Sine wave generation

Did you consider just using a MCU or DSP in addition to a decent DAC intended for audio applications to output the sine wave's samples as analog values? The DAC output could be fed into an op-amp or other amplifier as needed to generate the output voltage you require with a potentiometer adjustment. The most significant characteristic I noticed that you mentioned was the frequency stability so you will have to ensure your clock's voltage / temprature induced frequency drift is acceptable but that will not be difficult. If I recall correctly, there are many audio application DACs that can synthesize a waveform with the purity that your application seems to require in the frequency band you require. I wouldn't suggest generating a square wave which then must be heavily filtered when you can easily generate a much closer approximation (lower THD distortion) sampled output by using a DAC to begin with. Then the required filters to eliminate any residual noise will be much less stringent since the spectrum of the DAC samples themselves will just about meet your noise / THD requirements. I have no particular recommendation as to a device, but due to applications like PC sound cards, MP3 players, et. al. TI has very many inexpensive and good quality DSP and DAC devices for audio uses, and I'm sure many of those will either have the analog post filtering / amplification you require or have simple interfaces to sufficiently good quality amplifiers to provide the amplitude and quality analog output. http://focus.ti.com/docs/prod/folders/print/dsd1791.html http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?&familyAliasId=1100582&familyId=582&uiTemplateId=NODE_STRY_PGE_T&techFamId=null&rootFamilyId=null&sectionId=null&tabId=null&appId=null&viewDeviceCallingPage=null&totalCount=52&showAdditionalParameters=no&parameter=991467918&lc=3000195&lc=3000197&lc=3000196&lc=3000552&lc=2300088&lc=2001345&lc=2001339&lc=2200348&lc=2000084&lc=2001340&lc=2001341&compare=yes&download=yes&sort=yes&customize=yes&paramResults=yes&paramCriteria=yes&familyTree=yes&military=no&baSystem=yes&paramTable=no&sortOption=PA_SUB_FAMILY_NAME&sortMode=ASC&searchPaths=1100582&pageId=undefined&templateId=0&navigationId=0&family=analog&paramTable=no&military=no&&uiTemplateId=NODE_STRY_PGE_T&techFamId=null&rootFamilyId=null&sectionId=null&tabId=null&appId=null&viewDeviceCallingPage=null&resetCompare=true#sdp et. al.

Actually your amplitude stability specification is potentially a bit stringent also since that is in the range of uV/degC that even thermocouple effects may cause, so you'll want to be somewhat careful about those but more importantly also the temperature stability of the electronics you are using. For instance you mentioned a potentiometer to generate the gain for the output amplitude, and of course the temperature coefficient of that device may be a significant factor in the amplitude stability if you don't temperature compensate it. Due to that factor it might be more appropriate to consider using a digital gain control with the magnitude selected by a digital rotary encoder rather than a potentiometer for cost and signal quality vs stability reasons. Of course you could feed back the output signal from the DAC into the MCU via either an ADC or precision comparator/voltage reference and use that to get feedback about the generated signal purity, noise, amplitude, et. al. Many of the audio DACs are available as CODECs so you can get both ADC and DAC for low cost if they suit your needs. You didn't mention the actual temperature range over which operation is needed but of course if you expect significant temperature variations over a wide enough range to be problematic you could design the unit to have fairly high thermal mass and good heatsinking of the critical components so the actual operating temperature will tend to change more slowly and predictably which would aid any compensation mechanism's effectiveness (you could easily enough measure the temperature and apply a correction calibration affecting the gain or DAC LSBs as needed).