W3WN
12-14-2016, 08:53 AM
In regards to the Omni VI repairs, I thought these comments from Gary WØDVN might be of interest.
This was his recent post to the Ten Tec reflector, in context of a reply to comments about changing a stock Ten Tec PTO, or another 5 - 5.5 MHz VFO/PTO unit, to a DDS one. (Not because the VFO/PTO had an issue or wasn't repairable, just to 'improve' things and/or add other features.
Since I know some of us (hi Fred!) are looking into or doing these types of swaps, I thought this pertinent.
Gary's view are all his own. by the way. Not mine; I lack experience to make an informed post on the subject.
On 12/13/16, Dukes HiFi<dukeshifi@comcast.net> wrote:
Having just repaired an Omni 6+ 5.0 to 5.5 MHz PLL, I can see why many might like a simpler DDS offering.
However, there is a dark side.
Many of these do not use enough bits to draw the sine wave from the lookup table in ROM to provide anything nearly as good as what the Omni 5 and 6 PLL's provide.
You end up with some really odd looking distortion products that are only down from the full output by the bit resolution of the DAC. Thus, a 16 bit DAC can only offer 96 dB local oscillator purity below full output, even worse (84 dB) for the more common 14 bit D to A devices in low cost DDS circuits.
I had thought of putting one of these into my Paragon and even bought one. I decided that the performance was not even as good as the relatively poor PLL synthesizer in the original Paragon.
One COULD put the DDS synthesizer at high frequency and divide it down like Ten Tec did in the late Omni's.
I am not sure if these low cost devices work at 200 MHz. If they were available, then one could simply use the simple divider/filter that Ten Tec used.
Gary
This was his recent post to the Ten Tec reflector, in context of a reply to comments about changing a stock Ten Tec PTO, or another 5 - 5.5 MHz VFO/PTO unit, to a DDS one. (Not because the VFO/PTO had an issue or wasn't repairable, just to 'improve' things and/or add other features.
Since I know some of us (hi Fred!) are looking into or doing these types of swaps, I thought this pertinent.
Gary's view are all his own. by the way. Not mine; I lack experience to make an informed post on the subject.
On 12/13/16, Dukes HiFi<dukeshifi@comcast.net> wrote:
Having just repaired an Omni 6+ 5.0 to 5.5 MHz PLL, I can see why many might like a simpler DDS offering.
However, there is a dark side.
Many of these do not use enough bits to draw the sine wave from the lookup table in ROM to provide anything nearly as good as what the Omni 5 and 6 PLL's provide.
You end up with some really odd looking distortion products that are only down from the full output by the bit resolution of the DAC. Thus, a 16 bit DAC can only offer 96 dB local oscillator purity below full output, even worse (84 dB) for the more common 14 bit D to A devices in low cost DDS circuits.
I had thought of putting one of these into my Paragon and even bought one. I decided that the performance was not even as good as the relatively poor PLL synthesizer in the original Paragon.
One COULD put the DDS synthesizer at high frequency and divide it down like Ten Tec did in the late Omni's.
I am not sure if these low cost devices work at 200 MHz. If they were available, then one could simply use the simple divider/filter that Ten Tec used.
Gary