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John Linsley Hood's low distortion oscillator
John Linsley Hood's low distortion oscillator
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Old 20th May 2019, 05:18 PM   #1
MKerk is offline MKerk  Greece
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Default John Linsley Hood's low distortion oscillator

Some time ago I decided to build a low distortion oscillator for amplifier testing.After some research on the internet, I decided to try John Linsley Hood's design (Wireless World ,May 1981) due to its simplicity and good performance.The original article can be found here:http://www.keith-snook.info/wireless...distortion.pdf

Construction:
This design calls for the use of an optocoupler as the main AGC element.
In my build I tried to make my own optocouplers as described by Rod Elliott (Project 145), using some photoresistors and LEDs from my junkbox with really good results. So, I enclosed the two components of the diy opto facing each other inside a piece of heatshrink tubing and encapsulated the whole thing in epoxy.I finally constructed two boards , one for me and one for a friend.
As far as the Wien bridge network components are concerned, I matched them using my multimeter.I wanted the frequency to be about 1KHz calling for 10 nF(nominal) capacitors and 16.7KΩ (12K in series with 4K7 ) resistors.
I provide an image with the final values used on each board. The Wien bridge opamp on bard 1 is a TL072 while on board 2 is an LF353P. Also instead of the 741 op amps I used two LM358 ones on each board.Finally I enclosed the oscillator side of the board in copper for shielding.

Adjustment:
The adjustment of the oscillator is pretty straightforward.Adjust the trimpot until a stable output waveform without clipping is achieved (in the two boards it measured around 0.8 to 1 Vrms regardless of supply voltage). If the trimpot is misadjusted , the output amplitude will either be zero or it will reach the opamp output voltage swing limits and clipping will start.
It should be noted that the output voltage can be fine tuned using this trim pot over a range of 80 mVrms. However by increasing Vout , jitter is reduced while THD is increased.
Attached Images
File Type: jpg ldo schem.jpg (169.7 KB, 353 views)
File Type: jpg component measurements.jpg (182.3 KB, 361 views)
File Type: jpg opto components 1.jpg (542.0 KB, 322 views)
File Type: jpg opto components 2.jpg (573.5 KB, 299 views)
File Type: jpg Capacitor measurements.jpg (491.3 KB, 298 views)
File Type: jpg Oscillator section.jpg (838.8 KB, 87 views)
File Type: jpg complete board.jpg (758.0 KB, 90 views)
File Type: jpg Board back side.jpg (790.2 KB, 66 views)
File Type: jpg Back side oscillator shield.jpg (638.8 KB, 58 views)
File Type: jpg opto closeup.jpg (781.5 KB, 63 views)

Last edited by MKerk; 20th May 2019 at 05:29 PM.
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Old 20th May 2019, 05:35 PM   #2
MKerk is offline MKerk  Greece
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Measurements:
Supply is +/- 5Vdc. My board (board 2) outputs 830mVrms/ 976.4Hz on a output load of 10KΩ.
The output DC offset is 9.7mVdc.
I provide photos of the output waveform from my Tek 475 and from a handheld JYE Tech 150 DSO.
I also made 2 recordings (96KHz 24bit) with my soundcard, one of the oscillator settling (with a 10.5dB attenuator on the soundcard input) and one of it’s stable output (no attenuation). From the settling recording you can see that settling time is about 5s.
Finally I provide the results of the FFT analysis (FFT size: 2097152samples (resolution:92mHz) FFT Window: Kaiser20) of the stable output on RMAA.
From that follows, 0.0015%THD, 0.012%THD+N, 78.4dB S/N.
Also Jitter is +/-5Hz with sidebands 55dB below fundamental
Attached Images
File Type: jpg JLH ldo stable out TEK475.jpg (329.3 KB, 69 views)
File Type: jpg JLH ldo stable out dso.jpg (561.4 KB, 63 views)
File Type: png JLH ldo stable out FFT(FFTsize 2097152 92mHz res Kaiser 20.png (22.0 KB, 86 views)
File Type: png JLH ldo stable out FFT fundamental.png (40.5 KB, 77 views)
File Type: png JLH ldo stable out FFT fundamental(2).png (48.5 KB, 53 views)
File Type: png JLH ldo settling.png (45.4 KB, 44 views)
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Old 20th May 2019, 06:21 PM   #3
Vovk Z is offline Vovk Z  Ukraine
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John Linsley Hood's low distortion oscillator
Isn't it a little bit strange to make low distortion oscillator with high distortion opamps? Or it was a goal?
There are many much better ones, then tl071 and lf353, for example, a popular and quite cheap LM4562 and all that briliant family (lm49860, lm49720 ets).
I am shure that this oscillator can give much better results, with good enough opamps and sound card. As I can see - those results are similar to the soundcard results.

Last edited by Vovk Z; 20th May 2019 at 06:50 PM.
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Old 20th May 2019, 06:23 PM   #4
gpapag is offline gpapag  Greece
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John Linsley Hood's low distortion oscillator
Quote:
Originally Posted by MKerk View Post
I finally constructed two boards , one for me and one for a friend.
That second oscillator board is here with me. Thank you Marios.
FFT length has to be 262144 samples and longer for sidebands to become visible
What you see is with an FFT size of 2097152 samples and Kaizer window (beta 20). 10sec recording of oscillator's output (sampling rate 96kHz, 32bit float)
Jitter (freq stability) is +/- 3Hz and 60dB below fundamental.
Exceptional results IMO.

E-Mu 0404 soundcard (1MOhm input)
Rec SW: WaveSpectra Ver.1.51E
FFT SW: RMAA Ver 6.45


It is very encouraging to see young boys departing from the mobile phone imprisonment trend, delving into old electronics magazines and burning their fingers with the soldering iron.



George
Attached Images
File Type: jpg DSCN4981.JPG (284.7 KB, 52 views)
File Type: png FFT.png (63.2 KB, 79 views)
File Type: jpg DSCN4983.JPG (569.9 KB, 76 views)
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Old 20th May 2019, 06:55 PM   #5
MKerk is offline MKerk  Greece
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Quote:
sn't it a little bit strange to make low distortion oscillator with high distortion opamps? Or it was a goal?
Vovk Z,you are right about the opamps.However the goal of this project was to try and achieve good results with what I had in hand at that time and what was used originally by JLH.Also, I have put the oscillator opamp in a socket ,so, at any time in the future,I can replace it with something better.

Marios
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Old 21st May 2019, 12:00 AM   #6
Aridace is offline Aridace  Panama
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John Linsley Hood's low distortion oscillator
Quote:
Originally Posted by MKerk View Post
Vovk Z,you are right about the opamps.However the goal of this project was to try and achieve good results with what I had in hand at that time and what was used originally by JLH.Also, I have put the oscillator opamp in a socket ,so, at any time in the future,I can replace it with something better.

Marios
That signal generator is quite good already and as according to the article, the current through the LDR is a major cause of distortion, not much can be gained by changing opamps. What might give an improvement no opamp could provide is a simple passive filter. Already at 2 KHz it would provide 40 dB attenuation of the 2nd harmonic (and noise) and more than 80 dB at the 3rd.
Attached Images
File Type: jpg LPF 1k att.jpg (125.9 KB, 85 views)
File Type: jpg LPF 1k.jpg (18.2 KB, 112 views)
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Old 21st May 2019, 06:15 PM   #7
MKerk is offline MKerk  Greece
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Quote:
That signal generator is quite good already and as according to the article, the current through the LDR is a major cause of distortion, not much can be gained by changing opamps.
Thank you Aridace for reminding me of that parameter (It has been some time since I read this article).
In the future, I may try such a filter at the oscillator output and post the results.

Marios
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Old 22nd May 2019, 03:17 AM   #8
Aridace is offline Aridace  Panama
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John Linsley Hood's low distortion oscillator
Quote:
Originally Posted by MKerk View Post
Thank you Aridace for reminding me of that parameter (It has been some time since I read this article).
In the future, I may try such a filter at the oscillator output and post the results.

Marios
My pleasure Marios. I must have overlooked the article at work so this time I read carefully. I'll start working with such low pass filters as well because the project for which the 100 mH coils were meant, no longer exists.

Jan
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Old 22nd May 2019, 08:59 AM   #9
Vovk Z is offline Vovk Z  Ukraine
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John Linsley Hood's low distortion oscillator
Quote:
Originally Posted by MKerk View Post
Vovk Z,you are right about the opamps.However the goal of this project was to try and achieve good results with what I had in hand at that time and what was used originally by JLH.Also, I have put the oscillator opamp in a socket ,so, at any time in the future,I can replace it with something better.
I understand. I am impressed of such a good enough results with such a bad opamps. Will take it (this schematics) into consideration.

Last edited by Vovk Z; 22nd May 2019 at 09:02 AM.
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Old 22nd May 2019, 12:39 PM   #10
gpapag is offline gpapag  Greece
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John Linsley Hood's low distortion oscillator
Quote:
Originally Posted by Aridace View Post
What might give an improvement no opamp could provide is a simple passive filter.
Thank you Jan for the passive filter schematic.
I would like to ask you to provide some info on the coils.
Cored or air core? Dimensions? Are they off the self, or self made?

George
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