Precision low frequency generation

if you are making a linear summing amplifier, then YES you have missed something, the difference between addition and multiplication.

Mark

A whole lot simpler to produce the 2.5Hz signal directly from your computer.

"Dirk Bruere at NeoPax"

May take any number of wooden stakes driven through the vile zombie's heart.

Show no mercy - it is not human.

...... Phil

Wow. You don't have a slightest clue, do you?

VLV

I need to use any computer, without specialised hardware. Soundcards typically will not go down below 10Hz. I suppose I could simplify it by generating a high frequency sine wave

Dirk

"Human" is not something to aspire to, but something to transcend.

Dirk

Your usual helpful self. Why do you bother being such a jerk?

"Dirk Bruere at NeoPax"

May take any number of wooden stakes driven through the vile zombie's heart to kill it.

Show no mercy - it ain't human.

...... Phil

"Dirk Bruere at NeoPax"

It may take any number of wooden stakes driven through this vile zombie's heart.

Show no mercy - it ain't human.

...... Phil

You'd get no output at 2.5 Hz. You need a nonlinear mixer ahead of the filter, ideally a linear multiplier.

John

Dirk, to get the difference frequency you need to multiply, not add, the signals.

Here is a cheap man's solution:

C1 and C2 decouple the input. The potentiometer sets the voltage at gate2 so the drain is at about

Select C3 for adequate low pass filtering.

Does output need to be a precise sinewave? If so, use an EPROM with a binary counter on the address lines, driven from a 74HC4046 VCO, output EPROM into a simple DAC.

Given that you are adding "specialized hardware" in the form of external circuits, why not just make the external circuits be a ROM full of digitized waveforms and a DAC to spit them out? You can get a uP that has both enough ROM and a DAC on-board...

On a sunny day (Tue, 20 Nov 2007 07:25:00 -0800 (PST)) it happened snipped-for-privacy@yahoo.com wrote in :

Google screwed up my ASCII art, so here again from a real newsreader;

Dirk, to get the difference frequency you need to multiply, not add, the signals.

Here is a cheap man's solution:

C1 and C2 decouple the input. The potentiometer sets the voltage at gate2 so the drain is at about 7V. The 2200 Ohm together with C3 form a lowpass. The second gate in the BF961 dual gate MOSFET modulates the signal caused by the voltage on the first gate. Here is a similar diagram, note the mixer, it is for higher input frequencies however..

Select C3 for adequate low pass filtering.

Does output need to be a precise sinewave? If so, use an EPROM with a binary counter on the address lines, driven from a

signals.

C4

the voltage on the first gate.

however..

Thanks. Initially I need to test with sinewaves, but ultimately I want to use the computer to generate arbitrary LF waveforms. Modulating a HF signal with the LF waveform in the computer would seem the best way to go initially. Then it becomes an AM 'radio' problem. Cat's whisker?

Dirk

John, why did you sell him an ultimate arb generator?

VLV

Put a DAC on the parallel port, or buy a DAC dongle that plugs into USB.

That would be a good solution for a standalone unit. However, I like the idea of selling custom PC progs for specialised generation for the app I have in mind. The money, as usual, is in the s/w

Dirk

On a sunny day (Tue, 20 Nov 2007 07:33:19 -0800 (PST)) it happened Dirk Bruere at NeoPax wrote in :

We have precision rectifiers these days:

in -- 100nF ---------- + | opamp 10 kOhm out --------- | | /// --- - --- | 1/4 LM324 \\ / 1N4148 | --- | | |---------------------------------- LF | | 10 kOhm C | R1 | C1 /// ///

AM peak detector. R1 C1 timeconstant should be small enough so your highest frequency still passes.

The cheapest one we offer, the tiny new 4-channel serial thing, will be about $2K, probably not in his budget. It goes from 0.003 Hz to 2 MHz.

Actually, one doesn't need a linear multiplier. One of those simple

You could even do it with a tiny circuit that's powered from the sound card signals.

John