Shielding the receiver

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    • #11817
      Admin
      Participant
      • Posts: 5952

      It occurred to me that painting, with a highly conductive paint, the plastic cover and the body of the enclosure surrounding the Receiver PCB MAY improve the receiver (rcvr) sensitivity. Reduced EMI/RFI increases the chances of the rcvr picking out the intended signals over the noise. The local paint store will probably not have the correct paint. Look at the underside of the plastic enclosure for a laptop; mine has a gold colored paint scheme.

      More tips for this test:

      (1) bring the antenna out the back. Get that darn wire outside the enclosure as soon as possible to reduce the amount of coupling of noise to the antenna, which is fed back into the circuit or worse radiated. I don t understand why they bring the antenna wire from the back to the front (underneath the PCB), then back accross the circuit board (on top) again to exit the car from the rear. To me that just increases the odds that high speed switching transients will be coupled onto the antenna, thus reducing the effective sensitivity of the rcvr.

      (2) place a wire from the newly shielded enclosure to the battery negative terminal of the enclosure. Make the wire as short as possible. An alternative, would be to put a capacitor from the shield to the negative terminal of the battery … careful … too small a value will not help and too large will start removing the good sh!t (RF signal).

      (3) perhaps an easier paint scheme would be to paint the underside of the body shell. This would not be as effective as the previous suggestion (bottom has no shield except the earth, but you connect to the earth; the car would not go anywhere, thus eliminating the need to increase range), however, I think it would be better than nothing. The problem would be connecting that shield to the battery terminal w/o making the body shell captive to the chassis.

      That s enough words/ideas …

      BTW: I am not an EMC (Electro-Magnetic Compatibility) engineer, it s just a problem with a design project I am leading at work. Not a bonified RF engineer either, I tried to avoid it in school. However, I am working on my full, white beard (I think at 12″ from the chin you would be considered an apprentice :smiley2:) and I haven t seen any flannel shirts in the store lately (I guess I will have to wait for it to come back into style).

      Thank you for your time. 🙂

      Regards.

    • #47026
      micro_Amps
      Participant
      • Posts: 1290

      b007a, what you are saying is correct, but the Tomy (& clones) Rx circuits are just not that carefully set up in the first place, and significant gains in range can be obtained with just few mods and a little tuning.
      Whilst the ‘super regenerative’ receivers are a simple and highly sensitive RF receiver, the assembly and tuning seems to be a bit ‘hit and miss’ from the factory, no doubt they are made at break neck speed.
      Appreciable range gains are possible just by doing one or more of the following,…..
      – remove the ‘cripple’ cap from the Rx (if it has one)
      – remove the cripple cap from the Tx (if it has one)
      – lengthen the Rx aerial (length, ie distance up from the ground is crucial, a length of Cat5 wire or #9 guitar string is excellent)
      – lengthen the Tx aerial (again, greater distance from the ground allows wider transmission radius), although a correct aerial replacement would also require the inductances in the final aerial circuit to be changed. The original wire aerial droops down toward the ground and by doing this reduces the range.
      – rutune the variable inductor on the Rx pcb. These seem to be only ‘roughly’ tuned and even if the aerial is not replaced, retuning this can increase the range.
      – increasing the power supply to the Rx pcb. A dual cell car, or a Rx pcb with the DC-DC converter changed to a 5V one, has significantly greater range.

      Conversely, increasing the Tx output does not really have a great effect on range, its the Rx sensitvity, that makes the most difference. A higher power Tx will make a difference but it also causes more interference to cars on nearby frequencies, which is undesirable.
      Sheilding the Rx from unwanted stray signals would be a benefit, but there are all the above ‘issues’ that can be addressed first(for little or no cost) that will add meters of distance to the range of a micro rc car.
      Recently I had a car here that the owner (Barto) complained of short range. It was true, the car only went about 1 to 2 meters before stuttering to a halt. All I had to do was remove the Rx criple cap and retune the vari-inductor and the range increased to the length of my house, (10 meters plus) and it took 5 minutes to do.
      :)uA

    • #47028
      ph2t
      Participant
      • Posts: 2088

      I’ve started looking into the idea of changing the transistor in the RX stage of the car’s pcb with one that has a higher beta value (thanks b007a). We’ll see what happens.

      One other thing I have noticed is that the straighter the car’s antenna (ie: nice and perpendicular to the ground) the better the distance achievable.

      ph2t.

    • #47055
      jamiekulhanek
      Participant
      • Posts: 2563

      Is the cripple cap in the car the little capacitor just in series with the antenna?? I am pretty sure it is but i just want to be safe.

    • #47056
      micro_Amps
      Participant
      • Posts: 1290

      No Jamie, dont touch that one or you wont be going anywhere. Its in parrallel with the larger 100uF electrlytic cap.
      :)uA

    • #47060
      Admin
      Participant
      • Posts: 5952
      Quote:
      I’ve started looking into the idea of changing the transistor in the RX stage of the car’s pcb with one that has a higher beta value (thanks b007a). We’ll see what happens.

      One other thing I have noticed is that the straighter the car’s antenna (ie: nice and perpendicular to the ground) the better the distance achievable.

      ph2t.

      ph2t,

      I was referring to the transistor in the transmitter. I don’t quite understand the super-regen circuit. I have searched the web for a good description and all the circuit topologies were different.

      I was thinking that increasing the Q of the tank circuit would increase selectivity. I believe C3 in parallel with L2 is the tank circuit. From my research the Q is determined by the parasitic resistance in the inductor. I don’t think I am going to persue it though … don’t have any details on the inductor. If someone had some spare time might want to put a resistor in parallel with the inductor to reduce the parasitic resistance … the value is prolly pretty small.

      Selectivity is not really a problem for me anymore … the car I was dinking with had too many problems. I could not tune it to a freq, in that car, far enough away from the standard values to be of any use. That car’s rcvr components were too close to the edge of usefulness.

      Anyway … I got some of my cars away from the standard freq’s so the whole family can play at the same time (four microSizers, four freqs). I have to get back to the EE work that earns money or I will not be able to afford anymore stuff for this interesting hobby.

      BTW: The OSC resistor (R9 in Rx & R3 in Tx) can be put in parallel with another 220kohm resistor to make sure the other cars don’t rcv the commands … not to say that the other cars are not jammed by the carrier … the other cars just don’t start acting on the commands. Some possibilities here … Jamming for one.

      later dudes.

    • #47097
      Admin
      Participant
      • Posts: 5952

      BTW folks the booster cars now have the pads on the PCB for the crip cap.

      I just started using the boost microSizers today, they refer to them as microSizer Pro’s.

    • #47099
      jamiekulhanek
      Participant
      • Posts: 2563

      Okay where is the cripple cap then? I WISSSSSSHHHH i could find a schematic for the Rx PCB.

    • #47111
      ph2t
      Participant
      • Posts: 2088

      Jamie. Go through the tech forum. Search for two threads, yo might have to go back 3 or 4 pages.

      “The crip cap in tha car mod”

      “more frequencies for cars”

      Both of these threads have been done by myself and both have the info you need. Sorry, too lazy to find myself and link them here.

      ph2t.

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