[hpsdr] Hermes-Lite 2 and the Hermes Redesign with 5W Amp

[Steve Haynal]

Hello List,

Since there has been recent discussion on this list regarding a Hermes
redesign with 5W PA and we are wrapping up something very similar with the
Hermes-Lite 2, I thought it would be a good time to reintroduce the
Hermes-Lite 2 and also share some experiences. The Hermes-Lite firmware
shares roots with the Hermes, and we are very grateful to the openhpsdr
community for making their firmware opensource and giving us a leg up.
Perhaps there are ways now for the Hermes-Lite 2 to give back to any Hermes
redesign. Maybe just a few ideas from the Hermes-Lite 2 may be of interest,
or maybe people would be interested in the entire Hermes-Lite 2 design, or
maybe most of the Hermes-Lite 2 design but with higher performance ADC and
DAC.

You can find all things related to the Hermes-Lite at www.hermeslite.com.
Version 2 is a full DDC/DUC 5W HF SDR transceiver based on an inexpensive
mixed-signal frontend from Analog Devices, the AD9866. Our goal is an
inexpensive (target of <$200 for complete transceiver) yet decent HF
performance transceiver. This is how we differ from the "high performance"
goals of this list and why I forked 3 years ago. Although our ADC and DAC
are 12-bit, I think most would be hard pressed to notice much difference in
day to day use. I've used a Hermes in head to head comparisons. We are
finalizing a beta3 that will be available this summer and expect a final
version to be available soon after. Over 100 Hermes-Lite v1s have been
built. Almost 10 of the new Hermes-Lite 2beta2 boards have been built.

Here are some possible areas of overlap and interest with any Hermes
redesign:

* 5W PA. As discussed for the Hermes redesign, the Hermes-Lite v2 already
has a 5W PA based on some of the newer LDMOS devices from NXP. We are using
the AFT05MS003N:
http://www.nxp.com/products/rf/rf-power-transistors/rf-mobile-radio/3-w-cw-over-1.8-941-mhz-7.5-v-wideband-rf-power-ldmos-transistor:AFT05MS003N

We see >7W out raw and 5W with filters. Claudio Girardi, an RF test and
design engineer for Intel, has done numerous tests. You can find many of
these tests on his website:
http://www.qsl.net/in3otd/ham_radio/Hermes-Lite/Hermes-Lite.html

He also posts extensive and invaluable measurements to our Google Group:
https://groups.google.com/forum/#!topic/hermes-lite/AaX0BLAPdKY

Although this LDMOS device uses a lower power supply (7.5V to 9V), it is
inexpensive and overall was cheaper to go with it and add another switching
power supply. The lower voltage means less heat to dissipate, although some
would argue that IMD is slightly affected. Also, it is now a more
deterministic design as the PA voltage will not vary with whatever power
supply a user connects.

* Temperature sensing. We have inexpensive temperature sensing on the HL2.
This allows us to monitor how effective heat dissipation from the PA is and
to lower or shut off power if anything is too hot.

* PA Bias Current sensing and digital POT. We also have inexpensive PA
current sensing and digital POT adjustment. This turned out to ne of
equivalent expense to a manual trim POT. This allows software to set the
bias. In the future, we plan to experiment with dynamic adjustment and
tweaking of the bias.

* KSZ9031. We use the KSZ9031 rather than the KSZ9021 as in the original
Hermes. This newer version is less expensive and requires less power. We
run it at 2.5V.

* We use a newer switching power regulator, the ST1S10. This part is
inexpensive and readily available in China. This device also has the option
to provide a the switching clock as an input. We have this connected to the
FPGA if needed, but have found that no noticeable noise is being generated
at the default switching frequencies. Again, refer to Claudio's
measurements on the Google group. The supported switching frequencies are
high enough to allow for smaller less expensive parts, and also to miss any
amateur radio bands if harmonic spurs show up.

*100% opensource. The Hermes-Lite is not a commercial endeavor. We hope to
have several manufacturers make and sell the Hermes-Lite to drive down
costs, similar to other opensource hardware projects such as the Arduino
and those sold at Adafruit. All Hermes-Lite design files are posted on
github. This includes the PCB and other hardware design pieces which are
sometimes not fully shared in an "opensource" hardware project, for example
the Raspberry Pi or the original Hermes. Furthermore, we use opensource
tools for the design. This ensures others can easily take on and fork the
project. We use http://kicad-pcb.org/ for the PCB design and I would
encourage openhpsdr designs to start using this instead instead of the
suggested Altium.

We also use opensource myhdl, Python and Icarus Verilog simulator for
simulating pieces of the design. See
https://github.com/softerhardware/Hermes-Lite2/wiki/Receiver-Simulation . I
have not seen much simulation of openhpsdr design and such simulation is
useful to have. All commercial FPGA users I work with will simulate.

It would be nice to be truly 100% opensource with the tool chain and use
Yosys (http://www.clifford.at/yosys/) for synthesis, but we do use Intel
FPGAs and the free Quartus Lite edition. Although it is available to me
through my day job, I steer clear of using any paid version of Quartus as
it raises the entry bar for other amateurs.

* Intel FPGA. We use a Cyclone IV FPGA in the Hermes-Lite 2 because it is
inexpensive and readily available in China. We considered the MAX10, but
variants of the MAX10 do not support automatic programming of ROMs during
configuration and this would have required some big changes and
inconveniences to now load the filter coefficient tables.

We do not use any form of bga packaging so that builders can still build
the HL2 at home if desired.

There are two very interesting FPGA families on the horizon that we are
watching for future projects: the Spartan7 family from Xilinx and the
Cyclone 10 family from Intel. My contacts at Xilinx and Intel have informed
me that 25K LE versions of these FPGAs (the capacity we are interested in)
will sell for ~$20 in very small quantities. This is a significant price
reduction from the $35 we pay for a comparable Cyclone IV if bought at
Mouser now.

* Raspberry Pi. There always seems to be interest in connecting a SDR to a
inexpensive SBC such as the raspberry PI. The radioberry project:
https://github.com/pa3gsb/RadioBerry
which spun out of the Hermes-Lite project does that.

73,

Steve
KF7O
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