xbu.co.uk/websdr/setting-up-an-openwebrx-site

Mike W9MDB

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[Terry Bullett]

Dave,

Pulsars are a challenge to chase.  One often has to dig them out of the noise by a filter based on the period and frequency dispersion.   Thus the need for large bandwidth.

I already have a heavy duty equatorial mount, so the plan is to put a 2x2 array of crossed yagi elements into an array on a 'bedspring' type backplane that can manually
tilt in elevation and track sidereal time.  This should allow up to 12 hours of collect time each day.  With something like 5E12 samples per 12 hour period, one can dig a very small coherency out of the Gaussian noise.  So long as everything is stationary. 

Missing a sample throws off the timing of the filter and starts the coherent integration over again.

One of my passions at the Observatory is bringing astronomy to the visually impaired and other radio type people who have "good ears".  Or like me those who are audio learners.  Meteor scatter was remarkably effective at this.  I'm betting my volunteer hours that Pulsars will be so enlightening.  

Terry
W0SAP  

[Jonathan]

Dave Typinski,

Have others been successful at detecting pulsars below 30 MHz? To me, it just seems like a fruitless endeavor even for the most optimized system. Many of the non-millisecond pulsars are brightest at 300-400MHz, and as Terry said, a wide filter is needed. 

Terry,

I’m interested to see the RX888 with a disciplined sample clock used in amateur pulsar work. I’m even more excited to see the RTLSDR with a GPS frequency and timing ping method by the author of vlfrx-tools used in amateur pulsar work. The method uses a frequency reference that is fed into the RF input and and gated by a PPS, both sources from s uBlox M8T. The vlfrx-tools software knows where to look at the reference frequency and uses its PPS pulse for both frequency and timing calibration, effectively equivalent to a disciplined sample clock and with embedded timestamping. A successful, working application of this method is finding meteor pings in VHF beacons.

Jonathan 

KC3EEY 

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[Terry Bullett]

Jonathan,

The ionosphere severely messes with trans-ionosphere propagation below 30 MHz. That plus atmospheric noise and the near impossibility of making a low noise figure (< 1 dB) preamp than can handle the signal levels at HF are discouraging. 

The high natural noise level at HF allows us all to ignore many things about receive antennas.

If you want to learn about things like noise figure and transmission line loss and thermal stability there is no better way than to explore radio astronomy.
You don't need a huge antenna to get started.  Our LTO STEM students made a horn from aluminum backed insulation board with about a 1m^2 aperture.

Don't Panic!  Your hitchhikers guide to the Galaxy is Radio Eyes:
https://radiosky.com/radioeyesishere.html

Inserting switches at my receiver input is unacceptable. The best HP mechanical switches insert 0.5 dB of loss and noise.  In radio astronomy, 0.5 dB is HUGE.

I hope this inspires the community to pay attention to the details of RF. 

73,
Terry
W0SAP

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[Dave Typinski]

Jonathan, I'm unaware of any /amateurs/ making pulsar observations below 30 MHz.

Professional observatories have indeed observed pulsars below 30 MHz.

One example: in the 1980s, the 26.3 MHz 640-dipole array at the UF Radio
Observatory was used with an 8-kHz BW receiver and a 15-minute integration time
to observe four pulsars (reference attached).

Larger arrays could easily do it -- and probably have, although I don't have
refs handy for those. Thinking of HF band arrays like the UTR-2 (2,040
dipoles), LOFAR low-band arrays (100s to 1000s of dipoles depending on
configuration), and LWA (512 dipoles per station). The Nancay Decametric Array
(144 conical log spirals 10-80 MHz) might be able to.
--
Dave

On 7/28/24 22:23, Jonathan wrote:
> Dave Typinski,
>
> Have others been successful at detecting pulsars below 30 MHz? To me, it just
> seems like a fruitless endeavor even for the most optimized system. Many of the
> non-millisecond pulsars are brightest at 300-400MHz, and as Terry said, a wide
> filter is needed.
>
> Terry,
>
> I’m interested to see the RX888 with a disciplined sample clock used in amateur
> pulsar work. I’m even more excited to see the RTLSDR with a GPS frequency and
> timing ping method by the author of vlfrx-tools used in amateur pulsar work. The
> method uses a frequency reference that is fed into the RF input and and gated by
> a PPS, both sources from s uBlox M8T. The vlfrx-tools software knows where to
> look at the reference frequency and uses its PPS pulse for both frequency and
> timing calibration, effectively equivalent to a disciplined sample clock and
> with embedded timestamping. A successful, working application of this method is
> finding meteor pings in VHF beacons.
>
> Jonathan
> KC3EEY
>
> On Sun, Jul 28, 2024 at 9:22 PM 'Terry Bullett' via HamSCI

> <ham...@googlegroups.com <mailto:ham...@googlegroups.com>> wrote:
>
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[Dave Typinski]

I wish you great luck, Terry!

If you're into audio, here are some pulsar recordings collected from the
internet -- i.e., not made by me.

https://www.aj4co.org/AJ4CO%20Data%20Archive/Pulsar%20Sounds/
--
Dave

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