Antenna over ground plane -

In a two way radio (frequency range from 403-470MHz), when a 100pF
capacitor was being placed on the microcontroller reset line, the
radio was having problem to transmit at 436.025MHz. There was actually
no signal being transmitted out when the antenna was radiating over a
ground plane. No power could be seen from spectrum analyzer. If the
cap was replaced with other values, no problems were seen at that
frequency. Why? It only happened when transmitting over the ground
plane. Did the frequency transmitted from antenna cause somekind of
resonance with 100pF at reset line? But RF resonance would not affect
the DC level of the reset line right? Reset line is active low. High
voltage is 3.3V.

If 436.025MHz is the resonance frequency, then from the resonance
formula, we could derive the parasitic inductance of the cap. And
assuming the resonance was the culprit, we would see the same problem
if other cap values were used, depending on the self resonance
frequency. Let's say another cap cause a self-resonance frequency at
416.025MHz. But there's no problem seen at this transmission freq.

 

If the cap is resonant at that frequency in combination with a circuit
parasitic inductance, that will result in an r.f. a.c. voltage on the uP
reset line. If the negative peaks of this voltage, superimposed on the
d.c. level, result in a low enough level, this might cause a reset.

That shouldn't be difficult to diagnose. If the system goes through its
power-on boot up sequence, suspect the reset line. Throw a scope on that
line and see if a signal is present. You might have to be pretty quick
with the trigger, since it seems that the side effect of the reset is
the transmitter being shut down.

The resonant frequency is dependent on circuit parasitics in addition to
your cap and might not move to exactly the point you calculated. You
might have to inject r.f. into the circuit and sweep the frequency
around to hit the new resonant point.

 

Another thing he could do is to bring the power up slowly, while looking at
the reset line with a 'scope. Of course, there is the problem of the probe
adding to the capacitor value.

Tam

 

Another approach might be to scope the reset pin with an attenuator in the Tx
line to the antenna. Operating it below the threshold of interaction - if there
is a threshold - may well expose the mechanism.

 

Something else to consider: Excessive RF level on any IC pin can cause
damage and/or destruction.
The equivalent of a parallel resonant circuit being connected to an IC
pin can do this nasty (and maybe perplexing) damage.