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DDS spectral "purity" ?

N5IB
 

Steve and other have discussed the spectral purity, or lack thereof, from our inexpensive DDS modules and accessories. Playing around a bit today with a spectrum analyzer, I made a "home movie" of a sweep from one my my very early builds of a Type-2 PHSNA

PHSNA Type-2 (push-pull transformer output)
Type-2 AD-9850 module with on board filter removed
Standard 30 Mz version LPF on the PHSNA
The ERA-3+ running with a bit too much bias, at ~ 40 mA
Using the WinPHSNA app with 9600 baud Bluetooth communications
Sweep from 1 MHz to 40 MHz, with 100 kHz steps
Rigol DSA-815TG analyzer

SA vertical divisions 10 dB (top = +10 dBm)

SA horizontal divisions 10 MHz (0 to 100 MHz scan)


It's too big (12 MB) for Yahoo to allow into the Files or Photos areas, so here's the URL for it on my web page

http://www.n5ib.net/DDS_Spectrum.wmv


The cinematography will not earn me any Red Carpet time, but it's interesting to watch the wheat field march across the screen. Gives a good feel for the limitations of inexpensive test gear. Make popcorn, call the kids. It's rated "G" and is just over minute long.


Jim, N5IB


K5ESS
 

Jim,

Very Interesting.  Looks like the ERA3 generates lots of harmonics if the AD-9850 data sheet is to be believed.  Also found the latter part of the sweep very interesting.  I’m assuming that the responses shown marching right to left are from aliasing but they shouldn’t be showing up at output frequencies below 40 MHz with an assumed 125 MHz clock.  Maybe some funny business going on between the aliased frequencies and the harmonics generated in the ERA-3?  I’d be interested in seeing comments or just speculation on the likely source of these signals.

Mike, K5ESS

 

From: PHSNA@... [mailto:PHSNA@...]
Sent: Saturday, July 09, 2016 5:04 PM
To: PHSNA@...
Subject: [PHSNA] DDS spectral "purity" ?

 

 

Steve and other have discussed the spectral purity, or lack thereof, from our inexpensive DDS modules and accessories. Playing around a bit today with a spectrum analyzer, I made a "home movie" of a sweep from one my my very early builds of a Type-2 PHSNA

PHSNA Type-2 (push-pull transformer output)
Type-2 AD-9850 module with on board filter removed
Standard 30 Mz version LPF on the PHSNA
The ERA-3+ running with a bit too much bias, at ~ 40 mA
Using the WinPHSNA app with 9600 baud Bluetooth communications
Sweep from 1 MHz to 40 MHz, with 100 kHz steps
Rigol DSA-815TG analyzer

SA vertical divisions 10 dB (top = +10 dBm)

SA horizontal divisions 10 MHz (0 to 100 MHz scan)

 

It's too big (12 MB) for Yahoo to allow into the Files or Photos areas, so here's the URL for it on my web page

http://www.n5ib.net/DDS_Spectrum.wmv

 

The cinematography will not earn me any Red Carpet time, but it's interesting to watch the wheat field march across the screen. Gives a good feel for the limitations of inexpensive test gear. Make popcorn, call the kids. It's rated "G" and is just over minute long.


Jim, N5IB

 

Alfredo Mendiola Loyola
 

Hello,

I advice to use the CFB Opamp THS3201.
You can use a 75 Ohm or 200 Ohm Resistor on the input instead the 50 Ohm to get more output voltage.
See the datasheet for the second and third harmonics.

I'm using these opamps with my AD9851 DDS.

Regards.
Alfredo Mendiola Loyola
OA4AJP
Lima, Peru

Jaques Anthony
 

Hi all,
     Over the last week or so I have been doing quite a lot of measurements myself. They also confirm that the MMIC ruins what is otherwise quite a good spectral output, and I am thinking about various ways to fix it.
    So I would have been very interested to see Jim's video. Unfortunately though clicking on the link does not run the file.Evidently my browser does not know what to do with that extension. (I haven't come across it before either.)
     Related to this; I discovered a long time ago that the chinese modules (and possibly the AD985x itself) generates a whole forest of junk at about -60dBm (ish) while it changes frequency, and for about a second or so afterwards. In any sweep mode it does this all the time of course!
                                                                Tony (G3PTD)...

Tony G4WIF
 

Tony,

 

I have converted it to MP4 format and it’s a little smaller.

So Jim feel free to grab it and see if it will upload to the Yahoo group now.

 

https://dl.dropboxusercontent.com/u/1987387/DDS_Spectrum.mp4

 

Kind regards

Tony G4WIF

 

From: PHSNA@... [mailto:PHSNA@...]
Sent: 10 July 2016 10:35
To: PHSNA@...
Subject: [PHSNA] Re: DDS spectral "purity" ?

 

 

Hi all,
     Over the last week or so I have been doing quite a lot of measurements myself. They also confirm that the MMIC ruins what is otherwise quite a good spectral output, and I am thinking about various ways to fix it.
    So I would have been very interested to see Jim's video. Unfortunately though clicking on the link does not run the file.Evidently my browser does not know what to do with that extension. (I haven't come across it before either.)
     Related to this; I discovered a long time ago that the chinese modules (and possibly the AD985x itself) generates a whole forest of junk at about -60dBm (ish) while it changes frequency, and for about a second or so afterwards. In any sweep mode it does this all the time of course!
                                                                Tony (G3PTD)...

Steven Dick
 

Very interesting Jim. I don't have a spectrum analyzer here.  I wonder if you could rig your Type II PHSNA to eliminate the ERA-3+ per my writeup in the K1RF folder entitled "Bandpass Filter Measurement Anomalies R2.pdf"  I think you can greatly reduce the harmonics and still get about -8.7dBm output by:
1. Driving the antialias filter directly from the push-pull transformer with no attenuator per the AD9851 data sheet Page 11.
2. Take the output of the antialias filter to drive a 6dB attenuator.
3. Take the final output from this 6dB attenuator.

Then you are getting all your output from the AD9851 (or Type II AD9850) with no introduction of harmonics by the output amplifier. I just did not quantify the improvement but I see it in my measurements on some of the LC bandpass filters I designed.

If an output amplifier wants to be used, it really wants to be a low distortion type.  However, any wideband amplifier also needs to be carefully laid out per the manufacturer's data sheet. But like one of my bosses has told me many times, "Keep it Simple Steve" so I prefer no amplifier at all if the output power is adequate. The The OPA2674 or OPA2677 with better thermal package would work if more output power is required at low distortion.

Regards,
"Digital Steve", K1RF

Nick Kennedy
 


On Sun, Jul 10, 2016 at 4:35 AM, anthonyjaques94@... [PHSNA] <PHSNA@...> wrote:

Related to this; I discovered a long time ago that the chinese modules (and possibly the AD985x itself) generates a whole forest of junk at about -60dBm (ish) while it changes frequency, and for about a second or so afterwards. In any sweep mode it does this all the time of course!
                                                                Tony (G3PTD)...


​So I guess much of that extra junk wouldn't be present if the DDS were not sweeping then?  That's good news.  Although even when using the DDS as a​ VFO we might be doing a small jump from TX to RX for the CW and/or IF offset.  That's not within PHSNA's scope though.

73-

Nick, WA5BDU

Nick Kennedy
 


On Sun, Jul 10, 2016 at 6:59 AM, sbdick@... [PHSNA] <PHSNA@...> wrote:

If an output amplifier wants to be used, it really wants to be a low distortion type.  However, any wideband amplifier also needs to be carefully laid out per the manufacturer's data sheet. But like one of my bosses has told me many times, "Keep it Simple Steve" so I prefer no amplifier at all if the output power is adequate. The The OPA2674 or OPA2677 with better thermal package would work if more output power is required at low distortion.

Regards,
"Digital Steve", K1RF


​I see that the DDS-60 uses an AD8008. (I'd thought it was an LT1253 - maybe that's the DDS-30)

Anyway, they show fairly clean output. However, those are single frequency snapshots. And clean enough for TX specs might not be clean enough for test equipment​ if you want to push to the limit.

73-

Nick, WA5BDU

George N2APB
 

Hi guys,

FWIW, I tried many different output amplifier circuits in the early days of my DDS-xx plug-in modules (DDS-30, DDS Daughtercard) in order to provide clean-spectrum at the desired max output level (+16 dBm) … ranging from 2-stage discrete bipolar transistors, to 4 different MMICs, to the LT1253 op amp … and ultimately settling on the use of the AD8008, which is still with the DDS-60 module today.

I was convinced that the downfall of (my) attempts with the MMICS back then was the UHF parasitic oscillations (that are so hard to tame in homebrewer/experimenter designs) that were mixing down to create a lot of ‘spectral gudge’ in the HF region of interest. Further, those oscillations helped to make those MMIC devices run hot, and provided unwanted “energy” that would be detected in the wideband reflectometer circuits in which the DDS card was being used as a signal source, thus providing misleading measurements of Return Loss, Z, etc.

So, with the assistance of Jim Kortge back then, we carefully crafted a low-noise op amp design that seemed to provide the best results overall for the design goals … and that has stayed to this day. (And I also source the AD9851 chips directly from Analog Devices to ensure that I’m not getting noisy/fall-out parts that the offshore suppliers might provide.)

73, George N2APB

=========================
From: PHSNA@... [mailto:PHSNA@...]
Sent: Sunday, July 10, 2016 5:35 AM
To: PHSNA@...
Subject: [PHSNA] Re: DDS spectral "purity" ?


Hi all,
Over the last week or so I have been doing quite a lot of measurements myself. They also confirm that the MMIC ruins what is otherwise quite a good spectral output, and I am thinking about various ways to fix it.
So I would have been very interested to see Jim's video. Unfortunately though clicking on the link does not run the file.Evidently my browser does not know what to do with that extension. (I haven't come across it before either.)
Related to this; I discovered a long time ago that the chinese modules (and possibly the AD985x itself) generates a whole forest of junk at about -60dBm (ish) while it changes frequency, and for about a second or so afterwards. In any sweep mode it does this all the time of course!
Tony (G3PTD)...

N5IB
 

Yep, the combination of alias products and harmonics makes for interesting results. And of course the MMIC is downstream from the anti-alias filter.

I have several follow-up experiments in mind - along the lines of Steve's suggestions.

1. repeat the scan with a Type-2 9850 DDS on an Experimenter Board that has only the tapped transformer and a 50 ohm load - no attenuators or anti alias filter yet.

2. add the attenuators and anti-alias filter to the Experimenter Board and repeat.

3. return to the Type-2 PHSNA and vary the input voltage down to the drop out of the 5 V regulator - this will vary the bias current to the ERA-3+.

The darn little 1 minute movies with my digi camera are over 100 MB, and still take 12 MB after washing through Windows Movie Maker as a "windows media video" (wmv) file.  I'm going to hunt for a better way. I'll try using my tablet's camera at lower resolution and mp4 format. At 320x240 it's about 6 MB per minute

Jim, N5IB

Jaques Anthony
 

Hi Tony,
     Thank you for the conversion. After finding a suitable driver I was able to watch the MP4 version.
    (All my video editing is still done with .avi files!)
Tony (G3PTD)

Jaques Anthony
 


Hi Nick,
     Interestingly (I think) although I see the message header for your current posts, I do not see any content! I can see it in the e-mail version that I get though.
     This business of noise when changing frequency cam to light when I used a MK2 type module in an 80mtr receiver (the MKARS80). During tuning only with the rx at full gain (no aerial) there was a considerable increase in background noise. Looking at the output on a spectrum analyser showed a whole load of rubbish on the DDS output a few dB above its normal noise level when tuning.  I went back to an analogue VFO!
     But I still have the extracted DDS module so yesterday I tried to have another look. Unfortunately I no longer have access to the spectrum analyser. Instead I now have a Cushman CE15 "Spectrum monitor" This is similar to an analyser, but with limitations, and I can not see the effect on it.
     So was the effect real? I think so but I can not now prove it.
     But yes, it would not be present in single frequency mode.
                                                          Tony (G3PTD)

Jaques Anthony
 


Hi all,
    In a previous posting I referred to measurements that I have been making. Jim's very interesting video demonstration prompts me to put some of the results here.
     I am using a Cushman CE15 "spectrum monitor" for measurements. This is not as flexible as the true spectrum analysers that I no longer have access to, but does allow useful, if not definitive measurements to be made:

   Measured at f = 7.040 MHz
   Levels dBm
   on Boonton 92B millivoltmeter (50 Omhs)

Measurement at | f      2f      3f     4f      5f

    P101             -8     -50      *       -       -

    P102           -12     -53      *       -       -

    Output         +3     -25     -40    -56     *

   * = detectable in the noise
   - = not detectable
    This looks ok on my screen. I do hope that the system does not re-format it!
    The measurements at P101 and 102 were done with each link removed.

    Doing a frequency run from 0.5 to 30MHz does show the entertaining moving cornfield, but at a much lower level than in Jim's display, with much of it just above the noise. But with the Cushman (or my!) limitations not too much reliance should be placed on that.