[time-nuts] FE 5680A "new version" - Filtering the 10 MHz

[Bob Camp]

Hi

In the modern version of the 5680 the VCXO is at exactly 60 MHz. The PLD has
a fixed divide by 6 to generate the 10 MHz output. There's no dithering or
DDS stuff between the VCXO and the output. All of the DDS stuff is in the
loop that drives the gas cell. The DDS is an Analog Devices part.

Hope that helps.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Wednesday, November 16, 2011 1:26 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FE 5680A "new version" - Filtering the 10 MHz

On Tue, Nov 15, 2011 at 10:22 PM, Sylvain Munaut <246tnt at gmail.com> wrote:

> But a DDS would require a DAC somewhere. Here, I see can see the trace
> from the PLD pin, to the LC filter to the RF connector ... and I
> probed the PLD output, it's a square wave.

I don't know how this device works, an someone posted that how it
works dedends on which version of the device you have.  But many times
they will use pulse width modulation rather then a DAC.   Basically
you output a square wave but you vary the duty cycle and then after
filtering you have analog.  call it a combination of a one bit DAC and
dithering if you like.

This is also how "class D" audio amps work.  They use a one bit DAC
and a filter.  Sounds crude but it can work well and sound good.

Again I don't know how this device works, but do want to point out
that a "square" wave can in fact be the analog output (after
filtering)

Chris Albertson
Redondo Beach, California

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