[YARDStick] TX / RX amplifiers on Yardstick

Tue Apr 25 09:02:57 EDT 2017

Hi Dominic,

Thanks for all the info.  I'm not a normal Kicad user, so I didn't catch
that 'hidden' part number info.

Looks like RX LNA amp is about 18dB of gain over the YARD Stick operating
frequency (and the 3.3V supply voltage).  The TX amp is about 15-16dB gain
over the YARD Stick operating frequency (and the 3.3V supply voltage).
Assuming the amp can be driven by the full output of the CC1111 without
distortion (+10dBm), that would imply that +25dBm total output power is
available (about 300mW).  That's pretty good.  Odd that these specs are not
detailed anywhere for the YARD Stick.

Regarding message receipt, for the 433MHz packets, I have settled on these
radio settings for best performance (everything else is rfcat
firmware/software default)...

.setBSLimit(BSCFG_BS_LIMIT_3)
.setMdmChanBW(250000)
.setFsIF(352000)
.setPktPQT(0)
.setRFRegister(TEST2, 0x81)
.setRFRegister(TEST1, 0x35)
.setRFRegister(AGCCTRL0, 0x91)
.setRFRegister(AGCCTRL1, 0x00)
.setRFRegister(AGCCTRL2, 0x07)

I've found that the bigger receive bandwidth filter is needed for frequency
drift of the transmitters.  Raising the IF level also helped.  The RF
register settings are valid ranges as per the TI OOK app note for the CC1111
(their exact example didn't work).  With the above, I am able to get
excellent 433MHz receipt from Oregon Scientific temperature sensors (long 32
bit preamble, 2400baud Manchester).

There was much trial and error arriving at the above.  The Yardstick setup
is great for transmitting, but receiving is quite challenging.  Two things
about the Yardstick firmware and rfcat software additionally complicate
things.  First, despite using the .setEnablePktAppendStatus function, there
is a bug in the firmware where it does not append the RSSI and LQI to the
message payload.  I've logged a bug report for this on the altas site, but
it hasn't been fixed yet.  So without RSSI or LQI (receive signal strength
indication and link quality indication), it is hard to get a measurable
indication of whether settings changes have made receive performance worse
or better.  Debugging, recompiling, and reflashing the dongle firmware is a
bit beyond what I'm able to easily tackle - the available documentation is
generally not agreeable to a Windows user as myself.

Second, rfcat doesn't provide a function to enable the TX or RX amplifiers
on the YARD Stick.  The .setMaxPower function just sets the TI CC1111 to max
+10dBm output level, it doesn't enable the TX or RX amps.  I didn't realize
this for some time until taking a closer look at the firmware and Python
rflib source code.  Some alternate settings could work with the RX amp
turned on.  Easy enough to add a couple Python functions to get the proper
USB commands to the Yardstick - I'm experimenting with the RX amp on/off
now.

Using the above 433MHz settings that I had arrived at, I figured I could
just change the frequency to 915MHz and change the baud rate to 32.768kHz to
work on receiving the AMR messages from my water meter (changing the sync
word and packet length of course as well).  This is where I'm running into
problems.  These messages come in great with the rtl-amr program which uses
the RTL-SDR dongle, but I am getting absolutely nothing when trying to do
something similar with the Yardstick.  These 915MHz AMR messages only have a
5 bit preamble, where the 433MHz temperature sensors have a giant 32 bit
preamble.  From what I've come to understand, the TI CC1111 has trouble
getting the AGC settled when there are less than 2 preamble bytes.  The
915MHz signal is also not very strong vs. the 433MHz messages, but I'm
basically very close to the transmitter and get nothing. I can go pretty
much anywhere in the house with the RTL-SDR receiver and get the readings
from the meter, and even others close by.

Thanks again for the info.  I'll continue to work on gain / AGC combinations
to see if I can get something that works.  If yourself (or anyone else) has
other suggestions on what can be tweaked to improve the receive performance
for these packets, it would certainly be appreciated..

Best regards,
Dave

 From: Dominic Spill [mailto:dominicgs at gmail.com] 
Sent: Monday, April 24, 2017 5:42 PM
To: Dave Wakelee
Cc: yardstick at greatscottgadgets.com
Subject: Re: [YARDStick] TX / RX amplifiers on Yardstick

On 24 April 2017 at 13:32, Dave Wakelee <dwakelee at rochester.rr.com> wrote:
>
> Does anyone know the details of the TX and RX amplifiers used in the YARD
> Stick One? 

RX: Infineon BGA 420 H6327
TX: RFMD SPF5043Z

> The schematic just has general placeholders (TX_AMP / RX_AMP)
> without part numbers, and there is no BOM / parts list on github (unlike
the
> other greatscottgadgets projects).

If you right click on the parts in the schematic and select "edit", you can
find the part details.  I'm not sure why we don't have a BOM in the
repository for this project.

> I'm asking as I'm having some trouble receiving certain low power 915MHz
OOK
> messages with the Yard Stick, whereas the RTL-SDR dongle gets these
messages
> without trouble.  OOK settings are per the TI app note, and I can receive
> other OOK messages (433MHz, long preamble) without issue.

Are you able to receive some of the 915MHz packets?  But not all?  Does
enabling the RX amp make any difference, or none at all?  How about for the
433MHz packets?

> Although I have no problems transmitting, I'd like to also understand what
> constraints and capabilities exist when the transmit amp is enabled (what
is
> the dB gain, what is the max output that the TI chip can be set to when
> using the amp, etc).  I've not seen either the TX/RX dBm levels documented
> anywhere for the YARD Stick One, either with or without the amplifiers
> engaged.
>
> I'm fairly certain that my receive problem has to do with analog and
digital
> gain (and AGC), so any info regarding the TX and RX amplifier details
would
> be appreciated.

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