Low-distortion Audio-range Oscillator

[dirkwright]

For a readout on an audio oscillator, would you prefer dB or volts? I guess the standard is to have a 600 ohm output and indication in dB, but I should read more about this also.

[PChi]

Hello,
I agree with Samuel Groner,
If you want fast settling it's hard to beat the Sin^2 + Cos^2 approach. It's particularly impressive to see the operation at 10 Hz on an Oscilloscope but distortion is high.

If you want low distortion then the sample and hold or variant of works well.
There are low charge injection analog switches avaliable which don't cause much pedestal error, glitches or ringing.

I have put the circuit board shown in post #120 in a box and bought a ShibaSoku AD725C Automatic Distortion Analyzer off Ebay.
The Distortion measured was:-
At 10 Hz and 1 kHz = 0.0004 % measured with a 30 kHz low pass filter and RMS response. The residual is mostly noise with a hint of some harmonics.
At 10 kHz = 0.0006 % measured with a 100 kHz low pass filter and RMS response. The residual is mainly third harmonic.
At 20 kHz = 0.0007 % measured with a 100 kHz low pass filter and RMS response. The residual is third harmonic with others creeping in.

The ShibaSoku is a mass of close tolerance components and reed relays. The notch filter is based on 3 cascaded Bridged-T filters. Thanks to Dick Moore's web site for an explanation of the Bridged-T notch filter.

[davada]

Originally Posted by PChi

Hello,
I agree with Samuel Groner,
If you want fast settling it's hard to beat the Sin^2 + Cos^2 approach. It's particularly impressive to see the operation at 10 Hz on an Oscilloscope but distortion is high.

If you want low distortion then the sample and hold or variant of works well.
There are low charge injection analog switches avaliable which don't cause much pedestal error, glitches or ringing.

I have put the circuit board shown in post #120 in a box and bought a ShibaSoku AD725C Automatic Distortion Analyzer off Ebay.
The Distortion measured was:-
At 10 Hz and 1 kHz = 0.0004 % measured with a 30 kHz low pass filter and RMS response. The residual is mostly noise with a hint of some harmonics.
At 10 kHz = 0.0006 % measured with a 100 kHz low pass filter and RMS response. The residual is mainly third harmonic.
At 20 kHz = 0.0007 % measured with a 100 kHz low pass filter and RMS response. The residual is third harmonic with others creeping in.

The ShibaSoku is a mass of close tolerance components and reed relays. The notch filter is based on 3 cascaded Bridged-T filters. Thanks to Dick Moore's web site for an explanation of the Bridged-T notch filter.

Can you name a few low charge injection switches? Then we don't have to look for them.

Would you be willing to share a schematic of your design? Or did I miss that in a prior post.

[1audio]

Congrats on finding a Shibasoku. Do you have a manual?

In reviewing low distortion oscillators there is usually tradeoffs for fast settling vs. low distortion. The most obvious is the ST1700 series that have a relay that switches between fast settling an low distortion by switching in and out a Jfet agc alongside a led-cdse cell AGC module. The fastest settling are the Krohn Hite state variable oscillators. They use the sample and hold triggered at the peak solution and can settle in very few cycles at 1 Hz or less. They do suffer from feedthrough of the sampling pulse in the output, which is the major limiting factor in the distortion. I have the circuit and description of the KH4025 if anyone wants to see it. Its really complex since its all discrete but it will also be an education in the technology.

Make sure you are solving a problem that needs solving. For a single frequency solution even 10 seconds settling after power on would not be a problem in most applications. Also is low distortion or voltage accuracy the primary goal? If absolute voltage accuracy is less important and stability over a longer period less important something as simple as an incandescent bulb driving a CDS cell may be ideal since its free of transients. It would limit the low frequency possible however.

Adding a DSP would bring a real challenge keeping the digital stuff out of the output. You would almost have to seal it all in a can with a separate power supply. The agc dynamics would be easier to tune with a DSP but the same effect can be had with pretty simple circuitry.

[PChi]

The Oscillator schematic is attached to post #136. I had trouble exporting the KiCAD picture hence the poor qualtity.
The low charge injection switch is an Analog Devices ADG1221.
The original schematic and PCB layout is in KiCAD. If that is of use I can send them.

I don't have a manual for the ShibaSoku. There is no Oscillator only bucket loads of filters, reed relays and passives. I have traced out some of the schematic. It is very 1980s. It appears to mostly work OK apart from the 20 kHz filter.

[1audio]

I think the analog multiplier may be a limitation on the distortion. Its the only element I have not been able to make improvements on in the Boonton circuits and seems to be a constraint. The harmonics are low but not that low. There are trims possible but it then gets complex with a lot of parts and needs something like the Shibasoku to adjust.

Your 20 KHz filter is not working on the Shibasoku? Yours is much newer than mine. The oscillators are separate boxes with at least 4 variations I have seen. They are all very high performance.

[davada]

Hi PChi,

"There are low charge injection analog switches avaliable which don't cause much pedestal error, glitches or ringing."

Can you put some numbers to this?

Thanks,

[dirkwright]

Originally Posted by coluke

Have to check, but if I remember well I have a slightly more readable version of that - I'll let you know.

L.

Did you find the schematic for the AGC for the HP239? I'm curious...

[EssB]

Originally Posted by dirkwright

Did you find the schematic for the AGC for the HP239? I'm curious...

The schematic in fig. 8.4 is a bit fuzzy but readable enough.

Regarding your Q about meter scales: if it's either / or then I prefer Voltage readout, but I'd rather have both.
If you have an output attenuator with 10dB steps then you need 2 voltage scales but only one dB scale. see 2/3rds down this page Audio Millivoltmeter

[dchisholm]

Originally Posted by dirkwright

For a readout on an audio oscillator, would you prefer dB or volts? I guess the standard is to have a 600 ohm output and indication in dB, but I should read more about this also.

If you're going to include an audio millivoltmeter that can actually measure the voltage across the output terminals then the actual voltage reading is probably best. Unless it's being used in a telco application, or some pro sound applications, where there is widespread compliance with recognized reference levels for dB measurements, displaying some kind of dB value seems to waste more effort due to confusions than it saves in mental arithmetic. If you can't display the actual terminal voltage then showing the voltage that should appear across an open-circuit load would be the most useful.

Dale