[time-nuts] Update on prescaler board for HP 53131A-like counters

jimlux jimlux at earthlink.net
Sun Jan 23 20:01:32 EST 2011


On 1/23/11 4:27 PM, Samuel DEMEULEMEESTER wrote:
> Hi timenuts,
>
>
>
> Here is an update for all of you interested by this project. According to the emails I got, you are a lot ;-)
>
>

What if both inputs are terminated into a load (e.g. 50 ohms) rather 
than a short?  The input circuit in Fig 18 of the data sheet shows it 
going into a diffamp (which will be pretty Hi-Z differential) and tied 
through 500 ohms to the bias supply, which I assume would be (very) low 
Z to ground, so the input z is a few hundred ohms or so.

Tying that straight to ground might not be the best way to put "no 
input" in. That is, the amplifier isn't stable with a mismatch on the 
input.  Figure 27 shows a 51 ohm termination to ground (through a 
decoupling cap), fed by a resistive 2 way splitter.

When you say the eval board also oscillates, is that with the VCO 
removed from the circuit, or what?


>




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