[hpsdr] VHF/UHF downconverter for HPSDR

[Phil Harman]

>
> I'm probably misinterpreting something here, but… Shouldn't the 65 MHz
> LP filter (SXLP-65+) on the output take care of that? It has a loss of
> min. 40 dB in the stop-band above 96 MHz. Perhaps SXLP-45+, the 45 MHz
> version would be a better choice?
>
>

Hi Anti,

The issue with image rejection is this. Assume you want to listen on the
70cm band at 433MHz and choose an IF of 50MHz. So that the signal is not
inverted you set the local oscillator to 433-50 = 383MHz.

In which case an input signal at 383-50MHz = 333MHz, i.e. the image
frequency, will also give a signal at the IF frequency. If you want say
100dB of image rejection then your 70cm bandpass filter(s) before the
mixer will need to provide this level at 333MHz.

Probably not too difficult for a fixed frequency range (after all this is
what our current transverters do) but becomes progressively harder as you
move higher in frequency and/or wish to cover a wide frequency range.

73 Phil...VK6APH







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