Depends. If you are patient, you can get one at a better price simply by sticking around and finding a week when nobody's bidding on them - at least that *eventually* worked for me, and my cutoff point was a lot lower than $300, so I let a whole bunch go for more money than I was willing to put into them. I specifically did not want new and Chinese, and I'm quite happy with my 3325A, LEDs and all. I have not found old age to be an evil thing in well-built equipment, and 20 (up to 70, actually, with some limitations) MHz is not a problem for my current anticipated uses. I do wonder when I'm ever going to make use of the
0.000001 Hz end of the range; I am happy to have it, however, having run into a situation with a client's generator where something below its 5 Hz bottom end would have been useful. It does a far better sweep, too.
Plenty of 3325s selling for well below $300. Don't actually see any recent ones that got bid on (ie, more than one bid) selling for that much. Couple with high starting prices that did get bought for $325 and $399 with a single bid or BIN. One a couple of weeks back did get bid to $301, but plenty selling a bit above and even a bit below $200 (not really with shipping) in the past week. Also plenty of volume to indicate that another will be along if you don't get the one you want at the price you want to pay, if you can stand to wait.
Point being, by the time you've built the controls, power supplies, output amps, connectors, boxes, etc. for your $30 chip, you'll probably have spent more money on it. It will be newer, and perhaps it will go faster, but the old one is a fully built system with a well-fleshed-out user interface, etc. You can get new China junk to 20Mhz for $200 or less, if you're willing to go there. Building your own, unless you're sitting around looking for ways to fill your time, is something to do for the experience or the bragging rights, or else to design the thing well enough that you can sell them (built, as kits, board and a parts list...) and make money at it - otherwise it's not "worthwhile". It need only be "worthwhile" in one of those areas to be worth doing - it can be a non-economical experiential job, for instance. And perhaps you can even make it economical (ignoring time spent), if you can find a "known/presumed broken" hulk on ePrey and pick it up cheap (inclusive of shipping, border crud, etc) as a source of parts to build a user interface, box, power supply etc from.
Win's partner-in-book's lab has this up, if you're interested, though I think it's a bit out of date at this point in time:
Depends on the amount of time you have. The biggest difficulty is getting the programmable gain amplifier right to get various output amplitudes. Expect to get to 10MHz (square wave) or so using off the shelf parts.
My HP3314A (including the 3x output) was delivered yesterday. While it is not DDS based, it has some really neat features you won't find on most function generators.
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Which DDS chip are you using? Are you going to Ebay it? How much would you sell a complete PCB? Cost with and without an output gain/attenuation section? Cost with and without control software? Actually, I'll probably want the source code or write my own control software.
Of course, assuming the DDS chip manufacturer doesn't have evaluation boards for sale.
I think a modern PC controlled DDS signal generator on Ebay would blow away lots old clunky generators or Asian boxes (probably XR2206 based) in terms of price, range and quality.