Best design for connecting LEA-M8T module to active antenna

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I am embedding a LEA M8T module into a timing measurement system.  Since maintaining a good sky view will be problematic, I have opted to use an active antenna instead of a passive one.  In looking over the LEA-M8T Hardware Integration Manual, there are two circuits that appear applicable.  They are shown in figures 9 and 11 (pages 16 and 18).  - both use VCC_RF to provide antenna power.  The circuit shown in figure 11 is simpler - no blocking cap and no inductor.  But, if I understand the manual correctly, the antenna power supervisor circuit needs to be enabled for this circuit to work.  I have an M8T eval kit and have looked over the schematic carefully.  It seems to be wired as per figure 11  - but instead of using VCC_RF, it implements a separate regulated voltage source.  Does that imply that the eval kit has antenna power supvervisor enabled?  or maybe I don't have the correct picture of just what the supervisor does......

=> So, my question is the following:  If I want to use the default LEA M8T configuration on power up, can I use the circuit shown in fig. 11? (in other words, is the antenna power supervisor active by default...if I understood that correctly?)

Or, is there a reason to prefer the circuit shown in fig. 9 for a simple embedded solution using an active antenna?



by rtisys asked Jan 7
0 votes
Firstly - you should always use an active antenna with the M8T timing receiver since the system noise figure is reduced and precise timing requires the best signals possible - which cannot easily be achieved with a passive antenna.

The inboard antenna sensing circuit is disabled by default according to section 1.5.5 of the H.I.M.  the command UBX-CFG-ANT configures the receiver appropriately.

The power supervisor can be configured to detect open or short (ANT_DET_N configured),  or open circuit, overcurrent, normal mode, no VCC (four states require another input sense pin to your MCU).

The underlying expectation and most common use case for M8T timing receivers is that they are always on hence antenna on-off control is not required.  This allows the simplest design and allows use of external voltage supplies to power the more exotic roof-top-mount robust antennas that require more than a 3V supply. However, the LEA-M8S, also covered under this H.I.M, is expected to operate in power-save modes in which case the power to the external active antenna can be turned off when the receiver does not need GNSS signals.

In a less secure environment, (e.g. in remote rural cell sites where the timing antenna is sometimes used for ignorant target practice) the comprehensive antenna detection circuit is more critical.

The EVK implementation may be overkill for your application - it uses an external defensive current-limiting circuit that survives accidental application of a passive patch antenna in which the internal design includes a DC short between feedpoint and ground.  It also buffers any surges when an external antenna is disconnecte and reconnected.

In the simpler figure 11, the 10 Ohm resistor provides some protection but short-circuit can be up to 330 mA while my EVK appears to limit the overcurrent to about 70 mA.
Figure 9 is the simplest implementation for an slways-on M8T with all components under your control, in a low-risk environment.
by grampy answered 6 days ago
by rtisys selected 2 days ago
Best answer
Thanks for such a through answer.  Had not considered the problems of a user connecting a passive antenna to my connector.  Don't like the idea of 300+ma passing through the module - even if the power supply ultimately limits or shuts down.

I am thinking about incorporating both figure 11 and EVK design onto initial PCB design and populate to enable either.  This is mostly for debug purposes and to guarantee that initial boards are at least usable in a "benign, protected" environment.

I know that the antenna input PCB trace geometry is critical, but - with reasonable (?) care - is there a chance of getting it right the first time?  I plan a short connection to a SMD u.fl connector.  Alternatively, I would need to consult an experienced RF PCB designer which is not really in my budget.  Any pointers would be apprecated.

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