Perhaps the most two important measurements in electronics are Voltage and Frequency (1/time). The accuracy of which will eventually determine the go/no-go status of some project or another. This need not just apply to NBTV.
For Voltage I invested in the best grade LM4040 I could get at 5.0V, the LM4040-5.0A which is within ±0.1%. Ideal for checking your DVM on a monthly basis.
As for time/frequency, I went for broke. The GPS system. This is so accurate...forget it. 10<sup>-11</sup> is common, nay daily.
A chap in the UK sells 'sort of' DIY versions that provide the basis, the rest is up to you.
http://www.jrmiller.demon.co.uk/project ... frqstd.htm
I attatch how I interfaced James' GPSDO to my world, he has a newer version, but it should be very similar. I added a power-outage indicator as we have many here, a voltmeter to keep an eye on the PPL status, and an oven temperature indicator.
Also attatched is the view of the sky that the GPS receiver gets here over a 24hr period.
Steve A.