are all the function generators gone?

At audio frequencies? Why waste time, board space, money, and power dissipation on a DDS when even cheap-ass 8-bit processors come in 40MHz models?

Generate your sine wave on a PWM pin, filter it, and be happy. If you want to set mark and space frequencies with pots -- fine: cheap-ass 8-bit processors come with ADCs, that can be done. If the notion of using PWM to generate a "clean" signal makes you itch (XR2206? Clean?), then use a DAC.

You'll spend less time writing the software than you will trying to source the chips you want.

Then either use an ADC on-chip and implement your famous dual-tach signal processing in software, or just lick the back of that schematic and put it onto your board (since this is a charity job).

Do you need a lot? We've got several tubes left over from a discontinued gizmo*.

You could use a clocked 4017 with different R's into a opamp summing junction. (Phil or James suggested it a while back.) With ten steps you move most of the 'distortion' out to the 9th and 11th harmonic.

George H.

what frequencies?

-Lasse

Have you ever tried a cheap DDS? Here is a module with an AD 8950 for under $8. It can be driven by a small processor, and Analog Devices has a demo program with a simple parallel port interface to play with. They offer sample code for the 8051, (I think. It's been a while since I looked)

Close to Bell 202 (1200/2200 Hz) but not so close that DSP-based 202 chips will work.

Here are 100 for $276.00:

You can easily make a modem from active filter structures. That's what I used for the OmniComp/GenRad international-frequency-selectable modem circa 1981-82... dirt cheap but very accurate.

The discriminator was patented...

Much better performance in the presence of noise than the TI digital chips of that era. (I'll have to dig thru my lab notes, but I may even have used an LM324 :-)

I'm trying to remember now, but I most likely used something like this for the transmitter...

hard to beat a

You could hook the pots to the A/D inputs of a controller :-) Otherwise use an NE555-ish chip and filter the output...

Yeah the cost is all in the dam#, box, switch, connector, power supply...

I need someone to make thousands of 'em.

George H.

Neat, what was the mixer? (an XOR?)

George H.

ide quoted text -

Direct. It's audio, 1200/2200

They're available on eBay from a variety of vendors:

What about the XR2211?

Cheers

We did a Bell 103 for a fallback using a coordic to generate the CPFSK. Trivial if you know the algorithm. Demod was done computing the arcsin, again with a coordic, unwraping the phase, then doing a LMS line fit to get the incoming frequency. The slope of the phase is the frequency. This was circa 1985.

I can do the receiver with my tach circuit, probably a better FSK detector than a PLL.

It looks like there are lots of 2206's on ebay, from vendors with good feedback, so maybe I'll get a lifetime supply of them for the transmitters. Thanks to the guys for the tip.

Interestingly, the old Bell 202 protocol (1200/2200 Hz FSK) is still used in industrial (like HART) and RF applications, and several people sell modems. But most/all use a DSP modem chip, which don't like my somewhat-off frequencies. Old analog modems work.

I designed the systems in question about 30 years ago. The remote units have a software transmit-side UART+Modem (MC6802 with a 2708 1kbyte eprom) that transmits at 1200 baud with FSK frequencies 1200 and 2400 Hz, sort of synchronous FSK. That was really easy to do in software, but it freaks out the DSP modem chips.

I could make my own triangle generator and sine shaper with a few standard ICs, but the 2206 does it all.

I did essentially the same thing about 10 years prior, but used an RLC as the phase shifter, technically a very peaky lowpass filter. A jfet or (later) a cmos XOR gate was the mixer which fed the output filter, also an LC thing. This was for a series of narrowband FSK modems for pipeline and electric utility supervisory systems.

The tricky part was the input bandpass filter. It had to be tight, because many modems shared a leased phone line. And it had to have flat delay vs frequency. One of my more mathematical engineers came up with some clever elliptical filters where the notches also did the delay equalization.

XR2206 $4.95 - I know it is not your favorite type of supplier but since there is no profit in this job ...

sorry not currently in stock