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Re: RE: S band for 6 bucks



Hi Woody, KJ4SO

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

----- Original Message -----
From: "Woody" <kj4so@nc.rr.com>
To: <amsat-bb@amsat.org>
Sent: Tuesday, June 14, 2005 11:39 PM
Subject: [amsat-bb] RE: S band for 6 bucks


> Someone wrote:
>
> "Could one leave the output of the downconverter at 20 MHz if one had an
> IF rig that could receive at that frequency? Or is there perhaps an
> output
> filter that expects VHF output. Under these conditions, one would only
> need to retune the input filter, avoiding  what seems to be the most
> hazardous part of the refit".
> >>>>>>>>>>>>>>>>>>>>>>
>
> The Cal Amp 130215 S-band downconverter converts a 2.5 GHz. input freq.
> to an IF output of 120 MHz.
>
> If you try to use the downconverter to tune 2.4 GHz. without modifying
> the Local Oscillator (LO) frequency, you would have an IF of 20 MHz.
> While this may sound appealing to some, it is not practical with this
> downconverter without other modifications.
>
> I did not take detailed measurements at other IF frequencies, but I
> noticed a sharp drop in downconverter sensitivity once the IF output
> dropped below 50 MHz.
>
> The following chart shows a 20 dBm improvement at 2.4 GHz. in the
> downconverter once I converted the IF output from 20 MHz. to 144 MHz.
>
> Downconverter input level = -70 dBm
> (A) = Unmodified Cal Amp 130215 S-band downconverter
> (B) = Cal Amp 130215 - Retuned Combline filter only
> (C) = Retuned Combline filter & IF changed to 144 MHz.
>
>                               (A)              (B)           (C)
> Input (GHz.)
> 2.40                         x           -58 dBm     -38 dBm
> 2.42                         x           -60 dBm     -38 dBm
> 2.44                         x           -45 dBm     -40 dBm
> 2.45                   -58 dBm     -45 dBm     -40 dBm
> 2.46                   -54 dBm     -45 dBm           x
> 2.48                   -42 dBm     -42 dBm           x
> 2.50                   -38 dBm     -40 dBm           x
> 2.57                  -40 dBm             x                x
>
> 73,
>
> Woody
> KJ4SO
> AMSAT # 5562
> ----
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