Tracking N-path filter...

[bitrex]

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.

Something like: ADC -> crystal master clock & numerically-controlled
counter = voltage-controlled clock oscillator -> Johnson shift register
-> analog -> tracking switched-cap filter -> anti-alias filter.

Not expecting amazing performance but it\'s an interesting exercise; the
fact that on some models of this PLC (e.g.
https://www.dialog-semiconductor.com/products/greenpak/dual-supply-greenpak/slg46621)

you get a second VDD2 pin that only supplies the VDD of a set of
configurable NMOS/CMOS/PMOS outputs might make it possible to integrate
certain switched-capacitor topologies into the device aside from
external caps, using VDD2 as a signal input instead of for bridging
logic supply voltage domains

 

[Martin Brown]

On 01/04/2022 18:52, bitrex wrote:

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

Before the question is answerable you will have to describe what you are
actually trying to do and at what sweep rates and frequencies.

My instinct is that in the digital domain you would be better off doing
the sweep by subsampling and/or interpolating into a largish DAC lookup
table by the half cycle. Depends how much jitter you can live with.

There are surprisingly good DDS chips these days for not that much.

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.

4046 is hard to beat if you just want a cheap VCO
https://www.ti.com/lit/an/scha002a/scha002a.pdf

VCO out is triangle wave but diode shapers are easy enough provided that
you don\'t want a purist hifi quality sine wave. Good to ~1MHz.

Something like: ADC -> crystal master clock & numerically-controlled
counter = voltage-controlled clock oscillator -> Johnson shift register
-> analog -> tracking switched-cap filter -> anti-alias filter.

Not expecting amazing performance but it\'s an interesting exercise; the
fact that on some models of this PLC (e.g.
https://www.dialog-semiconductor.com/products/greenpak/dual-supply-greenpak/slg46621)


you get a second VDD2 pin that only supplies the VDD of a set of
configurable NMOS/CMOS/PMOS outputs might make it possible to integrate
certain switched-capacitor topologies into the device aside from
external caps, using VDD2 as a signal input instead of for bridging
logic supply voltage domains

--
Regards,
Martin Brown

 

[whit3rd]

On Friday, April 1, 2022 at 11:44:34 AM UTC-7, Martin Brown wrote:

On 01/04/2022 18:52, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter...

4046 is hard to beat if you just want a cheap VCO
https://www.ti.com/lit/an/scha002a/scha002a.pdf

VCO out is triangle wave but diode shapers are easy enough provided that
you don\'t want a purist hifi quality sine wave. Good to ~1MHz.

The VCO in a \'4046 gives a square wave; you\'d have to integrate that to
get a triangle, and it\'d have frequency-dependent amplitude. For controlled
triangle VCO, it takes voltage-current conversion and current polarity
switching (LM13700 and a comparator, for instance) and a capacitor.

 

[whit3rd]

On Friday, April 1, 2022 at 10:52:38 AM UTC-7, bitrex wrote:

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.

All the digital tricks need a low-pass filter, and a tracking low-pass is a LOT of work.
I\'d consider, first, doing two 10 MHz-ish oscillators, mixing down, and THEN the
low-frequency just has to stop 10MHz-ish, and 20 MHz-and-up. Something like
a classical IF strip of LC\'s is a very nice, passive, filter for a known stopband
that\'s not too wide, and there\'s SAW filters for a high end.

 

[John Larkin]

On Fri, 1 Apr 2022 13:52:30 -0400, bitrex <user@example.net> wrote:

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.

Something like: ADC -> crystal master clock & numerically-controlled
counter = voltage-controlled clock oscillator -> Johnson shift register
-> analog -> tracking switched-cap filter -> anti-alias filter.

Not expecting amazing performance but it\'s an interesting exercise; the
fact that on some models of this PLC (e.g.
https://www.dialog-semiconductor.com/products/greenpak/dual-supply-greenpak/slg46621)

you get a second VDD2 pin that only supplies the VDD of a set of
configurable NMOS/CMOS/PMOS outputs might make it possible to integrate
certain switched-capacitor topologies into the device aside from
external caps, using VDD2 as a signal input instead of for bridging
logic supply voltage domains

That is a very strange chip.

A cheap uP with an onboard DAC can do pretty good software DDS sine
wave generation up to at least 10s of KHz.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[Anthony William Sloman]

On Saturday, April 2, 2022 at 9:54:56 AM UTC+11, whit3rd wrote:

On Friday, April 1, 2022 at 10:52:38 AM UTC-7, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.
All the digital tricks need a low-pass filter, and a tracking low-pass is a LOT of work.

A shift register with the right value resistor on each tap can make a tracking low pass filter. You sum the currents through each of the resistors into a virtual, and what comes out can be a pretty respectable sine wave. It\'s a finite impulse response digital filter with most of the computation handled by the analog output.

I did it once for a gadget that was designed to confuse echo-locating bats in a predictable way. The customer recently got a one page obituary in Acoustics Today, so it was a while ago.

<snipped the rest, helpful though it is>

--
Bill Sloman, Sydney

 

[bitrex]

On 4/1/2022 7:16 PM, John Larkin wrote:

On Fri, 1 Apr 2022 13:52:30 -0400, bitrex <user@example.net> wrote:

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.

Something like: ADC -> crystal master clock & numerically-controlled
counter = voltage-controlled clock oscillator -> Johnson shift register
-> analog -> tracking switched-cap filter -> anti-alias filter.

Not expecting amazing performance but it\'s an interesting exercise; the
fact that on some models of this PLC (e.g.
https://www.dialog-semiconductor.com/products/greenpak/dual-supply-greenpak/slg46621)

you get a second VDD2 pin that only supplies the VDD of a set of
configurable NMOS/CMOS/PMOS outputs might make it possible to integrate
certain switched-capacitor topologies into the device aside from
external caps, using VDD2 as a signal input instead of for bridging
logic supply voltage domains

That is a very strange chip.

Looks like it\'s two separate chips that then got interconnected in the
same package, somehow. There\'s a \"Matrix 0\" and \"Matrix 1\" in the
designer, each have their own parts, and can only talk to each other
over a fixed number of lines. There\'s a lot of bits & bobs to make stuff
out of in there, though.

A cheap uP with an onboard DAC can do pretty good software DDS sine
wave generation up to at least 10s of KHz.

Done that and it\'s fine but this is a push-the-limits experiment,
hacking an analog filter into a part that\'s not really designed to do it.

The MF10 active filter IC was cool. The lead time at Mouser is May of
2023, not cool.

 

[Jan Panteltje]

On a sunny day (Fri, 1 Apr 2022 15:54:17 -0700 (PDT)) it happened whit3rd
<whit3rd@gmail.com> wrote in
<b122f741-72fe-48a3-8cae-2db17ea21547n@googlegroups.com>:

On Friday, April 1, 2022 at 11:44:34 AM UTC-7, Martin Brown wrote:
On 01/04/2022 18:52, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter...


4046 is hard to beat if you just want a cheap VCO
https://www.ti.com/lit/an/scha002a/scha002a.pdf

VCO out is triangle wave but diode shapers are easy enough provided that
you don\'t want a purist hifi quality sine wave. Good to ~1MHz.

The VCO in a \'4046 gives a square wave; you\'d have to integrate that to
get a triangle, and it\'d have frequency-dependent amplitude. For controlled
triangle VCO, it takes voltage-current conversion and current polarity
switching (LM13700 and a comparator, for instance) and a capacitor.

No idea what he is actually doing, but indeed 4046
My old signal generator was a 4046 driving a binary counter that was connected to an EPROM
programmed with a 256 steps sine wave...
Nice variable frequency sine wave output in the audio range.

 

[piglet]

On 01/04/2022 18:52, bitrex wrote:

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

Yes, a switched cap low pass filter could be all you need. If LPF is
high (like 5th) order then don\'t even need to premake a stepped sine
input, a square wave is enough. AoE has this since at least Ed 2 (poss
even Ed 1).

piglet

 

[server]

On Sat, 2 Apr 2022 00:39:17 -0400, bitrex <user@example.net> wrote:

On 4/1/2022 7:16 PM, John Larkin wrote:
On Fri, 1 Apr 2022 13:52:30 -0400, bitrex <user@example.net> wrote:

If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

I\'m doing an experiment with a mixed-signal PLC to put a sine VCO all on
one (stupid-cheap) chip.

Something like: ADC -> crystal master clock & numerically-controlled
counter = voltage-controlled clock oscillator -> Johnson shift register
-> analog -> tracking switched-cap filter -> anti-alias filter.

Not expecting amazing performance but it\'s an interesting exercise; the
fact that on some models of this PLC (e.g.
https://www.dialog-semiconductor.com/products/greenpak/dual-supply-greenpak/slg46621)

you get a second VDD2 pin that only supplies the VDD of a set of
configurable NMOS/CMOS/PMOS outputs might make it possible to integrate
certain switched-capacitor topologies into the device aside from
external caps, using VDD2 as a signal input instead of for bridging
logic supply voltage domains

That is a very strange chip.

Looks like it\'s two separate chips that then got interconnected in the
same package, somehow. There\'s a \"Matrix 0\" and \"Matrix 1\" in the
designer, each have their own parts, and can only talk to each other
over a fixed number of lines. There\'s a lot of bits & bobs to make stuff
out of in there, though.

A cheap uP with an onboard DAC can do pretty good software DDS sine
wave generation up to at least 10s of KHz.

Done that and it\'s fine but this is a push-the-limits experiment,
hacking an analog filter into a part that\'s not really designed to do it.

The MF10 active filter IC was cool. The lead time at Mouser is May of
2023, not cool.

MF10s were awful. They were super noisy and aliased like crazy.

N-paths should alias too.



--

I yam what I yam - Popeye

 

[whit3rd]

On Friday, April 1, 2022 at 7:04:19 PM UTC-7, bill....@ieee.org wrote:

On Saturday, April 2, 2022 at 9:54:56 AM UTC+11, whit3rd wrote:
On Friday, April 1, 2022 at 10:52:38 AM UTC-7, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

All the digital tricks need a low-pass filter, and a tracking low-pass is a LOT of work.

A shift register with the right value resistor on each tap can make a tracking low pass filter. You sum the currents through each of the resistors into a virtual, and what comes out can be a pretty respectable sine wave. It\'s a finite impulse response digital filter with most of the computation handled by the analog output.

Yes, that has good possibilities; if you use a table for the sine, there\'s going to be steps (jaggies) at the
table granularity, which means a 1000-point table of sines (one quadrant of a wave) gives at
least .001 of the peak amplitude as \'steps\'. DAC step size is also a granularity, assumed small.
A FIR filter with analog components only needs a
fast summing junction and a bevy of hold capacitors, and a tracking clock (easier to make
a clock track than a passive filter element) and thus smooth the steps. Expect
a bit of switching noise, still, but a CCD array of capacitors can make an on-the-chip
integrated implementation.

Is there an off-the-shelf implementation available? If nothing else, a modulated-capacitance
CCD array can be imagined, that would cheaply reproduce such a filter characteristic.

 

[Joe Gwinn]

On Sat, 2 Apr 2022 13:19:02 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Friday, April 1, 2022 at 7:04:19 PM UTC-7, bill....@ieee.org wrote:
On Saturday, April 2, 2022 at 9:54:56 AM UTC+11, whit3rd wrote:
On Friday, April 1, 2022 at 10:52:38 AM UTC-7, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

All the digital tricks need a low-pass filter, and a tracking low-pass is a LOT of work.

A shift register with the right value resistor on each tap can make a tracking low pass filter. You sum the currents through each of the resistors into a virtual, and what comes out can be a pretty respectable sine wave. It\'s a finite impulse response digital filter with most of the computation handled by the analog output.

Yes, that has good possibilities; if you use a table for the sine, there\'s going to be steps (jaggies) at the
table granularity, which means a 1000-point table of sines (one quadrant of a wave) gives at
least .001 of the peak amplitude as \'steps\'. DAC step size is also a granularity, assumed small.

What is commonly done is linear or perhaps quadratic interpolation
between table values.

Joe Gwinn

 

[bitrex]

On 4/2/2022 4:19 PM, whit3rd wrote:

On Friday, April 1, 2022 at 7:04:19 PM UTC-7, bill....@ieee.org wrote:
On Saturday, April 2, 2022 at 9:54:56 AM UTC+11, whit3rd wrote:
On Friday, April 1, 2022 at 10:52:38 AM UTC-7, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?

All the digital tricks need a low-pass filter, and a tracking low-pass is a LOT of work.

A shift register with the right value resistor on each tap can make a tracking low pass filter. You sum the currents through each of the resistors into a virtual, and what comes out can be a pretty respectable sine wave. It\'s a finite impulse response digital filter with most of the computation handled by the analog output.

Yes, that has good possibilities; if you use a table for the sine, there\'s going to be steps (jaggies) at the
table granularity, which means a 1000-point table of sines (one quadrant of a wave) gives at
least .001 of the peak amplitude as \'steps\'. DAC step size is also a granularity, assumed small.
A FIR filter with analog components only needs a
fast summing junction and a bevy of hold capacitors, and a tracking clock (easier to make
a clock track than a passive filter element) and thus smooth the steps. Expect
a bit of switching noise, still, but a CCD array of capacitors can make an on-the-chip
integrated implementation.

Is there an off-the-shelf implementation available? If nothing else, a modulated-capacitance
CCD array can be imagined, that would cheaply reproduce such a filter characteristic.

Y\'all got a schematic of what this \"FIR with analog components\" looks like?

 

[whit3rd]

On Saturday, April 2, 2022 at 2:16:28 PM UTC-7, bitrex wrote:

On 4/2/2022 4:19 PM, whit3rd wrote:
On Friday, April 1, 2022 at 7:04:19 PM UTC-7, bill....@ieee.org wrote:

A shift register with the right value resistor on each tap can make a tracking low pass filter. You sum the currents through each of the resistors into a virtual, and what comes out can be a pretty respectable sine wave. It\'s a finite impulse response digital filter with most of the computation handled by the analog output.

Yes, that has good possibilities; ...

A FIR filter with analog components only needs a
fast summing junction and a bevy of hold capacitors, and a tracking clock...
a CCD array of capacitors can make an on-the-chip
integrated implementation.

Is there an off-the-shelf implementation available? If nothing else, a modulated-capacitance
CCD array can be imagined, that would cheaply reproduce such a filter characteristic.

Y\'all got a schematic of what this \"FIR with analog components\" looks like?

I was imagining a CCD that instead of being a simple chain was a branched chain, portioning off
fractions of the DAC voltage/charge into a summing junction. That allows the FIR filter coefficients to be
printed on-chip. The single DAC output would be delayed differently for each tap, and it tracks
with the CCD clock (a multiple of the DAC clock) so it can effect its filtering up to that higher
clock limit.

 

[Anthony William Sloman]

On Sunday, April 3, 2022 at 6:19:06 AM UTC+10, whit3rd wrote:

On Friday, April 1, 2022 at 7:04:19 PM UTC-7, bill....@ieee.org wrote:
On Saturday, April 2, 2022 at 9:54:56 AM UTC+11, whit3rd wrote:
On Friday, April 1, 2022 at 10:52:38 AM UTC-7, bitrex wrote:
If I have an adjustable-frequency stepped sine wave can I use a tracking
switched-cap filter to band-pass the low-frequency steps, and push the
steppies up to a higher frequency that can be filtered with a a
fixed-frequency low pass?
All the digital tricks need a low-pass filter, and a tracking low-pass is a LOT of work.

A shift register with the right value resistor on each tap can make a tracking low pass filter. You sum the currents through each of the resistors into a virtual, and what comes out can be a pretty respectable sine wave. It\'s a finite impulse response digital filter with most of the computation handled by the analog output.
Yes, that has good possibilities; if you use a table for the sine, there\'s going to be steps (jaggies) at the
table granularity, which means a 1000-point table of sines (one quadrant of a wave) gives at
least .001 of the peak amplitude as \'steps\'. DAC step size is also a granularity, assumed small.
A FIR filter with analog components only needs a fast summing junction and a bevy of hold capacitors, and a tracking clock (easier to make a clock track than a passive filter element) and thus smooth the steps. Expect a bit of switching noise, still, but a CCD array of capacitors can make an on-the-chip integrated implementation.

The device I put together didn\'t have any hold capacitors. It would have produced a stepped waveform if it had been perfect, but the steps were at 32-times the clock frequency, and small, so it didn\'t take much low-pass filtering to make them inperceptible.

> Is there an off-the-shelf implementation available?

Never heard of one.

> If nothing else, a modulated-capacitance CCD array can be imagined, that would cheaply reproduce such a filter characteristic.

After crunching through the sinc values to get the 32 resistances, and then having to do it again to throw in Hamming windowing to get rid of the Gibbs oscillations on the output, I\'d be a bit surprised if any mass market application could throw up enough customers to pay for a mask set.

--
Bill Sloman, Sydney