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[Linrad] Re: Future SDR Developments

The appears to be a misunderstanding.   The Mercury A/D is operating at 
130 msps and it has capture aperture well above the nyquist rate for 
bandpass sampling.  As Leif says for the other models mentioned,  a 
bandpass filter is required to eliminate aliasing and because you are 
using aliasing as your friend here,  you have a fall off of sensitivity 
as you go higher in frequency.  For effective operation,  they need a 
bandpass filter and a preamp to lower the system noise figure.

Mercury is good for DIRECT SAMPLING to about 55 Mhz and bandpass 
sampling much higher.  The limits depend on your construction of the 
required front end to make it useful and the specs, which may be found 
from linear technologies for the LTC2208.


You can see from here:


that it has full power operating bandwidth (responds to) signals up to 
at least 700 MHz according to the data on the URL.  There is NO 
filtering in front of the A/D and the buffer amplifier (LTC6400-20) is 
good to well over 1 GHz.


VK2KU wrote:
> Hi Leif,
> Thanks for your useful comments.
> Yes, I was aware that the SDR-14 will work at up to 200MHz using its
> direct input, and presumably the alias response of the unit with
> appropriate filtering.
> However the SDR-14 (and SDR-IQ) are not built in a way which enables
> GPS locking of the sampling frequency, and the error in this (in my
> case) is some 300Hz.
> I do not really have the technical background for detailed discussion
> of this, but it seemed to me that if we have 2 identical Mercury
> boards, both locked to the same GPS signal, then for all practical
> purposes it is as if they had a common LO. Is that indeed the case?
> The other lack in the SDR-14 is that it is a receiver only, with no
> provision for transmitting on 144MHz. A transmit function is necessary
> if we are to dispense completely with a 28MHz transverter.
> I appreciate your comments on the needed transfer rate of 2x1MHz. I
> had not understood that.
> And yes, of course it would be necessary to synchronize the transfers
> from the 2 Mercury receivers. I have no idea what is involved in that,
> but has anyone actually communicated these needs to the project
> leaders in the HPSDR project?
> 73  Guy  VK2KU

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