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Re: RE: S band for 6 bucks

i8CVS Domenico wrote:

> If you use the above downconverter with an IF of 20 MHz to tune
> 2400 MHz without modifying the LO frequency 2380 MHz you get
> 2380 + 20 = 2400 MHz = the wanted frequency.
> 2380 -  20 = 2360 MHz = the unwanted frequency or the image frequency.
> The wanted frequency is separated only 40 MHz by the image frequency.
> At 2.4 GHz a separation of only 40 MHz between the wanted signal and 
> the
> image signal make the image rejection very small or non existing at 
> all.
> As a consequence the noise at 2380 MHz will be not attenuated by the 
> input
> filter and it will be added to the noise at 2400 MHz
> In this conditions if you measure the noise figure with an automatic 
> noise
> figure meter you may measure a good NF= 1 dB for example wich is a DSB
> noise figure measurement but in real work you must add to it 3 dB of 
> noise
> due to the contribution of the noise at the image frequency and your
> NF= 4 dB in a SSB noise figure measurement.
> Best 73" de
> i8CVS Domenico

Hi Domenico:

Thanks for your very accurate and clear explanation of the problem in 
trying to use a 20 MHz. IF on a 2.4 GHz. downconverter.  I had stated in 
an earlier message that there would be an image problem with a 20 MHz. 
IF.  I was trying to take the easy way out and not have to explain 
microwave receiver mixer and LO theory.  This project has devoured all 
of my spare time, and then some, for the past four days.  I am not 
complaining, it has been very enjoyable.  It was just unplanned for.


AMSAT # 5562
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