- Sun May 30, 2010 3:51 am
#102091
I wanted to give folks a warning to watch out for ESD damage on the antenna input of those inexpensive ISM-band receiver modules (315/434MHz). I recently bought a couple which use the HiMark RX3400 chip. The manufacturer does not have anything on the module to protect against ESD and I was connecting the module to an external antenna through several feet of coax cable.
At first, the module worked nicely, but over time it became less and less sensitive as I removed and re-connected the antenna. Static damage. Eventually, it would only work when it was right next to the transmitter.
Luckily, I bought two of them. On the second one, I soldered two back-to-back microwave PIN diodes between the antenna input and ground. I used Avago HSMP-3812 diodes -- two series connected diodes in an SOT23 package. The center leg on one side can go to the antenna terminal, and the two pins on the other side are close enough together they can both be soldered to the ground pad next door. These diodes have very low capacitance and can withstand a decent static discharge (but would probably die under a really large zap). To be bullet-proof, you'd probably want to put a high frequency inductor (e.g. 120nH) between the antenna terminal and ground -- but make sure the inductor's self-resonant frequency is higher than 500MHz.
Anyway, the new module with the diode protection is MUCH more sensitive than the original one was which I think proves my point. If it seems like the receiver just isn't as sensitive as it used to be -- there just might be some ESD damage.
At first, the module worked nicely, but over time it became less and less sensitive as I removed and re-connected the antenna. Static damage. Eventually, it would only work when it was right next to the transmitter.
Luckily, I bought two of them. On the second one, I soldered two back-to-back microwave PIN diodes between the antenna input and ground. I used Avago HSMP-3812 diodes -- two series connected diodes in an SOT23 package. The center leg on one side can go to the antenna terminal, and the two pins on the other side are close enough together they can both be soldered to the ground pad next door. These diodes have very low capacitance and can withstand a decent static discharge (but would probably die under a really large zap). To be bullet-proof, you'd probably want to put a high frequency inductor (e.g. 120nH) between the antenna terminal and ground -- but make sure the inductor's self-resonant frequency is higher than 500MHz.
Anyway, the new module with the diode protection is MUCH more sensitive than the original one was which I think proves my point. If it seems like the receiver just isn't as sensitive as it used to be -- there just might be some ESD damage.