...under perpetual construction.
I have built the nice QRP2000 USB-Controlled Synthesizer to have a simple and versatile RF generator to use as a general purpose RF source; it's not expensive at all and provides quite good performances (see graphs below).
The version I built uses the Si570 LVDS version that is specified from 10 MHz to 280 MHz (in practice it might go a little higher in frequency).
The measured output power of my USB synthesizer over its entire frequency span is shown in the following graph; it includes also the (small) losses of the connection cable. Output power is about 0 dBm at 10 MHz and drops to -5.5 dBm at 270 MHz; most of the high-frequency drop is probably due to the homemade output transformer.
I had also the opportunity to have its phase noise measured with a professional equipment. For such a small device, the phase noise performances are very good, in line with the Silicon Labs specifications until around 1 MHz offset from the carrier where some discrete spurs appears, maybe due to the USB-supplied power supply.
Note that for small offsets (< 10 kHz) the phase noise might be even a little better than the venerable HP 8640B...