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Old 22nd March 2012, 03:24 PM   #141
coluke is offline coluke  Italy
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Join Date: Mar 2009
Hi PChi,

I took too quick a look
Anyway, the best way to achieve both high acquisition speed and low droop is employing a two stage peak detector, i.e. cascading a track&hold and a sample&hold, or two sample&hold - this way you may have the first stage optmized for speed and the second for accuracy. Take a look at the December 1985 issue of the HP Journal, in which you can find a description of the audio modulation section of the HP8642 RF generator (which is a true fast-settling, low THD BF generator). The same topology has been employed by HP itself in the 8903, and by AP and Boonton - I've tried it, and it actually works quite well. At first I tried the all-monolithic way via a pair of LF398, but did end up with a cascade of two open-loop discrete S&H (each consisting of a pair of buffers and a jfet) - I've found out that open-loop S&H are less DC-accurate than closed loop ones, but they are faster and more ripple-free. The zero-crossing detector is a LM311, and the S&H are driven by a bunch of hex inverters plus pair of bjt each; the first S&H actually tracks the signal from the negative peak to the positive one, and the second samples peak value for a fixed - small - amount of time. I'm not completely satisfied with this design (have to try a real peak-detector with reset in place of the first T&H - I hope this will further lower ripple), nevertheless it rocks.

L.
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Old 22nd March 2012, 07:24 PM   #142
davada is offline davada  Canada
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Hi Coluke,

How much ripple do you observe at the output of a THSH and for variations of this type of system?

David.
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David.
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Old 22nd March 2012, 09:25 PM   #143
PChi is offline PChi  United Kingdom
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Location: Staines, UK
Hi L.

Thanks for informing me about the December 1985 issue of the HP Journal. I wasn't aware of it.

I have an oscillator based on the state variable / quadrature architecture of the type that is described in the HP Journal I used analogue multipliers to square the Integrator 'Sine' and 'Cosine' outputs and then combined them to produce a relatively low ripple control voltage. It's fast settling but not low distortion at about 0.2 % at 1 kHz. The state variable oscillator works well but I don't know how much the gain varies with frequency setting with the matching of a typical dual ganged potentiometer. That's why I chose the All Pass Filter circuit. Possibly not the best choice though.

I have been looking at reed relays, digital potentiometers and Multiplying DACs because the PCB mounted switches and potentiometers that I used are so expensive.

It's a pity that there are so few sample and hold options apart from the LF398.

Regards Paul
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Old 22nd March 2012, 10:06 PM   #144
coluke is offline coluke  Italy
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Hi David,

I've attached difference amplifier output as it appears both on the scope and in the spectrum analyzer; the 3rd graph shows ALC multiplier output, while the last one is the generator output itself scaled down to 1Vrms or so. I think T&H/S&H output ripple can be further reduced by replacing the first T&H with a true peak detector with reset, and maybe optimizing somewhat the layout (although I'm already employing local regulators and careful decoupling for both the ring itself and the peak detector), but what does actually matter is the analog multipler output signal, and it is quite clean, as you can see. This generator is based on a ring of 318s and a single AD633 in the ALC loop working at a decoupling factor of about 21, or 26.5dB; its unloaded output level is about 9.1Vpp - the operating Q @ 1kHz is slightly more than 60dB, and it runs smoothly from 10Hz to 150kHz (although its Q lowers quite a bit at high frequencies). It's a little bit noisier than the version with 2 AD633 and a 4-phase rectifier, but 318s are noisier than 5534s; anyway it seems to have quite a bit lower THD - something in the -130dBc range @1kHz, but I still have to check thoroughly these preliminary results - I know they are a little bit impressive...

Ciao,

L.
Attached Images
File Type: png TH-SH Ripple Scope.png (8.6 KB, 479 views)
File Type: png TH-SH Ripple.png (35.8 KB, 475 views)
File Type: png ALC-multiplier output.png (35.2 KB, 438 views)
File Type: png Generator output.png (33.5 KB, 432 views)

Last edited by coluke; 22nd March 2012 at 10:09 PM.
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Old 22nd March 2012, 10:53 PM   #145
benb is offline benb  United States
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Quote:
Originally Posted by PChi View Post
I have been looking at reed relays, digital potentiometers and Multiplying DACs because the PCB mounted switches and potentiometers that I used are so expensive.

It's a pity that there are so few sample and hold options apart from the LF398.

Regards Paul
I was looking at that 12-position 2-pole switch, and was wondering about adding a third pole to switch in different values for the sample-and-hold capacitor along with the oscillator capacitors. With relays and appropriate steering logic (diodes), you'd only need three or four extra relays to switch in several s/h caps so each works over a 10 to 1 range, as opposed to the current 1000+ to 1 range of a single capacitor. Reading about the sample time, you could increase and decrease it as well along with switching the capacitor.
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Old 22nd March 2012, 10:54 PM   #146
coluke is offline coluke  Italy
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Ho PChi,

Quote:
Originally Posted by PChi View Post
Hi L.

Thanks for informing me about the December 1985 issue of the HP Journal. I wasn't aware of it.
You may also want to have a look at the August 1980 issue - it's entirely devoted to the venerable 8903 THD analyzer, and it's well worth a careful read.

Quote:
I have an oscillator based on the state variable / quadrature architecture of the type that is described in the HP Journal I used analogue multipliers to square the Integrator 'Sine' and 'Cosine' outputs and then combined them to produce a relatively low ripple control voltage. It's fast settling but not low distortion at about 0.2 % at 1 kHz. The state variable oscillator works well but I don't know how much the gain varies with frequency setting with the matching of a typical dual ganged potentiometer. That's why I chose the All Pass Filter circuit. Possibly not the best choice though.
I did try the same sin^2+cos^2 peak detector approach, but it turned out to have too high a ripple to keep THD low without heavy filtering and very slow response. Besides this, my only APF test generator was a slightly modified version of the one cited in the first post, but after having had a look at your work I've decided to try a version with a much better ALC loop to see what comes out - I'll let you know.

Quote:
I have been looking at reed relays, digital potentiometers and Multiplying DACs because the PCB mounted switches and potentiometers that I used are so expensive.
So it seems: where did you get them?

Quote:
It's a pity that there are so few sample and hold options apart from the LF398.
As far as I know they are the lowest cost monolithic S&H in the market; and they do work pretty well indeed, but they do not seem to suit the needs of this specific application. Boonton generators employed a HAI-5320 by Harris (a fast precision integrating S&H) and a SMP-11 by AD (a diode-bridge S&H with very low droop rate), but I've not managed to find both the first and the last - don't even know if they are still in prduction (although HAI-5239 DS is dated March 2012...).

Ciao,

Luca

Last edited by coluke; 22nd March 2012 at 11:06 PM.
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Old 22nd March 2012, 11:56 PM   #147
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Luca, you seem to have reached a nice -102dB THD+N now.
What ADC do you use to read that?
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Old 23rd March 2012, 01:22 PM   #148
coluke is offline coluke  Italy
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It's an EMU0204 - a low cost USB soundcard with outstanding performances: at 120$ or so it's hardly beatable.

L.
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Old 23rd March 2012, 05:54 PM   #149
PChi is offline PChi  United Kingdom
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Hi L,

Thanks for the suggestion of looking at the August 1980 issue of the HP Journal for the description of the 8903 analyzer.
Thanks also for the Oscilloscope waveform and spectrum analysis plots. The THD that you are achieving is impressive.

I have now set the circuit up again so can take some more detailed measurements of the control voltage ripple.
Measuring at the Loop error header, component reference CON6 pin 1.
The Loop gain was adjusted using VR2 to give 0 V error at 1 kHz.
At the Oscillator Frequency = 100 Hz, control voltage ripple 1 mV peak to peak.
1 kHz, 1 mV peak to peak ( -60 dBV). I believe that this is feedthrough via the switch (IC7) off capacitance.
10 kHz, 3 mV peak to peak.
25 kHz, 5 mV peak to peak.
100 kHz 30 mV peak to peak dropping to about 7 mV peak to peak when the PCB is lifted from the desk. This is partly due to the length of the sample period. I think that the 2 stage approach of a peak detector followed by a sample and hold is best. I didn't think of it.

The frequency control dual ganged potentiometer is a Vishay Cermet 149 series. Part Number 14920F0GJSX13502KA, price from Farnell = 10.66 plus VAT.
I originally ordered a Vishay 148 series conductive plastic, greater drift with temperature but supposedly better resistance linearity (not matching of the gangs) for low distortion.
The frequency selection switch is an Elma 08-2104 form Farnell at 54.47 plus VAT.

Hi Benb I have used a PCB mounted switch which limits the number of poles to 2 (more are listed by Elma but are probably unavailable in small quantities). I agree another pole would be useful for the error amplifier control. The cost per switch contact is greater than the cost of a reed relay.

Top trace: Oscillator output at 5 V / div
Bottom trace: Sample and Hold capacitor C52 (increased to 10 nF) 5 mV / div
Attached Images
File Type: jpg S_and_H_at_1kHz.jpg (25.7 KB, 402 views)
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Old 24th March 2012, 06:05 PM   #150
coluke is offline coluke  Italy
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Quote:
Originally Posted by PChi View Post
Bottom trace: Sample and Hold capacitor C52 (increased to 10 nF) 5 mV / div
Hi PChi, your S&H output seems very clean indeed (although a spectral analysis could reveal quite a bit of HF content), but it's a little bit higher than optimal, too - it's likely to significantly raise the 2nd harmonic level in output signal. What's the signal level at the multplier output?

I'm trying to measure my T&H/S&H SVF generator, but - as expected - it seems to be a truly challenging task. More on this later.

Ciao,

L.

Last edited by coluke; 24th March 2012 at 06:08 PM.
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