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

[Samuel DEMEULEMEESTER]

> What if both inputs are terminated into a load (e.g. 50 ohms) rather than
a short?  

I tried to terminate the inputs with 50 ohm between the pin and the DC block
cap (and even 1 Kohm).  auto-oscillations are gone, but the loss of
sensibility is catastrophic. From -30 dB at 2 GHz, it drops down to +5 dB.
Tried with and without amp input stage. 

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

I removed C18, so VCO was disconnected. The "best practice" for this chip
seems to decouple the RFINB input from ground with a 100 pF cap and use a DC
block cap on RFINA directly to the pin. In that configuration, the ADF4107
oscillate. 


-----Message d'origine-----
De : time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] De la
part de jimlux
Envoyé : lundi 24 janvier 2011 02:02
À : time-nuts at febo.com
Objet : Re: [time-nuts] Update on prescaler board for HP 53131A-like
counters

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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