Separation of RF board and microcontroller board caused prob

D

Dummy

Guest
As an attempt to shrink the size of radio, RF and microcontroller
board are separated with a flex PCB, instead of residing on the same
PCB. RF ground is connected to microcontroller ground through the flex
PCB. Besides ground, the are still a number of pins connected from RF
to microcontroller board. Both PCBs share the same 7.5V supply.

Separation of RF board and microcontroller board has caused a problem
which certain amount of distortion was measured from the transmitted
DTMF tone. A check on the radio without RF and microcontroller
separation showed that the distortion level was far lesser, 13% vs 5%.

The distortion disappeared when separate power supply was used for RF
and controller board. Can somebody shed some light on this?
 
Dummy wrote:

As an attempt to shrink the size of radio, RF and microcontroller
board are separated with a flex PCB, instead of residing on the same
PCB. RF ground is connected to microcontroller ground through the flex
PCB. Besides ground, the are still a number of pins connected from RF
to microcontroller board. Both PCBs share the same 7.5V supply.

Separation of RF board and microcontroller board has caused a problem
which certain amount of distortion was measured from the transmitted
DTMF tone. A check on the radio without RF and microcontroller
separation showed that the distortion level was far lesser, 13% vs 5%.

The distortion disappeared when separate power supply was used for RF
and controller board. Can somebody shed some light on this?
Click to expand...
Welcome to the world of mixed signal design where GND is not GND,
but a few mV above plus some noise on it depending on the spatial location.
To start with, you first have to know what exactly you're measuring
where and how.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
 
What thickness of copper was used on the flex board ?
What's the power consumption of the radio-part ?

Possible solutions (one or all):
Use a stranded connection for ground connection between both boards, better
ground connection between both boards
Use extra caps on the power lines on the boards (say 1nF for RF in parallel
with say 47uF for power stability)

Ultimate solution
Revise the layout, putting power-consuming parts closer to power supply
(or power the boards from the side of the most power-consuming end)



"Dummy" <ahkit1021@yahoo.com> wrote in message
news:74bb84c0.0409010237.43c4b745@posting.google.com...
As an attempt to shrink the size of radio, RF and microcontroller
board are separated with a flex PCB, instead of residing on the same
PCB. RF ground is connected to microcontroller ground through the flex
PCB. Besides ground, the are still a number of pins connected from RF
to microcontroller board. Both PCBs share the same 7.5V supply.

Separation of RF board and microcontroller board has caused a problem
which certain amount of distortion was measured from the transmitted
DTMF tone. A check on the radio without RF and microcontroller
separation showed that the distortion level was far lesser, 13% vs 5%.

The distortion disappeared when separate power supply was used for RF
and controller board. Can somebody shed some light on this?
 
Probably the ground trace on the flex isn't wide enough and now the
power trace acts as a ground connection as well. This can introduce
noise if the power supply isn't very clean or becomes modulated by heavy
load changes. Such load changes can happen when, for example, memory
banks on the controller are read or written to.

Bottomline you may have to add ground connections, ideally a plane
although that isn't easy on a flex. Also, look at your power supply with
a scope set to AC. Check it down to the millivolt level.

Regards, Joerg

http://www.analogconsultants.com
 
ahkit1021@yahoo.com (Dummy) wrote in message news:<74bb84c0.0409010237.43c4b745@posting.google.com>...
As an attempt to shrink the size of radio, RF and microcontroller
board are separated with a flex PCB, instead of residing on the same
PCB. RF ground is connected to microcontroller ground through the flex
PCB. Besides ground, the are still a number of pins connected from RF
to microcontroller board. Both PCBs share the same 7.5V supply.

Separation of RF board and microcontroller board has caused a problem
which certain amount of distortion was measured from the transmitted
DTMF tone. A check on the radio without RF and microcontroller
separation showed that the distortion level was far lesser, 13% vs 5%.

The distortion disappeared when separate power supply was used for RF
and controller board. Can somebody shed some light on this?
Click to expand...
Is it FM? What is the transmitter output power, and where is the
antenna? Where is the circuit generating the RF output? Is this a
transceiver ... receiver? Please give more information. Maybe you can
get more specific help.

You may be dealing with issues like injection locking. Changing layout
may not be enough.

Regards,

Frank Raffaeli

http://www.aomwireless.com/
 
Joerg <notthisjoergsch@removethispacbell.net> wrote in message news:<llqZc.14217$5J5.2209@newssvr27.news.prodigy.com>...
Probably the ground trace on the flex isn't wide enough and now the
power trace acts as a ground connection as well. This can introduce
noise if the power supply isn't very clean or becomes modulated by heavy
load changes. Such load changes can happen when, for example, memory
banks on the controller are read or written to.

Bottomline you may have to add ground connections, ideally a plane
although that isn't easy on a flex. Also, look at your power supply with
a scope set to AC. Check it down to the millivolt level.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
I've checked the power supply with scope before. There were some
noises on the line, with Vp-p ranged from 150mV to 200mV. These noises
were found on 'normal radio' (without separation of RF and
microcontroller) as well. But amazingly, there was no DTMF distortion
seen in 'normal radio'.

A tweaking on audio PA to reduce its gain helped, but the audio volume
was slightly low from rated audio. Distortion was mostly seen when
audio knob was turned to max.

Audio PA resides in microcontroller board.
When audio PA supply (7.5V) was connected with separate external
supply, no problem was observed. By disabling the speaker from audio
PA, there was almost no distortion seen at DTMF tone.
The crux is, audio PA supply. Additional info is the audio PA circuit
was similar with 'normal radio'. Can the problem occurs when RF and
controller share similar supply? In 'normal radio', they are sharing
same supply as well, but no problem was seen.
 
Hi,

150mV is a whole lot of noise. When you split the portions with a flex
and there isn't too much ground in the flex then part of the ground
return will inevitable be via the supply. That's when some of this 150mV
noise can be modulated back onto RF signals, clocks, PLL loops and so on.

Can you try to regulate the voltage going to the DTMF circuitry, or to
all the circuitry except for the PA? The idea would be to feed the PA
raw power and then the rest of the unit gets clean regulated power. You
can use a low drop-out regulator so you don't lose much in voltage. It
looks like there is some noise caused by the PA stage that ends up on
the supply rail and then enters the rest of the circuit. Alternatively,
you could try to decouple the PA but that's harder. There you either
need lots of capacitance and there may not be enough space for that, or
some kind of nifty current regulator that evens out the spikes being
sent from the PA.

One test I'd do for sure and it's easy: Lay a short and very wide copper
path, for example 3M copper foil, from one board to the other and see
whether the DTMF distortion drops to acceptable values.

Regards, Joerg

http://www.analogconsultants.com
 
Joerg <notthisjoergsch@removethispacbell.net> wrote in message news:<alMZc.15094$vL1.14839@newssvr29.news.prodigy.com>...
Hi,

150mV is a whole lot of noise. When you split the portions with a flex
and there isn't too much ground in the flex then part of the ground
return will inevitable be via the supply. That's when some of this 150mV
noise can be modulated back onto RF signals, clocks, PLL loops and so on.

Can you try to regulate the voltage going to the DTMF circuitry, or to
all the circuitry except for the PA? The idea would be to feed the PA
raw power and then the rest of the unit gets clean regulated power. You
can use a low drop-out regulator so you don't lose much in voltage. It
looks like there is some noise caused by the PA stage that ends up on
the supply rail and then enters the rest of the circuit. Alternatively,
you could try to decouple the PA but that's harder. There you either
need lots of capacitance and there may not be enough space for that, or
some kind of nifty current regulator that evens out the spikes being
sent from the PA.

One test I'd do for sure and it's easy: Lay a short and very wide copper
path, for example 3M copper foil, from one board to the other and see
whether the DTMF distortion drops to acceptable values.

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
Finally, the root cause of the distortion had been found.
There were four screws on the radios. The screws serves the purpose of
holding radio PCB and chassis together. When one of the screws was
removed, somehow, no distortion was seen at DTMF tone! That screw
(contacted to ground) resided very near to 7.5V supply line in third
layer of PCB layout. But I was still a bit bewildered on what happened
actually. Could you please shed some lights?
 
Dummy wrote:
Finally, the root cause of the distortion had been found.
There were four screws on the radios. The screws serves the purpose of
holding radio PCB and chassis together. When one of the screws was
removed, somehow, no distortion was seen at DTMF tone! That screw
(contacted to ground) resided very near to 7.5V supply line in third
layer of PCB layout. But I was still a bit bewildered on what happened
actually. Could you please shed some lights?
Click to expand...
The four mounting posts aren't coplanar, so when you tighten the fourth
screw, it warps the board.

Cheers!
Rich
 
Rich Grise <null@example.net> wrote in message news:<r3d_c.743$fF2.454@trnddc03>...
Dummy wrote:
Finally, the root cause of the distortion had been found.
There were four screws on the radios. The screws serves the purpose of
holding radio PCB and chassis together. When one of the screws was
removed, somehow, no distortion was seen at DTMF tone! That screw
(contacted to ground) resided very near to 7.5V supply line in third
layer of PCB layout. But I was still a bit bewildered on what happened
actually. Could you please shed some lights?

The four mounting posts aren't coplanar, so when you tighten the fourth
screw, it warps the board.

Cheers!
Rich
Click to expand...
I don't think that's the problem. When washer or insulator was mounted
on the screw ground pad, no DTMF distortion was seen after the fourth
screw been tightened.
 
Hi,

I don't think that's the problem. When washer or insulator was mounted
on the screw ground pad, no DTMF distortion was seen after the fourth
screw been tightened.


That sounds like either the screw may be touching some traces down in
Click to expand...
the PCB or, more likely, you have a serious case of a ground loop. Do
both boards have a full ground plane?

Regards, Joerg

http://www.analogconsultants.com
 
Joerg <notthisjoergsch@removethispacbell.net> wrote in message news:<HCp_c.15883$AN4.10024@newssvr29.news.prodigy.com>...
Hi,

I don't think that's the problem. When washer or insulator was mounted
on the screw ground pad, no DTMF distortion was seen after the fourth
screw been tightened.


That sounds like either the screw may be touching some traces down in
the PCB or, more likely, you have a serious case of a ground loop. Do
both boards have a full ground plane?

Regards, Joerg

http://www.analogconsultants.com
Click to expand...
The screw only touches the ground pad at first layer.
No full ground plane is used. Insulation of screw from ground pad or
removal of screw can solve distortion problem. It's still a mystery to
me.
 
I read in sci.electronics.design that Dummy <ahkit1021@yahoo.com> wrote
(in <74bb84c0.0409051903.1e90a0f@posting.google.com>) about 'Separation
of RF board and microcontroller board caused problems', on Sun, 5 Sep
2004:
The screw only touches the ground pad at first layer. No full ground
plane is used. Insulation of screw from ground pad or removal of screw
can solve distortion problem. It's still a mystery to me.
Click to expand...
It's almost certainly a ground loop. But exactly what is happening
depends on the details of the schematic and PCB layout, which is not
practicable on the net.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Hi,

The screw only touches the ground pad at first layer.
No full ground plane is used. Insulation of screw from ground pad or
removal of screw can solve distortion problem. It's still a mystery to
me.


I agree with John, this sure sounds like a ground loop. It can be very
Click to expand...
hard to make a RF/controller combination work reliably without a full
ground plane. I would probably spring for a relayout with a full ground
plane even if that means one or two more layers on the boards. The other
issue with this unit might be EMC, especially with a plastic enclosure.

Regards, Joerg

http://www.analogconsultants.com
 
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