Those cheap DDS devices do not use sufficient bits of data to generate the sine
wave output. This gives rise to severe quantization error, resulting in a sine
wave that looks more like a sign-ish staircase waveform.
This has a very complex Fourier spectrum, which plays out in the rather high
“phase noise”. It isn’t really phase noise,it is the convolution of the Fourier
spectrum of the DDS and the incoming undesired signals.
To make matters worse, while phase noise tends to decrease rapidly as you move
away from the center frequency, quantization noise induced noise can extend
quite far from the center frequency.
You really need a MINIMUM of 16 bits in the DDS to even come close to an analog
PTO in terms of this garbage. 24 bits would get close enough to reality that
you’d probably prefer it over your PTO.
You can get around this in a 14 bit DDS by doing the DDS at VHF or microwave
frequencies and then dividing to get to the desired operating frequency. You’d
still need a low pass filter to strip away the harmonics of the scare wave that
comes out of the divider.
Gary
> On Jun 12, 2018, at 1:24 PM, Patrick <flymooney@gmail.com> wrote:
>
> I'll second what Barry said. I had a DDS in a Corsair II and ended up pulling
> it all back out after a few weeks. It wasn't really "objectionable" but it
> was definitely noticeable and thus, objectionable to me. YMMV
>
> Patrick
> K4PNR
>
>
> On 6/12/2018 4:47 AM, Carlos wrote:
>> Hi Barry,
>> You are absolutely right. I have been using the Corsair to monitor
>> local traffic in 40 meter, where signals are so strong that any
>> off-frequency parasitic response due to the spurious tones generated
>> in the DDS seem to be negligible.
>> What I'll do next weekend is to make proper screen captures of the
>> spectral distribution of the 5 MHz PTO Vs. the 5 MHz DDS. The stuff I
>> have at home does not have much dynamic range (probably 75 dB at
>> best), but I'm sure that the differences are easy to notice.
>> 73,
>> Carlos VK1EA
>> On 6/11/18, Barry N1EU <barry.n1eu@gmail.com> wrote:
>>> FWIW, the PTO has a MUCH cleaner spectrum - noticeably lower transmit and
>>> receive spurs.
>>>
>>> You can easily see this if you put the PTO vs. DDS Corsair in transmit mode
>>> and look at the signal on an SDR.
>>>
>>> Barry N1EU
>
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