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

[Lyle Johnson]

>> 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.

 > I figured out that the OPA1632 is a combination of two inverting
 > amplifiers, so why not they could not work as two integrators.


What bandwidth/risetime impacts are there on the op-amps used as the 
integrators in this topology?  If, for example, the switches are running 
with a 30 MHz clock, do the op-amps have to be very wide bandwidth to 
effectively integrate the input?  Are these characteristic of the op-amp 
similar to the QSD switching time issues?

I note, for example, that the AD8139 has a -3dB bandwidth of 410 MHz and 
a slew rate of 800 V/usec, so I  would expect it work well at HF, 
probably acceptably all the way to 30 MHz.

The OPA1632, on the other hand, has a Gain/Bandwidth product of 180 MHz 
and a slew rate of 50V/usec.  I suspect this part would be sub-optimal 
for use as the QSD integrator at the higher end of the HF spectrum.

73,

Lyle KK7P


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