Low-distortion Audio-range Oscillator

Administrator
Joined 2004
Paid Member
I'm puzzled as well. I found CMF60 the best working with power amplifiers, the CMF55 should perform great in your application. They have a low voltage coefficient and I think that is why I get lower distortion numbers using them in those locations. I wouldn't think tighter tolerance would help, but lower VC definitely will depending on what else you are comparing with.

Mind you, if you're using surface mount you have a higher voltage gradient from end to end. That may matter and change the picture.
 
  • Like
Reactions: 1 user
I now have some interesting comparative data.

I built a bridge from 4X 10KΩ resistors and drove the upper and lower arms of the bridge with ULDO at +26 dBu.
The center of the bridge was connected to the balanced input of the notch filter.
The post-filter gain is 40 dB with a CM rejection stage following the post-filter amp.
The CM voltage was +20 dBu; 7.75V RMS.
The test Yageo was value-matched within about 4 Ohms to the near-perfect value of the CMF55.

The lower two arms were 1984-era Dale RN55s that I had previously matched to 0.01% with a 4 wire Kelvin bridge.

In the following FFTS one or both of the upper bridge arms contained a test resistor.
Remember that the drive level is +26 dBu, attenuated by 6 dB in each arm, and that there is 40 dB gain.
A -60 dB indicated 1 kHz FFT level is actually about -120 dBu.
A 1Mpt flat-top FFT was averaged 10 times.

The first FFT is with 2X Yageo MFR-25 in the upper arms and 2X Dale RN55 in the lower:

Resistor_Bridge_Test_2X_10K_Yageo_MFR-25_Upper_Arms.jpg


As you can see in the above it appears "distortionless." If the distortion signatures of the upper arms' resistors match each other - and the lower two arms' signatures match there is cancellation. This doesn't mean any of the resistors are "perfect" it just means that the upper pair distort similarly to each other and the lower pair distort similarly to each other.

This FFT is with 4X 1984-era RN55s:

Resistor_Bridge_Test_4X_RN55_All_Arms.jpg


Again there is cancellation.

The following FFT is with 3X Dale RN-55 and a single Yageo MFR-25. One of the RN-55s is in the upper arm and we can now see a relative difference in the signature.

Resistor_Bridge_Test_3X_RN55_Plus_10K_Yageo_MFR-25_Upper_Arrm.jpg


OK, so the Yageo distorts differently than the Dale RN-55s.

The final FFT may be the smoking gun. It has 3X Dale RN-55 and a single CMF55:

Resistor_Bridge_Test_3X_RN55_Plus_10K_CMF55_Upper_Arm.jpg


Relatively speaking the CMF55 has about 5 dB more HD3 and introduces HD2 compared to the Yageo. The insertion loss of the notch at HD2 is about 9 dB so the HD2 introduced is far worse than it appears.

This is a comparative test but may well explain the increase in HD3 when I use the CMF55 in the oscillator.

UPS arrives in a few minutes and I'll test the Holsworthy and Vishay MBB0207.
 
Last edited:
God bless UPS and DHL.
Yes. UPS came bearing resistors from Holsworthy and Vishay.

The FFT results for 3X RN55 and 1X Holsworthy Y1RB:

Resistor_Bridge_Test_3X_RN55_1X_YR1B_Holsworthy.jpg


A near-perfect match to the Dale RN55s and perhaps all are near-perfect or again maybe are equally bad LOL.
Note that the Holsworthy isn't introducing the HD3 compared to the RN55s like the Yageo MFR-25 does.

And the runner-up is the Vishay MBB0207:

Resistor_Bridge_Test_3X_RN55_1X_Vishay_MBB0207.jpg


Again we have a near-perfect match to the Dale RN55s.

It's starting to look like the CMF55 is the culprit. It's added HD2 and additional HD3 compared to the Yageo is elevating HD3 in the oscillator.

To bust -140 dBc I think I need to install either the Holsworthy Y1RB or the Vishay MBB0207.

I'm running 50 FFT averages now to see if I can dig anything out of the mud.
 
  • Like
Reactions: 1 user
So did you buy the Precision, Professional or VG06?
I bought MBB0207VD1002BC100
Mouser PN 594-MBB0207VD1002BC1
https://www.vishay.com/docs/28766/mbxsma.pdf

The datasheet does not show what the "V" means nor does it show the 0.1% "D" tolerance to be available despite me holding 0.1% tolerance parts in my hands. So honestly I'm clueless though the data sheet shows the MBB0207 to be the Professional version.

A 50 sample average gave me about another 5 dB of measurement range and no additional spurs poke out with the MBB.

One thing I do know is that I want more.
 
  • Like
Reactions: 1 user
Member
Joined 2004
Paid Member
Hi Demian,
The test frequency must be high in that case. Clue?

Anyway, looks like a very cool instrument. I wish I knew about these decades ago!
The test frequency is 10 KHz. It has a 5W amp and a passive low pass filter using ferrites the size of bowling balls. I have managed to find 2 of them. Necessary for testing pots. My primary unit seems to have an HD3 residual of -170 dB.
 
Administrator
Joined 2004
Paid Member
Hi Demian,
Well, I am very impressed. Not only that, you're a lucky dog!

-170 dB, really? Not that I doubt you, but at those levels noise is a major factor. I guess they bandpass the signal ... or?

A 10K test frequency makes sense. Proximity at 1 KHz wouldn't do nearly as much. When I test at frequencies of 10 KHz or higher, lead positioning makes a large difference, so a measurement jig or fixture is required to keep errors low and repeatability high.
 
  • Like
Reactions: 1 user
Member
Joined 2009
Paid Member
Hi mediatechnology,
You bought the professional series it looks like.
No, Mouser states that it is from the precision series, which also is needed if you want the 0.1% tolerance. But the datasheet Mouser has on that page is for the professional series!

The datasheet for the precision series can be found here: https://www.vishay.com/doc?28767

It also contains an explanation for the "V" in the part number.
 
  • Like
Reactions: 1 user
This keeps getting weirder.

I also tried 2X of the "RN55" 1X Y1RB and an unmarked "SSL" 0.5% 10KΩ that I had in my Solid Stage Logic console service stock. The 0.5% appeared to be Mil, not Dale and probably of British origin.
The combination of three different manufacturer's in this experiment also did not produce HD2 or HD3 spurs.

So the dirty outlier - excluding the Yageo MFR - are the modern 2023 date code CMF-55s when compared to four other different lines from three different manufacturers of "precision" product. All the others in various combinations don't distort measurably.

Here's the weird part. I bought the clean RN55s as mil resistors in the 1980s. The date code is 1984. As I was putting the box away I took a good long look at the label. It's a CMF-55.

Dale_RN55_CMF-55_1984.jpg


Oh and just to rule out the modern CMF-55 being a bad one I'm not using the ones I removed from the oscillator. The 10K was from another unit in the same lot.
 
  • Like
Reactions: 1 user
I just checked a 15K 0.1% modern CMF-55 in the upper arm compared to a 15K MBB and also measured a 15K MBB to a 15K Y1RB pair as a control.

The MBB/Y1RB combo has a SFDR of about -150 dBc mostly HD2.

The MBB/CMF-55 combo has an SFDR of about -135 dBc strong HD3 with HD2 about 10 dB less.

So its not just the 10K value. The 15K CMF-55 was in the Wein bridge so no wonder it measured so bad.
 
  • Like
Reactions: 1 user
The test frequency is 10 KHz. It has a 5W amp and a passive low pass filter using ferrites the size of bowling balls. I have managed to find 2 of them. Necessary for testing pots. My primary unit seems to have an HD3 residual of -170 dB.
Very impressive and unusual: inductive components are incredibly practical and useful, but as soon as a magnetic material is involved they generate some level of distortion.
I try to keep my own builds quite compact, but if you impose yourself no limit, I imagine you can have magnetising inductances close to infinity and near-zero flux excursions, all of which can certainly help.
However, I don't think that brute-force alone can bring such a level of performance; I'd be curious to see the innards of such a beast
 
Member
Joined 2004
Paid Member
A quick check of some 10K resistors on hand- I had to go up to 30V to get out of the noise floor. The HD3 residual on 2 Dale 1/4W and a Vishay S102 were the same at around -168. I did not want to go to 100V before doing some calculations to make sure I don't fry the resistors. Basically nothing to see here.
WRT CLT-1 some pictures and the schematic for the low pass filter.
 

Attachments

  • 20180804_094123.jpg
    20180804_094123.jpg
    209 KB · Views: 102
  • 20180804_094328.jpg
    20180804_094328.jpg
    211.3 KB · Views: 91
  • 20180804_094536_Burst01.jpg
    20180804_094536_Burst01.jpg
    255.9 KB · Views: 94
  • LP filter 2.PNG
    LP filter 2.PNG
    360.4 KB · Views: 101
  • LP filter.png
    LP filter.png
    38.2 KB · Views: 104
  • Like
Reactions: 1 users