[hpsdr] HORTON

[KD5NWA]

The switch time is the killer, most FET switches can handle 100's of 
MHz but are slow at switching which makes them unusable.

Give consideration to using a Doubly Balanced Diode Mixer, they are 
inexpensive, and good to GHz frequencies. They have a 6dB loss but 
that can be made up in the pre-amp.

At 03:02 PM 6/10/2006, you wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>I've been doing some thinking about HORTON.
>
>It seems to me that the biggest problem with getting the QSD to run
>fast is finding switches/muxes that are fast enough.
>
>In my searching, the fastest CMOS FET mux I've come across to date
>is the TI SN74CB3Q3253.
>http://focus.ti.com/docs/prod/folders/print/sn74cb3q3253.html
>
>The problem with these isn't the "High-bandwidth data path (upto 500 MHz)",
>but rather it's the relatively slow speed at which you can switch
>them.  According the specs, this part maxes out at 20 MHz, but perhaps
>it could be pushed a bit past that.
>
>The ADG901 has a similar problem.  It's flat out past 1 GHz, but
>the typical switching time is 4.7ns (worst case 7.75ns: 129 MHz).
>
>I'm interested in getting a QSD running at at least 100 MHz, but why
>not try for 145 MHz while we're at it.
>
>To run at 100 MHz (passive radar application), the switches need to
>work at 400 MHz.  You're feeding it quadrature LO at the center freq,
>but the difference between I & Q is 90 degrees.
>
>Sythesizing the control of the switches shouldn't be a problem.  E.g.,
>use a DDS to generate the I & Q LO, filter, run that into one of the
>very nice ADI clock distribution parts (AD9511 and friends).  These
>will give us differential LVPECL out.  A bit of LVPECL fanout
>buffering and some AND/NAND ECL will generate the switch enable
>signals, no problem (modulo level translation).  We should be able to
>run this at 700 - 800 MHz, no problem.
>
>So, then we return to the original question: What's the fastest analog
>switch we can find?
>
>Are there faster FETs available in technologies other than CMOS?
>E.g., GaAs or SiGe?
>
>I don't think we need the signal path to be flat out past 150 MHz,
>but we do need them to switch quickly ;)
>
>
>Regarding the converter, the AD7762 (parallel) or AD7763 (serial) look
>like a better fit than the AD7760.  The '62 and '63 sample at 625kS/s
>(or lower, it's a delta-sigma part), with better dynamic range than
>the 7760.
>
>
>I'd like to end up with a system that contained 4 HORTONs, a board to
>generate the quadrature LO and sample clock for all 4 HORTONs, and
>some way to get it all back into the PC.
>
>Comments, suggestions, etc?
>
>Eric
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Cecil Bayona
KD5NWA
www.qrpradio.com

"Windows the worlds most successful software virus" 


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