Re: [time-nuts] FE 5680A "new version" - Filtering the 10 MHz
Sylvain Munaut wrote:
It's not generated from a DDS. It's from a xilinx PLD, then through an L and then through a C to the RF connector.
The signal at the L output is fairly sinusoidal (at least monotonic near the crossing), but after the cap, it's like that ...
While I'm waiting for my own FE-5680A to arrive... that "sine" wave certainly looks awful. What kind of signal is coming from the Xilinx PLD pin, before the L? Is it a simple 10 MHz square wave, or some kind of >> 10MHz PWM intended to approximate a 10 MHz sine wave?
If it is a 10 MHz square wave, can I use that as a 10 MHz clock directly (probably through a digital buffer) or does it have a lot of jitter, which the LC filter was supposed to smooth out?
On Tue, Nov 15, 2011 at 4:25 PM, beale beale@bealecorner.com wrote:
Sylvain Munaut wrote:
It's not generated from a DDS. It's from a xilinx PLD,
I think the DDS is implemented inside the PLD.
Sying "that is not a DDS, it's a PLD, is like saying "That is not an
amplifier, it is a transistor."
In the description of how the device works in the data sheet they say
there is a DDS inside the programmable synthesizer. It seems
reasonable that several functions are implemented inside the PLD and
this seems to be supported by the data sheet.
Chris Albertson
Redondo Beach, California
It's not generated from a DDS. It's from a xilinx PLD,
I think the DDS is implemented inside the PLD.
Sying "that is not a DDS, it's a PLD, is like saying "That is not an
amplifier, it is a transistor."
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.
In the description of how the device works in the data sheet they say
there is a DDS inside the programmable synthesizer. It seems
reasonable that several functions are implemented inside the PLD and
this seems to be supported by the data sheet.
There is a DDS, but not there. Someone described those new units in a
previous post.
Cheers,
Sylvain
Hello,
El 16/11/2011 07:22, Sylvain Munaut escribió:
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.
A good DDS does, but (as was discussed here some time ago) you can use a
1-bit DAC (i.e. only the MSB). The output frequency would be correct,
but quite jittery. Have you performed any jitter measurement at the PLD
output? In any case, it seems to me a small PLD to have implemented a
NCO, but of course that depends on the NCO implementation and perhaps
could be there... but it is an speculation.
In any case, if there is only a simple LC filter from the PLD to the RF
connector, I would not be too surprised for artifacts in the output of
the PLD. The ones and zeros have usually a lot of noise coupled from the
remaining logic.
I hope I can play also soon, when I got mine :)
Regards,
Javier
On Tue, Nov 15, 2011 at 10:22 PM, Sylvain Munaut 246tnt@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
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@febo.com [mailto:time-nuts-bounces@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@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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