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[Linrad] Re: Future SDR Developments

Hi Guy,

> It is no more than 2 years since the SDR-IQ appeared, following its
> sibling the SDR-14, with a clock frequency of 66.7MHz allowing direct
> sampling at up to 30MHz.
Oooh! The SDR-14 works nicely up to 200 MHz. You just have to
put a selective amplifier in front of it to remove aliasing and
(of course) use the direct input.

> The Perseus SDR soon followed with direct sampling up to 40MHz.
> I have been following the HPSDR project with considerable interest:
> http://openhpsdr.org
> The Mercury receive module has direct sampling up to 65MHz, already
> more than double the SDR-IQ.
> And there is a transmit module Penelope to match, with a GPS locking
> module Excalibur soon to be released.
> Surely in a year or so, two at the most, we will have direct sampling
> at 144MHz, with no further need for transverting down to 28MHz, just a
> need for a bit more front-end gain.
We already have that. SDR-14 or a modified Perseus.

> Already I can see the shape of my next major 144MHz upgrade:
> Penelope transmit board with direct output at 144MHz,
> Twin Mercury receive boards, sampling directly at 144MHz, one for each
> polarity,
> and Linrad as the main receiver, followed by MAP65.
I am afraid that the units are not designed for that. It will be
necessary to synchronize the the digital downconversion FPGAs
and to provide for synchronization of the USB transfers as well.

> USB3 will be the standard for interconnection long before then.
Well, I do not think it is wise to force everyone to buy new hardware.
USB 2.0 will be adequate for most applications.

The reason Linrad is not working with the openhpsdr hardware is: "Perhaps
the most exciting of all the modules, the Mercury module will enable direct 
sampling of the 0-65 MHz spectrum. Based on a Linear Technology LTC2208 
130MSPS 16-bit A/D converter, the board will contain it's own FPGA to 
undertake Digital Down Conversion (DDC) to 250 kSPS or less for transfer 
over the Atlas bus to the USB interface on the OZY board."

To become interesting the OZY board would have to combine the synchronized
output of two Mercury boards and send the two I/Q data streams at a user
selectable bandwidth to the PC. I would not invest my time in it unless
the maximum bandwidth is at least 2x500kHz. 2x1MHz should be no problem
since Perseus gives 2 MHz on a single channel. The high bandwidth is 
required for the removal of static rain.

I am pretty sure someone will make such hardware available and I will
buy one unit and make sure Linrad works with it as soon as I get a chance:-)

> Doubtless there will be minor problems with sampling frequency being
> incompatible with MAP65,
> but such details should not present any major difficulties.
Yes. We can do fractional resampling in the PC. (Linrad already has the 
code for that but I am not aware anyone tested it with MAP65.)

> Do others share this vision?
In a broad sense, yes:-) But differ in some details...




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