RE: Local Osc. for Linrad rec.

[leif.asbrink@xxxxxxxxxxxxxxxx>]

Hi All,

Jim wrote:

> I am developing the hardware that I hope to use on Linrad for 2M 
> EME receive.
> I am using a lot of parts on hand to keep down cost.
> My questions at this time only relate to the local oscillator.
> The basic plan is to receive 144 mhz using a 10.7mhz IF and a 
> 154.7 mhz (high side injection),
> local oscillator. I have a 5 pole filter tuned to 154.7 mhz that 
> is very selective and should 
> provide a spur free local oscillator signal. The problem is to 
> provide a quiet, stable LO.
> My plan is to use a 13.667222 MHZ crystal and multiply it 9 times 
> to produce 123.005 mhz.
.
.
> 1) Given that the output from the CB at 31.695 mhz may have some 
> spurs, is it best to
> limit the amount of drive to the mixer to provide for better 
> selectivity or is it better to
> badly overdrive the mixer.

When you mix 31.695 with 123.005 to get 154.7 you will also
get things like 2*123.005-3*31.695 = 150.925 and 5*31.695 = 158.475.
These may be stopprd by your filters but there will be higher order
products that generate spurs.
If you limit the power to both mixer ports (if you use a Schottky mixer)
the higher order products will be more suppressed than the desired sum
frequency. I do not think the power level at the mixer will affect
the influence of spurs and phase noise that may be present on
the 31.695 MHz signal.

> 2) Currently the 123.005 MHZ oscillator/multiplier chain only 
> triples to 41,001667 MHZ.
> This 41.001667 MHZ signal is fed into the mixer with the 
> expectation that the 41.001667 MHZ
> signal will triple to 123.005 mhz inside the mixer. This is how 
> the X9 multiplication is 
> accomplished in the 155 mhz VHF Micors. Is this an acceptable 
> solution or would
> my LO be more pure if I were to interupt the signal path between 
> the 41.001667 MHZ
> signal and the mixer to triple it first to 123.005 MHZ and 
> provide some selectivity?
If your desired signal is 3*41.001667 + 31.695 you will have 
many more potential spurs. Things like 5*41.001667-2*31.695=141.618
or 7*41.001667-4*31.695=160.231. You would have to drive the 41MHz
input close to saturation in order to get the third overtone mixing
products strong enough which means that large currents will
circulate inside the mixer at higher overtones also. I suggest you
add a separate tripler and a filter. 

 
> 3) Is high side frequency injection a problem for Linrad? The 
> frequencies will be backwards.
No problem.
If you can set the LO frequency with a command from a PC serial port
or with your own bit-pattern on a few of the parallel port pins
you can set the frequency with Linrad and always get the correct
frequency scale on screen.

73

Leif / SM5BSZ



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