My not-so-supercharged PL-380

gary_thorburn <gwt@...>

Having some trouble 'supercharging' my Tecsun PL-380.  Opened it up, disconnected its internal ferrite antenna and connected a new one I wound on an 8-inch R40C1 ferrite rod from Ming Mak.  The signal strength was greatly improved over the internal antenna.  However, it is picking up noise which I believe is produced by the LCD display and its associated circuitry.  The lead connecting to AN1, which I think is the 'hot' lead, seems to be the one picking up the noise.  I played with the routing and the length of these leads, but could only slightly improve the situation.  Even tried a short piece of litz headphone cable for the internal routing.  Though not truly shielded, with this cable the AN2 lead closely hugs the AN1 lead.

The noise is an audible whine.  On a stock PL-380, as with some other radios, this can be heard on a weak MW station by grasping the top of the radio, apparently conducting the noise to the ferrite antenna.  Furthermore, I believe this noise confuses the Si4734 AGC, worsening the 'pumping' audio level which I find to be a common annoyance in DSP radios.  I was hoping the stronger signal would actually alleviate this. 

For now, I've reconnected the internal ferrite.  I wonder why the stock coil and its leads do not pick up the noise as much?  Maybe because of its lower impedance? 

Any ideas/comments are welcome!    A few more details below, for those interested:

  • Coil wound with 10/44 Litz.  Not ideal, I know, but its what I had on hand to experiment with, and has worked nicely on passive ferrite antennas I've made. 
  • About 120 turns, spread over about 60% of the 8-inch ferrite length.  Lots of turns, since R40C1 has lower permeability than Amidon 61.  My coil was checked by an RF engineer on an HP induction analyzer that cost way more than my car.  At 1MHz:  430 microhenries, Q=400. 
  • My coil is wound in the same direction as the stock coil, though I don't know if this is called a left-hand or right-hand helix, or if that would affect noise pickup anyway.
  • PL-380 purchased late 2018, SN: 36920181006731
Gary Thorburn 

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