[time-nuts] Temperature and signal amp for 'Bay FE-5680A?

[shalimr9 at gmail.com]

Bruce,

That is at first counterintuitive (conventional wisdom and good design practice suggests good decoupling caps across the supply pins), but it makes perfect sense.

I have to try this (a small resistor in series with the positive supply rail) when I get home.

Thanks for the tip.

Didier KO4BB

Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Sender: time-nuts-bounces at febo.com
Date: Wed, 11 Jan 2012 07:30:27 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] Temperature and signal amp for 'Bay FE-5680A?

David wrote:
> On Tue, 10 Jan 2012 21:57:49 +1300, Bruce Griffiths
> <bruce.griffiths at xtra.co.nz>  wrote:
>
>    
>> Hal Murray wrote:
>>      
>>> The TADD-3 uses 3 AC drivers in parallel, each going through a 51 ohm resistor.  Changing those resistors to 150 ohms should work.  Maybe a bit lower to account for the impedance in the drivers.  I'd probably check it with a scope.
>>>
>>>        
>> That approach doesn't do anything for the Vcc and GND bounce exhibited
>> by the driver chip.
>> GND and Vcc bounce is the cause of the high frequency ringing exhibited
>> by the TADD-3 outputs.
>> This ringing can even be observed at the outputs of inverters whose
>> inputs are tied low or high in the same package
>>
>> Damping the crossover current induced transient in the supply leads
>> (bondwire and lead frame) inductance is one way to minimise this.
>> A small resistor in series with the Vcc pin often works well, the
>> resistor value being chosen for near critical damping.
>>
>> Another problem with the TADD-3 is the sharing of a driver chip by
>> different input frequencies which leads to intermodulation between the 2
>> outputs.
>>      
> I have never seen that much ground bounce before so assumed it was a
> termination problem.  Was the driver chip decoupling inadequate?  That
> at least would be easy enough to fix.
>
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>    
No, its due to the high switching speed of the output stage.
Such ground bounce is typical for ACMOS devices without a staged output 
device turn on.
Low inductance decoupling has no effect on internal bondwire and 
leadframe inductance.
Apart from redesigning to chip to have a more gradual output stage turn 
on, damping of the circuit is the only effective cure.
An example of the effectiveness of this can be found in the SRS FS730 
distribution amplifier CMOS output option.

Bruce

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