[Linrad] Re: Future SDR Developments

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Guy,

> 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 am pretty sure you could easily lock these units with
injection locking. Make a PLL that locks a crystal to
66.666666666666 MHz and couple some (small) power at that
frequency capacitively to one or the other pin of the crystal 
inside the SDR-IQ. (I do not say it is a clever way of locking
the unit, but it is surely possible - and technically uncomplicated)

> 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?
Yes. But that is not enough.

The downsampling has to be synchronized as well and one will
need a way to synchronize the USB transfers also.

> 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.
Yes. But there is no reason that the transmitter has to be sold
by the same manufacturer as the receiver;-)

> 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?
Yes. I think so.

I also think the other manufacturers RFspace and Microtelecom are
well aware of the merits and complications of a two channel
(or multichannel) receiver.

I have a feeling we will not have to wait many years for this
kind of product to become available:-)

73

Leif



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