[TenTec] OMNI-VI problem

[Ken Brown]

Hi Gerry,

>
> <SNIP>
>
> Actually the synthesizer is up in the 450 to 500 MHz region for better
> phase noise and then divided down to 5 to 5.5 MHz where its used alone
> for 80 and 20 and mixed with a crystal for the other bands.
>   
Actually the synthesizer generates 4.97 to 5.53 MHz as I said. The way 
it does this is to use a PLL operating at 200 - 220 MHz. The VCO in this 
PLL is actually a VCXO using a 39.940 MHz crystal. The 200 - 220 MHz 
output (a harmonic from the 39.9 MHz VCXO) is divided by by 40 to get 
the 5.0 - 5.5 synthesizer output. It is done this way for the reason you 
stated, to minimize phase noise.

Maybe it is the Omni V that does not use a crystal oscillator on 80 and 
20 meters. The Omni VI uses a 18.0030 MHz crystal oscillator.
> Loss of either oscillator will cause a dead receiver.
>
> The 160m crystal is 5.8 MHz. 
The crystal used for 160 meters in my Omni VI XTAL OSC board schematic 
is 16.3030 MHz. With the 5.0 - 5.5 MHz synthesizer working 16.303 - 
5.503 = 10.800 and 10.800 mixed with 1.800 will give a product at the 9 
MHz IF.
> Add 1550 that's 7350, quite a ways from the
> 9 MHz IF, but twice 5.8 is 11.6 subtract 1550, that's 10.050, still
> outside the filter. 5.8 + twice 1550 is 8.900 MHz. Still a supersonic
> beat note. Twice a 1600 MHz BC station would mix with the 5.8 MHz
> crystal to pass through the IF at 9 MHz.
>   
I guess that would be the case in an Omni that uses a 5.8 MHz crystal on 
160 meters.

In an Omni VI like mine that use a 16.303 MHz crystal, and the 5.0 - 5.5 
MHz synthesizer not working, the most straightforward mixes to 9 MHz 
would be from 25.303 MHz and 7.303 MHz. Since Joe identified the MWBC 
station that he hears at 1550 kHz, I would say there is some other 
component involved in some IMD product he is hearing.

It is also possible that there is another station with the same program 
on 7.303/5 = 1.460 and he is hearing the 5th harmonic of that. That is 
not too hard to believe if you tune around the MWBC dial and hear how 
much duplication of programming there is on multiple stations. I expect 
Joe would have heard that one too if it were th

> I'd concentrate on the synthesizer.
>   
Yep. First determine whether the synthesizer has any output at all. If 
it does, then try to determine whether it is making it to the XTAL OSC 
LO MIXER BOARD. The simplest explanation is a failure of the cable and 
connectors between boards. I hope it is that simple.

DE N6KB