correct, but without the harmonics compensation 3rd one would be about -150-155db because LPF MFB 2ord with, a 4ord would wipe out the 3rd harmonic lower.
Has anyone explored using somthing like this: https://www.soundsemiconductor.com/downloads/ssi2161datasheet.pdf for AGC in a low distortion oscillator? I'm looking for a modern replacement for the analog multipliers used in some of my instruments.
That's one of those "impossible" chip makers - for about 10 years starting in the late 70s or so, there were analog synthesizer specific chips made by SSM and CEM, used in most all the famous analog polysynths of the time. Then the all-digital Yamaha DX-7 came out, which sucked the sales from analog synths.
But more recently, a miracle occurred:
https://electricdruid.net/analog-renaissance/
Be aware that page hasn't been updated in recent years, so there are likely more available from those manufacturers than listed, for example:
https://www.alfarzpp.lv/eng/sc/application.php
It appears the best performance (specifically distortion-wise, but likely overall as well) VCAs are from THAT corp, previously the semiconductor division of dbx.
http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
Most of these makers don't appear to be carried by the major distributors, but I at least I found THAT parts at Mouser.
But more recently, a miracle occurred:
https://electricdruid.net/analog-renaissance/
Be aware that page hasn't been updated in recent years, so there are likely more available from those manufacturers than listed, for example:
https://www.alfarzpp.lv/eng/sc/application.php
It appears the best performance (specifically distortion-wise, but likely overall as well) VCAs are from THAT corp, previously the semiconductor division of dbx.
http://www.thatcorp.com/THAT_IC_Selection_Guide.shtml#VCAs
Most of these makers don't appear to be carried by the major distributors, but I at least I found THAT parts at Mouser.
Mediatechnology posted an Oscillator with a THAT Corp. VCA (THAT2180) a few pages ago: https://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=7&t=1265
I considered doing a similar design with SMD parts, but at least the THAT2180 is not stocked by the THAT Corp. distributors that I have access to (Mouser and ProfusionPLC).
I considered doing a similar design with SMD parts, but at least the THAT2180 is not stocked by the THAT Corp. distributors that I have access to (Mouser and ProfusionPLC).
Thanks for the link. I may be able to use the THAT part. Swapping for a current mode analog multiplier is not obvious but should be possible.
yeah, I prefer to use ADAU1701 with a potentiometer to set the frequency if really need such gen 😉 Actually, $50 ES9038Pro will outperform AP as a low distortion generator even without LPF.
You know, that I have JKGEN 🙂 Which is able o generate various signals on it's I2S/SPDIF/Analogue outputs.
Maybe in the next revision it will make sense to replace the AK4490 with ES9038 to reduce the THD on the analogue output, but even a tunable LPF is not suitable for non-sinusoidal signals. The same problem with the ADC notch - only for THD with a sinusoidal signal. Not even IMD is possible.
By the way, when I played with notch+ mdac (not successful due to THD and SNR), I used a slightly different technique than AP:
AP uses the second channel for the direct signal, then digitally filters it and mixes it with the second (notched) signal in the microcontroller.
I used only one channel of a "good ADC" for notch+amplifier and a simple 12-bit microcontroller ADC for the direct signal - only to determine its amplitude and frequency. Then I generate a pure 32bit sine wave (DDS) with this amplitude and frequency and add it to the notch channel.
I can not say that it is better in terms of signal, but it requires significantly less resources of the Microcontroller.
This is for AGC. Its not the full signal. Its contribution is attenuated a lot. Similar for a Jfet in that application.
I have used Victor's oscillators to test higher speed SAR ADCs and appreciate that you don't have to worry about out of band noise or clock feedthrough. Maybe the LPF on an audio DAC is enough, but I would want to see a wideband FFT.
Every DAC I have checked has serious out of band grunge. I use a 4 pole passive filter to deal with it. Opamps just are not adequate filters for 25, 50, 100 MHz stuff. Victor's oscillators are good to -150 dB. I can't imagine anything that needs a lower distortion source and I would want to see several verification methods for validating those really low numbers. You are in the range where the passive parts can be the limitation.
yeah, I tried sallen key filter and MFB as well, both are differential and low impedance(biggest cap is 200nF C0G), they provide about -150db 3rd harmonic with ES9038Q2Mx2(the DAC's 3rd about -135db, LPF is second order with -15db@3kHz).