My HP 33120A Function Generator died. (I'm still trying to fix it, but it isn't obvious what's wrong ... rails check so I'm suspicious the EEPROMs gave up the ghost.) I bought a new one on epay week befor last for a fair price. It arrived and I was surprised/excited to find it had the external reference option. Just what a TimeNut needs. The external ref option wasn't mentioned explicitly in the listing, although I can see it (in retrospect) in one of the pictures. Sweet! -- Christopher. 73 de AI6KG

On 3/5/17 9:14 PM, Christopher Hoover wrote: > My HP 33120A Function Generator died. (I'm still trying to fix it, but it > isn't obvious what's wrong ... rails check so I'm suspicious the EEPROMs > gave up the ghost.) > > I bought a new one on epay week befor last for a fair price. > > It arrived and I was surprised/excited to find it had the external > reference option. Just what a TimeNut needs. > > The external ref option wasn't mentioned explicitly in the listing, > although I can see it (in retrospect) in one of the pictures. > I don't know about on the 33120, but on the 33620 series, the external reference doesn't actually phase lock the internal clock for the ARB. It's more of a frequency reference, and the output is steered to match the frequency of the external reference. (counters and NCOs) The phase noise of the 36620 is entirely determined by the internal oscillator, and the ADEV is a bit tricky - it's like the ADEV of a disciplined oscillator, not a phase locked loop. There are ways to sync multiple 33620 and 33120s (and the earlier 3325) so that you have multiple outputs with a phase coherent relationship. You have to do it manually.. feed them all the same reference, then adjust til they're synced, then they'll stay in sync. Here's Agilent's ap note on externally syncing 33120s http://cp.literature.agilent.com/litweb/pdf/5988-8151EN.pdf about your option 001 http://www.keysight.com/upload/cmc_upload/All/33120A_Opt001.pdf?&cc=US&lc=eng service manual and schematic of your Opt001 http://literature.cdn.keysight.com/litweb/pdf/33120-90022.pdf (From Keysight about the 33620 series: When using an external 10 MHz time base with this instrument, a PLL syncs together the internal oscillator with that external signal, as you have surmised. To my understanding, because of this however, the stability of the function generator’s time base is still dependent on the internal oscillator. If an OCXO is being used in the 33600, the instrument will be limited to a input frequency range of 10 MHz ± 1Hz. Therefore, even if an external input signal at 10 MHz was stable to 10 ppb, for instance, the instrument would still be limited to a 100 ppb stability because of its’ own internal oscillator. This means feeding a more stable reference signal into the function generator, will likely not improve performance. Again, this is to my own understanding of how this instrument works and I am going to double check this with our factory to see if they agree or disagree. I will update you when I have that confirmation. )