Here is a simple circuit I played with using an XR2206 function generator IC as a VFO that could be used with a homebrew transmitter. It covered the entire broadcast band with the twist of the 10k pot. I've used this IC as an audio generator in the past but never as an RF generator. I chose a 330 pf mica cap as the value for "C". Stability was touchy using the pot but once a desired frequency is found, the pot could be replaced by a fixed resistor in series with a low-value trim-pot for fine tuning. Another idea might be to use a variable (tuning) cap for C. Output voltage is about 3 volts for a sine wave and much greater if the square-wave output is used. I looked on the internet and found that these chips are still available at reasonable prices. The chip will actually function as an AM transmitter by connecting audio to pin 1. I connected a 3' antenna and sent a decent signal across the room to a portable radio.



Jameco has these for $6.49 but I've seem them at lower prices from other sources.

http://www.jameco.com/webapp/wcs/stores ... 1_34972_-1

Dave

I am pretty sure that the XR2206 is no longer in production. My function generator uses a MAX038, which like the 8038, is no longer made.

Get 'em while you can.

Rich

This is true. I have some NOS left over from other projects. I've never had trouble buying from Hong Kong, though. I saw one on this list for $2.39 with free shipping.

http://www.ebay.com/sch/items/xr-2206?_ ... 2206962139

I never knew these could go up to 2 mHz. If I were going to build one as a Part 15 exciter, I'd put it in small, enclosed metal box to act as a shield from the rest of the transmitter circuit. I have a tube LPB college transmitter that I want to try this with.

Dave

My MAX038 will give good output to about 18 MHz. Really disappointed that Maxim discontinued it.

Rich

The spec sheet I can find says max "typical" 1 MHz, although I see lots of 'kits' with it claiming 2MHz. Maybe this is a case of "well, it'll do it despite the spec."

Btw, Mouser lists it, but has no stock, marked EOL.

A search at Exar comes up with dead links.

You can read about this IC here:

http://www.jaycar.com.au/images_uploaded/XR2206V1.PDF

Dave

That's one of the copies I found. Jameco has it too. It says max 1MHz like the others (which were all the same sheet anyway).

However, these folks have a kit claiming 2MHz http://www.electronics-diy.com/function-generator-kit-xr2206.php

The spec sheet chart that I read didn't give an absolute maximum frequency but they did say 1 mHz is typical. They claim it will go to "more than 1 mHz" in the introduction. I saw those DYI kits boasting 2 mHz. That's why I decided to give the circuit a try. My test circuit (which was on an old perf-board I built years ago set for 400 Hz) did go to 2 mHz after I changed the timing cap but it's output at that frequency was only about 1 volt compared to 3 volts at 1 mHz.

Interestingly, the chip can be amplitude modulated. I was surprised how good it sounded. After perfecting the circuit to provide the best stability, the next step might be to design a 100 mW linear amplifier.

Dave

Well, that is, pardon the pun, 'typical' for a 'max' performance spec as you can't guarantee an "absolute maximum frequency," because not all can do it. What you guarantee is a 'minimum' max, what all are guaranteed to do, and that number is 500 KHz. Then they say 'typically' they can do 1 MHz, but that's not guaranteed. What the distribution is between the extremes and 'typical' I don't know but it's possible the curve has shifted upward as the process improved. However, if you were building something to sell the best the spec guarantees is 500 KHz so I'm not sure what the kits are basing the 2MHz on. Maybe they do their own component screening and then sell the others as individual parts, which would mean they're unlikely to hit 2 MHz.
That's another possibility. It might be they'll do 2 MHz but not be 'in spec' for the rest, like output swing, at that frequency. And maybe it can't handle the spec'd output load (overheat) at that frequency. Don't know and can't tell.
If they'll do 2 MHz but just not at full output then that's workable since one could compensate for it. The problem is there's no way to know from the spec because it only guarantees 500 KHz.

Yep - that's why I figured I'd experiment with it first before committing myself to do a complete project.

Dave

While I've been silent, I am watching this project with keen interest...

tom

Looks like that MAX038 would be a better choice.

http://datasheets.maxim-ic.com/en/ds/MAX038.pdf

Hong Kong price is around $11.00 but probably worth it for better stability at our frequencies of interest rather than trying to push the Exar unit beyond it's limit.

http://www.ebay.com/sch/i.html?_from=R4 ... Categories

I still plan on trying the XR2206 in a shielded box.

Dave

The MAX038 was originally introduced to be a better 8038, the function generator pioneered by Intersil. It will indeed go out to 18 MHz. Sadly, Maxim discontinued it.

Probably the gain of some of the chip's transistors starts to fall off as the freq. increases. Eventually, it just won't have any output at some extended frequency. Another issue is parasitic capacitances on the chip that can limit HF performance.

So, if you can find these chips, they are useful and fun to work with. Don't design anything that you need to replicate, however.

Rich

Along with 35Z5, watching with interest.
Ordered two of these from Hong Kong, waiting on delivery.

I'd like to see the MP3 Player discussed in this thread ( http://www.antiqueradios.com/forums/viewtopic.php?f=12&t=184399 ) combined with this XR2206, a cheap/common wal-wart and a nice small enclosure. But the MP3 player wants five volts, XR2206 wants 12.

-Sam T.

A friend just ordered (and received in 5-days) one of these.

http://www.ebay.com/itm/1KHz-to-33MHz-A ... 5d323133f4

Dave

Dave, that's a neat piece...

A nice sine wave can be produced at BCB freq with 150 to 390uh in series with the output...

Tom

When this thread started I ordered a couple of the chips on eBay. They were about $2.25 each free delivery, which took about 3 weeks from China. Interesting chip with a lot of possibilities.

How about taking a fixed oscillator and mixing it with the output from the function generator to get a variable output at a higher frequecy? Use the function generator chip like a PTO.

arnie

I ordered one of these $10 modules on the 3/18 and it came today. I haven't tested it out yet but the specs look promising



Dave

And then there is this. Ive been playing around with the AD9850 awhile for a different project, its nice to work with and even better ones are available.

Carl

http://www.ebay.com/itm/AD9850-Module-D ... 3897800052

I looked at those and they seemed ideal. But they looked like you need to be computer savvy to program them for your desired frequency.

http://www.analog.com/static/imported-f ... AD9850.pdf

Dave