[hpsdr] Proposal for a 'low-end' RX/TX board for HPSDR

[brainerd at wildblue.net]

  The 74AUC2G53 looks like an excellent switch.  I am really excited to see what 
results Phil gets with it and the active integrator circuit.  One additional thing I noticed 
in a data sheet on the cmos switches is that the cross talk goes up a lot with a higher 
impedance load.  It also increases with frequency by 20dB per decade(according to 
the data sheet)! 

Dave - WB6DHW

> > Phil,
> >
> > Good idea to use that SPDT switch. No need for inverted pulses. I may
> > modify my schematic for that component, but not want to go to the
> > active integrators. Too many components!
> >
> > 73, Ahti OH2RZ
> 
> The active integrators are an attempt to provide a reasonably well
> defined range of impedance to terminate the BPF filters both in and
> out of band.  I also wanted to remove the relatively large capacitive
> load on the switch by the sampling capacitors in the QSD.
> 
> In the SDR-1000 QSD the sampling capacitors look like a short well
> outside the bandwidth of the QSD.  The series resistors before the
> switches are an attempt to provide a defined impedance looking into
> the QSD.  This terminates the preceding BFP properly but only out of
> band.  Within the bandwidth of the QSD, the sampling capacitor's
> reactance increases to infinity the closer you get to the sampling
> frequency (or DC).  The BPF does not have a well defined impedance
> looking into the QSD in band - it is actually very very high due to
> the input impedance of the instrumentation amp ( I think it is around
> 60 Mohm).
> 
> I did some experimentation with various analog switches with various
> capacitive loads to ground on one side of the switch.  Each different
> switch and even the same switch within the same family or part number
> behaved differently in the tests.  I could find no predictable
> response when the switches were operated at > 10 MHz and the
> capacitive loads were varied within the values needed for a 48-192 kHz
> BW QSD... it depended on the particular (physical) switch used.  This
> is why I wanted to avoid the sampling capacitors hanging off of the
> switches.
> 
> The fully differential amp in my circuit can be replaced by the OPA
> part the Bob uses... the AD8139 is probably overkill.
> 
> 73 de Phil N8VB


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