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

[Jerry]

Hi Ahti

	Looked at your schematic and have a question. 

	  Why are you biasing the inputs above ground with R1 and R2? The OP amps
are balanced around ground by the +- 5 volt supply. This means that there
a loss in dynamic range for positive going signals. Also as pointed out on
this reflector -5 volts is no longer being supplied by the computer power
supplies. Thus an addition source of 5 volts would be required.

	Keepng the offset and running the op amps between +5 and ground seems OK.

	73 de Jerry NO2T

	 


At 07:09 PM 7/27/2006 +0300, you wrote:
>***** High Performance Software Defined Radio Discussion List *****
>
>On 27/07/06, Philip Covington <p.covington at gmail.com> wrote:
>
>> 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
>>
>Phil,
>
>You are talking so nicely about the active integrator, so I have no
>alternative  but admit. Being a simple and silly person myself, I love
>KISS. Here is my latest simple schematic
>diagram:<http://kotisivu.dnainternet.net/ahti/sdr-1000/newmix4.pdf>
>
>I figured out that the OPA1632 is a combination of two inverting
>amplifiers, so why not they could not work as two integrators.
>Unfortunately, I am still waiting for the ordered components to come,
>so no tests made yet. This is just a paper exercise. If this idea
>works the QSD (and QSE likewisw) can be put to a really small area.
>
>By the way, I like isolating tranformers to separate the dirty digital
>world from the sensitive analog circuits.
>
>What do you say, and what am I missing this time?
>
>73, Ahti OH2RZ
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