DDS Selection

te:

legroups.com...

quare wave

ics where the

e wave is the

ransform at the

the results.

attenuation at

at the carrier

any op amps.

get rid of the

p. Too much

etworks to

ir phase

ay. =A0Long

ot a factor

esonance of

_ win.

pen all

the

en the

ncy

e

re

I meant before the demodulator, where the temptation is use a notch that only lets through the modulation frequency.

You pointed out that you can safely notch out other frequencies - which is true - but when I agreed you then broadened the discussion to include the post-demodulator low-pass filter, where a notch could be useful to take out the ripple at twice the modulation frequency, to which I responded by pointing that it;s rarely a sensible choice.

Now that we've anatomised the problem in some detail, you now want to ignore all that detail.

What I actually said was

"Notches aren't a good idea inside a lock-in amplifier set-up. Too much phase shift.

You start needing to add all-pass networks to your filter networks to keep the phase-shift low and stable."

and one could ague that "inside a lock-in amplifier set-up" implies before the demodulator, though it certainly isn't explicit.

Not necessarily - whence my mention of all-pass networks.

Obviously untrue.

Hoping that misleading over-simplification and text-chopping are inoffensive enough not to compromise your "dispassionate expert" image.

Pity about that.

-- Bill Sloman, Nijmegen

here:

That was me I think.

Overkill for most applications, expensive, power-hungry and awkward to use. But it is the king of references. It is still unbeaten after nearly

30 years for long term drift and probably noise too.

--

John Devereux

I am expecting switching noise. I have seen this in past experience with them. The frequency of this noise will be an exact integer multiple of the generated sine wave and so it will be taken out by the FFT.

The sine wave frequency needs to be adjustable and so then must also be the filter. Analog filters are not easily adjusted.

The switched capacitor filter I have chosen for the task is the MAX294. To get the square wave's third harmonic in the notch shown at

1.8 kHz in the frequency response curve on page 4 at: I figure I will need to clock it at 167 times the frequency of the square wave. The lowest needed frequency is 100 Hz. So the lowest clock frequency is 16.7kHz. In addition to there being an FFT, I intend to put a 4.5kHz four pole filter at the output of the MAX294. This would suppress the 5mVpp clock feed through (page 3) down to 26uV. This and the FFT should be sufficient.

The capacitors in the switched capacitor filter are referenced to ground. I do not see that there is any direct mechanism to couple power supply noise to the output. Coupling would be by stray capacitance and the limitations of the op amp's power supply rejection. I expect this coupling to be weak. I also intend to use regulators designed for low noise.

What is left is to find a way to generate all the needed clocks and square waves. I will start another thread for advice on this.

I changed my mind about it. Its voltage is too high.

Like I said, awkward to use. That is the least of it.

There is LTC6655 if you are looking for low noise.

--

John Devereux

BUT if I understand your application you should have no need for a reference at all - everything can be ratiometric to the supplies.

--

John Devereux

at:

quoted text -

That's not really a valid generalisation for LEDs any more - there is a wide selection of single colour LEDs with a few hundred mW of output power.

Right now I'm pretty impressed with the LEDEngin range and using some of there parts in a couple of projects. I'll be building a driver soon to control their 40 and 90W parts and modulate them at a few hundred Hz.

These higher power parts are multple die devices but very impressive.

cheers

David

ht

es

d r

p://

e

ng

in

se

y

ut

s t

Hide quoted text -

s

r.

Cool, Thanks. and in stock at Mouser. I'll have to order some of the ~$3 660nm ones... ~500mW! (I'll need a heat sink.)

Run at low current ~200mA it's almost 25% efficient. (~100mW of photons at 2.3Vf)

George H.

light

does

modulated

or

one

beginning

in

response

frequency

cut

this

it

bit

in

a

DDS

expect

All nice. Somewhere along the way i learned that the sample time for the =46FT produced some potentially useful artifacts. If the sample time was

18.18 ms there would maximum orthoganality notches in the spectrums at 55 Hz spacing, likewise for 9.09 ms and 110 Hz spacing. Um! This was exploited in military Link 11 modems. It is also used in v.90 and v.92 modems.

?-)

at:

quoted text -

Mostly I'm using the short wavelngth units - 405nm (and some 365nm from Nichia), but have a selection going upto 660nm. These have all been the

5W (max. input power) versions but hooekd up a 10W device yesterday - very impressive and high intesity when I've added some optics to it.

Got a couple of 40 and 90 W devices sitting in the lab and in the proces of designing soem drivers for them so I can apply analogue modulation ( probably do one for digital too)

cheers

David

Plenty. the TI launcpad, capacitative touch accessory uses underside mounted SMD leds shining though unmasked FR4.

you'll get leakage to/from adjacent leds too.

--
?? 100% natural