Re: RF out with RF OFF: Clock feed through


The 180 MHz component is intrinsic to the DDS design, any frequency generated by the DDS is sampled by the 180 MHz reference clock. Maybe you are right and this type of connection causes the DAC to output more clock energy that should be.  Probably using the transformer or the opamp the clock component will be reduced but a trap at 180 MHz could suffice. In fact my external 30 MHz LPF removes the clock (leakage or whatever) quite well.


Ignacio, EB4APL

El 10/07/2017 a las 1:29, Clifford Heath clifford.heath@... [PHSNA] escribió:

Filtering is the wrong solution here. You need to fix the problem at the source.

The evaluation board (and the Chinese clones) use asymmetrical outputs,
with one DAC output terminated to ground and one to the filter. The output
of this kind of DAC is fully differential current-steering, and you cannot get
good performance by taking one side to ground.

Instead, you should fit a transformer or a suitable op-amp (with good CMMR)
to subtract the two outputs. You can see this in the schematic for the AD9959
evaluation boards. Note that the transformers they use there are $10/each,
so the Chinese boards generally don't provide them, or only provide 2 of the
four channels.

Alternatively, here's a cheap op-amp that'd do the job:

Use a normal differencing setup (four resisters) and feed the output into the

Clifford Heath.

> On 10 Jul 2017, at 3:10 AM, EB4APL eb4apl@... [PHSNA] wrote:
> This is exactly what I did, I have a connectorized 30 MHz LP filter and I inserted it between the input and the output of the PHSNA. This time it is not a harmonic problem, it is the 180 MHz refclock frequency which is not well filtered out.
> Ignacio, EB4APL
> El 09/07/2017 a las 18:44, mendiola_loyola@... [PHSNA] escribió:
>> Try putting a low pass filter of 30mhz at the output of the dds-60. The amplifier of the dds60 also adds harmonics.
>> Posted by: mendiola_loyola@...

Posted by: Clifford Heath

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