[hpsdr] Hermes update

[Kevin Wheatley]

Since the announcement of the Hermes project there has been quiet a  
bit of traffic ‘in the background’, in summary the hardware details  
are now reasonably stable. What follows are some snippets of traffic  
to provide readers with some insight to the development process ......

Phil VK6APH has provided several suggestions, namely:

There needs to be an audio out from the TLV320.
JTAG - the JTAG programming can come from the FX2, perhaps we have a
standard JTAG header and jumpers to connect the relevant pins on the FX2
so we can use either?
Need to show the LPF at the RF input.
We could take the forward power from the 15w PA as well since I think
there are enough spare inputs to read all we need.
ICD header needs to connect to ADC78H90.
If there are spare inputs should we bring them out - perhaps for use  
with an external amplifier etc?
122.88MHz clock directly feeds DAC.
Phil is also trying to find a packaged LPF so we can get rid of the  
toroids on
the DAC output.
Add a header for I2C signals from the FX2 to control external equipment.

Graham KE9H has made an interesting suggestion regarding the gain  
distribution of the existing Penelope 1/2 W PA strip :

As far as the gain re-distribution to get the transmit frequency  
response flat out to
50 MHz, you basically change the transformer T3 from a TC4-1T to a  
TC1-1T
(which will maximize linearity of the AD9744,) and add a second OPA2674
in series with the current OPA2674 OpAmp, and redistribute the gain  
between them.
The current OPA2674 is being run at 24 dB gain, and is hitting gain- 
bandwidth
limits around 30 MHz. With two in series, they would each be set for  
around
12 dB gain, and the combination should be flat well over 60 MHz.

Graham is also investigating the use of a THS6012 as a substitute PA,  
this provided a very clean output, at a higher level, a test board has  
been drafted and a few boards have been ordered to enable to testing.  
I’m not sure this chip will form part of Hermes as the supply  
requirements are beyond the current spec, but I guess the final  
decision will be made once the test results are in :)

While I’m discussing the PA strip, I must reiterate that the  
PennyWhistle PA is really just a dream for Hermes, I really don’t  
think there will be enough room on the board, however I will squeeze  
everything one first, and if there is room, PennyWhistle will be  
considered.

Graham has also suggested some form of audio PA be included, this has  
now been added to the hardware block diagram.

Scotty WA2DFI, has made several suggestions and between us we have now  
settled on some of the connecting hardware. To save board edge space a  
number of the connectors are now ‘stack’ connectors. Scotty has also  
made a good start on the schematics, I’m not going to provide a  
guesstimate for release for comment, mainly due to Scotties Dayton  
commitments, but it’s going to be weeks not months :)

Phil N8VB has provided some valuable information, namely:

Take a look at how I connect the FX2 to the FPGA using port E for 8  
bit parallel configuration.  After configuration I use this as the 8  
bit communications bus between the FX2 and the FPGA.  This allows  
faster configuration of the FPGA and faster read/writing to the  
registers in the FPGA (especially for tuning word).  You might want to  
use this scheme in Hermes.

Also, instead of using an Altera configuration device (especially for  
the larger '40 Cyclone III), consider using a SD card which can have a  
serial interface to the FX2.  The FX2 can read the SD card and  
configure the FPGA via the parallel mode mentioned above.

Phil’s advice regarding the use of port E will be incorporated in  
Hermes. The move to an SD card is still in the balance as board real- 
estate is of a premium and I don’t understand / can’t see any benefits  
for the change? Can someone out there enlighten me?

Phil N8VB, has also enlightened me on capability of the FX2 (although  
I’ve had to delve into the technical reference and ‘Google’ to  
‘partially’ understand USB end points):

The FX2 has 4 high speed endpoints.  This is enough for full duplex  
operation of the receiver and transmitter via USB.

The current command and control scheme will need to be restructured to  
achieve this and is well worth pursuing.

Well I guess that pretty well brings everyone up to date with the  
Hermes story so far, so in summary, the hardware is close to being  
finalized, Scotty has made a start on the schematics, and I’m climbing  
the learning curve of OrCAD, in preparation to layout the board :)

Kevin - M0KHZ

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