[hpsdr] Odyssey Active QSD Rev. A Schematic

[Ahti Aintila]

Steve (and the Gang),

It seems to me that your circuit should work as such. Here are  just
few comments to be taken with a grain of salt, because I am an analog
designer from the vacuume tube era:
1. Keep the extra component pads for easy changes during the prototyping.
2. 74HC74 may be too slow for 42.76 MHz, try 74LVC74A.
3. According to my quick checking of truth tables, maybe, Q-Clock is
leading 90° to I-Clock. It should be opposite. In any case, it is just
a matter of reversing the output wires.
4. R5 and R7 should match the input impedance to 50 ohm. The exact
value depends on the actual duty cycle of the sampling pulses, the
internal resistance of the switches and the values of R6, R3, R8, R21,
R18, R23. Possibly some of these resistors should be removed in the
final design. Hoever, due to any parasitic coupling of switching
spikes and internal capacitances of the semiconductor components and
circuit board traces, maybe that R5 and R7 are partly or totally
tranferred after the swithes to the places of R3, R8 and R18, R23
respectively. R6 and R21 may be unnecessary.
5. It is useful to have the output capacitances C4 and C16 for
improving the filtering of the  transients that may get coupled in due
to the eventual unbalance of the 0.01µF capacitors.
6. If the common and/or differential mode components in the output I
and Q signals are problems, output coupling capacitors may be used.

I hope, these comments help.
Good luck and 73,
Ahti OH2RZ


On 29/09/06, Steven Bible <srbible at earthlink.net> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
>
>
> Gang,
>
> Here it is, revision A version of the schematic using Ahti's active QSD circuit connected to a TI Codec.  The TI codec will be connected to a dsPIC33FJ256GP710.
>
> You will notice that I added some extra components and labeled them with ? values.  These parts are just a guess as to weither they are needed or not.  I thought to place the foot prints for prototyping purposes.  However, I do know the circuit well, so I'll allow the group to comment.  If they are not needed at all, I'll removing them.
>
> Comments please.
>
> - Steve N7HPR
>
>
> \r\n- Steve\r\n  (n7hpr at tapr.org)\r\n
>
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