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Experimenting the Mobile Bug Circuit
I emailed you some time ago about the circuit and suggested that it would be a good idea to reduce the supply voltage so that it could be powered from a Li Po battery (3.7V) and therefore integrated into a handheld type product.I have carried this idea even further and developed the complete circuit on a PCB measuring25 x 50mm, using the same fundamental design as the mobile bug. I incorporated a few changes to make the circuit LiPo battery powered and stable as the battery voltage drops from 4V to 3V. The design works very well on a solderless breadboard prototype (quite surprisingly!)but i have had problems with the pcb implementation. Basically, the PCB works perfectly in close
proximity to a transmitting DECT phone source (2.4GHz) but has very low output with a WCDMAmobile phone transmitting in equally close proximity. This tells me that my pcb implementation is tuned to too high a frequency (>= 2.4GHz) with very low output at WCDMA ( ~ 2GHz). I therefor need to re-tune for approx 2GHz but i have quite limited RF experience. I did not expect it to work
perfectly on the PCB implementation but i also did not expect it to work so well at 2.4GHz DECT frequency but not 2.0GHz WCDMA. I have attached the design with pcb layout and hope you can possibly suggest some changes to try to bring the sensitivity back down to the lower frequency.
Thanks for the mail. I expect this kind of experimentation to make a circuit more reliable and perfect. As you know, the 0.22 capacitor is the tricky component of the circuit. I have found out this by testing with a number of caps.0.22 capacitor is a large sized one with large surface area and long leads. The important thing I found out during experimentation is that, the frequency detection varies with the lead length of the cap. I tried to get 2.5 GHz by trimming the lead to 18mm length. If it is 20 mm or 16 mm, the frequency detection changes.So experiment with 0.22 cap by trimming its lead length. You can fix it as 2GHz.
Thanks very much for your help in trying to trouble-shoot the design
and the very useful document.
I was working late last night after i sent my email to you and actually got
the PCB implementation to function nearly as good as the solderless breadboard
I was so puzzled that the solderless and PCB versions both seemed to work well with the DECT phone but not the WCDMA mobile that i placed both solderless breadboard and PCB version together side-by-side on top of the mobile phone. Viola! Both started to work!I removed the solderless breadboard and the PCB version still worked. I then discovered that the PCB version had to be placed in very close proximity to the antenna on the mobile phone(usually at the top) and i had just so happened to have set it up this way while testing with the solderless breadboard.
I now understand more about how it works and can explain the differences in our PCB designs which are probably giving rise to differences in operation.
a) I use a 100nF disc ceramic, not being able to easily source a 220nF. I guess the larger diameter 220nF will probably pick up more signal and increase the detection signal and range.
b) On my PCB, the 100nF disc leads are cut very short, soldered to the end of the pcb trace (marked CAP) and the pcb trace acts as the 18 x 8mm capacitor leads. This seems to work very well (as proven by the 2.4GHz DECT!) but capacitor is in the horizontal plane of the PCB so appears to make the overall detection less sensitive and i need to place the GigaHz antenna directly above/close to the WCDMA mobile antenna. In your version, the capacitor is mounted vertically and positioning may not be as important (as i found out from the same tests carried out with the solderlessbreadboard – vertically mounted 100nF capacitor).
c) I have top/bottom layer ground planes on my pcb.
d) I use the CA3140 instead of CA3130 and have a different biasing circuit to comply with LiPo battery powering. The spec for CA3130/40 says it is guaranteed to work at Vs = 4V but i found that they are operational down to about 3.2V which is fine for LiPo battery.
Hope i have been able to give you more insight into your original design. As you rightly say, you can only really learn by experimenting and trying things out. Where would we be if Maxwell did not play with electro-magnets or Edison with electricity and filaments?. Hope you continue to make more interesting discoveries.