Author Topic: Red Pitaya for a beginner  (Read 16436 times)

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Offline piranha32

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Re: Red Pitaya for a beginner
« Reply #25 on: August 02, 2015, 01:03:29 PM »
Low voltage (+/- 1V).
Well, ±1.095V is 0 dBm.
I just checked with "High Voltage" setting. The peaks remain the same, what indicates that the noise is not picked up by the input circuit, but appears somewhere closer to the ADC.
The ADC sample rate is 125 MHz, right? If the peak is at 2.5 MHz, then it's possible that the ADC is clocked by 2.5 MHz (which leaks into the ADC inputs) and uses a x50 PLL to get the sample rate. How did you measure the frequency of the peak?

The frequency is more of a "ballpark" figure. At this frequency the spectrum analyzer app detected the peak, and the exact frequency detected is 2.35MHz. According to the schematic from http://wiki.redpitaya.com/index.php?title=External_ADC_clock the system is clocked from 125MHz XO, so it's rather unlikely that the ADCs are clocked using lower frequency.

Offline tggzzz

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Re: Red Pitaya for a beginner
« Reply #26 on: August 02, 2015, 05:35:34 PM »
The idea is very nice, but implementation - not so much.

There's another area where the implementation is unnecessarily poor: the signals and grounds on the external connector
http://www.eevblog.com/forum/microcontrollers/red-pitaya-hd-images-for-getting-the-schematic/msg541053/#msg541053

All the "NC" pins were grounds, and they were were distributed amongst the signals - not good for signal integrity. And it is compounded by not being able to use LVDS on those pins, due to the choice of Zynq IO bank voltage :(
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline markd

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Re: Red Pitaya for a beginner
« Reply #27 on: February 12, 2016, 10:57:30 PM »
I recently bought one and I now lament the other things I could have bought with the money I spent on the RP.
The graphical IDE is not free.
There's no IDE that's similar to Arduino.
Connection to the RP is through the network, which is a royal pain in the arse  for 2 reasons. 1- if your going to use it at work and don't have admin access to routers etc. 2. I found it very difficult to set up for a fixed IP Address.
You would want to be very familiar and comfortable using LINUX to be comfortable using RP.
I found the web page and Wiki contradicted each other to some degree.
The Wiki page doesn't appear to be well maintained.
When RP talks about beginners, they must mean University Student beginners, because the surely can't be talking about hobbyist level beginners.
 

Offline 1design

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Re: Red Pitaya for a beginner
« Reply #28 on: February 14, 2016, 05:46:39 AM »
Has anyone gotten their hands on a Red Pitaya and put it through its paces?
I have looked around and can't find much in terms of reviews and what I can find is very basic.
Would it be worth getting one or would it only really be useful for simple projects.

I've got RP on Kickstarter, and I'm disappointed. The idea is very nice, but implementation - not so much. I'd have a lot to say about very crude official software, not improved much since the first release making the tool more of an educational toy than a real measurement instrument, short buffer, etc. However, the most important reason for me to leave it on a shelf is noise. The image below shows how the spectrum of the signal from ADCs looks like with no signal connected to the inputs (inputs terminated with 50ohm SMA terminators). 25dB spike at 2.3MHz, a bit lower one at second harmonic, plus quite a bit of noise at low frequencies. Of course it is well visible as noise on scope view, making RP useless for measuring weak signals.

I tried to check this but couldn't manage to reproduce, my spectrum is clean even without a termination, what you see is a screenshot without anything on the connector. Where you using a signal generator at the time on the transmitting channels? It could be a crosstalk issue?

BR
 

Offline piranha32

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Re: Red Pitaya for a beginner
« Reply #29 on: February 14, 2016, 06:23:58 AM »
I tried to check this but couldn't manage to reproduce, my spectrum is clean even without a termination, what you see is a screenshot without anything on the connector. Where you using a signal generator at the time on the transmitting channels? It could be a crosstalk issue?

BR

No signal generators, nothing transmitting anywhere nearby. The issue is 100% reproducible, at many locations, so the issue of interference from local noise sources can be crossed off the list.

Offline sfox7076

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Re: Red Pitaya for a beginner
« Reply #30 on: February 18, 2016, 01:53:31 AM »
I tried to buy the calibrated Red Pitaya kit last week when it was on sale.  They ignored my order and now want me to pay full price.  After seeing this post, I am glad the order didn't go through.
 

Online Howardlong

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Re: Red Pitaya for a beginner
« Reply #31 on: February 21, 2016, 08:23:23 PM »
I'm developing a supposed quick and dirty solution for someone and recommended a Red Pitaya rather than build hardware from scratch bearing in mind the short timescales and need to de-risk. I received the device on Friday. I got the board to work fairly quickly, but then to develop on it... not so much.

Here are my thoughts to Red Pitaya.

o Make the unit easily visible through the network with a hostname out of the box, don't rely solely on the end user having to go through hoops (admittedly small, but irritating) just to connect to board. Thank God it has a USB/Serial port.

o The Visual Programming thing doesn't even work... I assume it/s a cloud development system, but I don't know as I click the link and bugger all happens. Tried a few browsers including Chrome, Firefox and IE in Windows and Ubuntu, nothing. The scope didn't work, nothing seemed to want to make the traces do anything other than flatline. The spec an works, but that was it.

o Blinky. FFS, how hard do you have to make this? I've been at it since Friday on and off. It's now Sunday, I reckon I've had about eight hours at this, in between building various Linux versions in VMs to try to make the instructions work. I still can't flash a ****ing LED. Not in Python, not in C. And no, I'm not an academic outfit with cheap Matlab licences. Why can I not get a Blinky to work? the documentation is woefully out of date/wrong/doesn't tell you what versions of what to install. You end up pissing in the wind trying stuff out. I have now found out that they switched from soft floating point to hard floating point making their very very very long Eclipse instructions incorrect. All well and good having detailed instructions, that's appreciated and to be applauded, but when they're wrong because you've deprecated it..... well I think I'll leave it there.

Their web site is all very flashy and whatnot, sadly the content is very broken :-(

True, there are warnings that the instructions are inconsistent since a Sept 2015 release... but we are now five months later.

With regret, as it stands it's hardly for a beginner if you want to use it as a development platform.
« Last Edit: February 21, 2016, 08:27:31 PM by Howardlong »
 

Offline piranha32

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Re: Red Pitaya for a beginner
« Reply #32 on: February 24, 2016, 05:25:03 PM »
I have an impression that the company has been hijacked by the marketing department. The default apps (especially what they call "oscilloscope") shipped with the basic image suck big time. They're enough to verify that the hardware works, but completely useless for everyday use. Even shady Chinese manufacturers of crappy oscilloscopes would be ashamed of them.
While there was some justification for shipping such low quality apps at the beginning, especially with the Kickstarter units while they were still working on the software, including them with the new system image released recently is a disgrace. Of course you can purchase much better looking "premium" apps. You'll just have to part with €49 for the oscilloscope part, or €39 for spectrum analyzer on top of what you paid for the hardware.
I don't deny Redpitaya the right to make money from selling software. It's a commercial company after all, not a charity; However they should fulfill first what they promised, and then look for ways to make money on extra options.

Online Howardlong

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Re: Red Pitaya for a beginner
« Reply #33 on: February 25, 2016, 12:06:49 PM »
To put this into perspective, I finally got a blinky going this evening. I'd say that took about 25 hours of seemingly endless random button pushing when none of the instructions worked as described. Even then I had to compile the code on the board as there seems to be a problem with cross compiling shared libraries in the recommended toolchain, but that's just one of dozens of banana skins I encountered.
 

Offline Carrington

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Re: Red Pitaya for a beginner
« Reply #34 on: February 27, 2016, 06:57:37 AM »
I'm curious, just to compare the noise floor, here are the results of one Analog Discovery I (worst case possible and nothing in the inputs):
My English can be pretty bad, so suggestions are welcome. ;)
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Offline Carrington

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Re: Red Pitaya for a beginner
« Reply #35 on: February 29, 2016, 05:29:13 AM »
Wow! Seems that the Red Pitaya has half noise floor that the Analog Discovery:



And even so it round 5 mVpp.
My English can be pretty bad, so suggestions are welcome. ;)
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Online Howardlong

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Re: Red Pitaya for a beginner
« Reply #36 on: March 02, 2016, 09:42:04 AM »
If you're lucky enough to get the scope app to work at all of course! No luck with that one here I'm afraid although the spec an seems to run.

When travelling at the weekend, I used an Analog Discovery to debug code on the Red Pitaya. I can't imagine ever having to do it the other way around.
 

Offline Carrington

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Re: Red Pitaya for a beginner
« Reply #37 on: March 04, 2016, 07:02:21 AM »
If you're lucky enough to get the scope app to work at all of course! No luck with that one here I'm afraid although the spec an seems to run.

When travelling at the weekend, I used an Analog Discovery to debug code on the Red Pitaya. I can't imagine ever having to do it the other way around.

Yeah, relative to the SW, the red pitaya is a different history (at least for now).
For the Red Pitaya I guess that is a matter of time or money (if one want to buy the SW).
With the Analog discovery one can put to work it quickly, just using the provided SW.

I know that isn't easy to comply with the USB specification, but both devices have a lot of noise floor, a shame.

It could be as good as this:



But instead, you get something like this:

My English can be pretty bad, so suggestions are welcome. ;)
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Online Howardlong

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Re: Red Pitaya for a beginner
« Reply #38 on: March 04, 2016, 08:02:09 AM »
It turns out that unlike the SA that comes with the Red Pitaya stable release, the scope app included with the "stable" build doesn't work. I had to install the one from their online store.

To add insult to injury, for most of this week I've been unable to log in to their systems.

Perhaps this is scare mongering, but I do have concerns about their business model. I think they were betting that they'd make their mint on their app store purchases. Like the Analog Discovery, there can't be much margin on the hardware once the retailers have had their pound of flesh.
 

Offline Carrington

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Re: Red Pitaya for a beginner
« Reply #39 on: March 05, 2016, 03:49:06 AM »
It turns out that unlike the SA that comes with the Red Pitaya stable release, the scope app included with the "stable" build doesn't work. I had to install the one from their online store.

To add insult to injury, for most of this week I've been unable to log in to their systems.

Hey! Red Pitaya team, takes note. :--

Perhaps this is scare mongering, but I do have concerns about their business model. I think they were betting that they'd make their mint on their app store purchases. Like the Analog Discovery, there can't be much margin on the hardware once the retailers have had their pound of flesh.

More than likely.
My English can be pretty bad, so suggestions are welcome. ;)
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Offline austnais

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Re: Red Pitaya for a beginner
« Reply #40 on: February 28, 2017, 09:37:06 PM »
Hi guys. Not much interest in the Pitaya here!
I have a sort of general question, but the application is a Red Pitaya. What do you use the RF outputs for? Say I want to drive a mosfet to create a switchmode application where adjustment of the dutycycle, frequency, waveform etc is really easy, is this possible. The output is only +-1Vpp so I assume it might be difficult.

If not, what to use instead? A 555 will for sure do the job, but it's a bit tedious to modify for experimental work.
 

Offline Kean

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Re: Red Pitaya for a beginner
« Reply #41 on: February 28, 2017, 10:15:53 PM »
I have an RP, but I've never seriously used it beyond an initial play.  It has been much easier to use my bench equipment than stuff around with a Linux environment with web interface and poor support.  Maybe one day I'll have a specific application I can put it to so as to jusity the original purchase  :palm:

The RP Sig gen output would be used just like any other Sig gen.  It can go up to 50MHz which isn't bad, but the output of only +/-1V (and no offset?) is a bit of a limitation.
As a sig gen it can be used for their LCR application, or testing HF/VHF filters, etc.

For drivng a MOSFET from either a 555 or a signal generator, you going to want to use a MOSFET driver chip - maybe something like the TC4428A (logic level input).
You can get a MHS-5200A 25MHz sig gen which is more functional then the RP one for $70, and the MHS-5200P is similar spec but higher power (can put out 1A).  :-+
I'd still use this with the driver chip as that would generally be needed in the final design.
 

Offline austnais

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Re: Red Pitaya for a beginner
« Reply #42 on: February 28, 2017, 11:11:45 PM »
Thanks a lot Kean!

Regarding the TC4428A, HIGH-threshold is 2.4V which means the RP is not enough just to drive directly. Can one use a pull-up to 5V on the line and let the pitaya sink the curent? Or does there exist a chip with lower HIGH-voltage threshold?
 

Offline Kean

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Re: Red Pitaya for a beginner
« Reply #43 on: February 28, 2017, 11:34:38 PM »
Yep, glad you spotted that - that's why I mentioned "logic level input"  :)
A pull-up isn't ideal, as you want reasonably quick transitions from low to high, and that requires active circuitry.  Also I don't recall the RP output circuit, but it may not like having the 5V pullup either.

Personally, I'd suggest building a small output amplifier board for the sig gen that also lets you add a DC offset.  It wouldn't have to handle the full 50MHz bandwidth, just whatever range you think you'll be working with.  That would then allow you to take a signal from the RP, then amplify and offset it to say generate a 0-3V signal from the RP sig gen.  You can feed it back into one of the RP inputs (in HV mode) to check it, and would be quite a useful general purpose add-on for experimenting I think.  It would need separate power supply, something like +5V & -5V, depending on your desired output range.

You'll want to read up on using opamps if you're not too familiar - something like this explains the concept
http://electronics.stackexchange.com/questions/93423/how-to-amplify-and-offset-the-voltage-in-an-opamp
 

Offline austnais

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Re: Red Pitaya for a beginner
« Reply #44 on: February 28, 2017, 11:50:21 PM »
Great! Op-amps is clearly the solution for me!
Thanks :)
 

Offline Kean

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Re: Red Pitaya for a beginner
« Reply #45 on: March 01, 2017, 12:06:40 AM »
Maybe just start with a common one like http://www.ti.com/product/TL072
 

Offline boffin

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Re: Red Pitaya for a beginner
« Reply #46 on: March 03, 2017, 05:11:40 PM »
it's not fast, but it's cheap, and at least the API is open http://www.nscope.org/
 

Online janoc

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Re: Red Pitaya for a beginner
« Reply #47 on: March 04, 2017, 03:22:08 AM »
it's not fast, but it's cheap, and at least the API is open http://www.nscope.org/


At 4 megasamples/s you are going to be have difficulty to even debug basic SPI with that thing. That's a toy or perhaps cheap educational tool. Real usable bandwidth is going to be perhaps 400kHz and if all 4 channels are in use, 100kHz or so. Is is basically a PIC with some opamp buffers + DDS for signal generation.

NScope doesn't come anywhere close to the capabilities of Red Pitaya or Analog Discovery. Open API is cool but of little use when the hw is not up to task. And it costs $100 on Amazon. That's fairly expensive for what it can do, IMO.

« Last Edit: March 04, 2017, 03:26:17 AM by janoc »
 

Offline irakandjii

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Re: Red Pitaya for a beginner
« Reply #48 on: March 21, 2017, 02:54:28 PM »
I had one, (early adopter) found the performance had been overstated and the support terrible.  At the time, the promised applications were buggy and the community very small.
I really wanted it to work but,  a tool should never consume more of your sweat and effort than the task for which it was acquired. It did!
I sold it on ebay at a significant loss, and still happy I did..
 


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