On Mar 14, 8:17 pm, "Wimpie" wrote:
Hi Paul,

About your 868 MHz project (probably european [ETSI] SRD application).

The datasheet gives impedance requirement that should function when
connected directly to 50 Ohms. I saw (little bit surprised) the
datasheet doesn't give a reference design.

If the design is to be terminated with 50 Ohms, and you run a coaxial
cable from bulkhead to PCB (both sides bread of cable connected), it
should be OK (irrespective of cable length). I assume that the antenna
that you are using is a 50 Ohms antenna (a quarter wave?). The antenna
will us the metal casing as ground plane avoiding RF currents inside
the housing.

Sometimes, circuits may become instable (out of band or just out of
band) when changing the cable length. You may check this by adding a 3
or 6 dB attenuation. Normally spoken, adding 6 dB will reduce the
range with about a factor 2 (assuming free space, not close to
ground).

Other things can be interference on the IF (directly onto the chip)
from a switching regulator or digital HW. Is it possible that you are
close to other applications (for example 2W UHF RFID, or UHF broadcast
stations). You should see that on the analog I&Q outputs, If you move
with the antenna, with no transmitter present, the output noise should
not change (so not become more or becoming less).

When this a commercial application, and have some budget, you may hire
a specialist. Many of the single chip TRX are difficult to get in
line with EN300220, though the datasheet does mention compliance.

Best Regards,

Wim
PA3DJS.

Thanks for the Info. We do use UHF and VHF systems as well however I
don't believe this is the problem here as turning of the transmitter
causes the IQ signals to go to close to DC. There is some ripple due
to ripplet from a switcher though we have reduced this to 5mA and a
large cap by the transmitter reduces this further. In addition the
switched is i a separate compartment to the transceiver.

Cutting the tracks at the tx module to the SMA connector and wiring
coax directly seems to do the job, so I guess that the PCB package did
not do a good job of calculating 50 Ohm. Mind you I think it was
probably my fault I should have copied the module and tracks were on
the inside surrounded by planes where as from the module to the SMA
connector the tracks were on the outer layers of PCB. I assume the
50Ohms would also be affected once the board was in the metal box.

Paul.