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

[Philip Covington]

On 7/27/06, Lyle Johnson <kk7p at wavecable.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> >> 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
>

Hi Lyle,

Interesting question...  I don't really know the answer myself.

When I simulate these circuits (which basically means nothing) using
an op amp model with a low GBW or slew rate I seem to get equivalent
results to using a high speed op amp.  From this you could conclude
that the op amp integrator only acts as a LPF and as long as the gain
is enough for the < 200 kHz bandwidth you are interested in, it will
work.  Where I am pretty sure it does matter is how the GBW and slew
rate of the op amp affects the impedance looking into the RF input
port over the whole operating range of the switch (0-50 MHz say).
Since this is what we are really trying to control then an answer to
your question is very important.

Obviously the way to resolve this is to build up the circuit, try
different op amps for the integrator, and make measurements.
Simulation is not very useful for this... :-(

73 de Phil N8VB

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