[CQ-Contest] Diode-Generated Crud

[Marijan Miletic, S56A]

Single Si input protection diode would start conducting around 500 mV which
is 59+80 dB of a local signal.  Such a signal would cause a lot of RX crud
even without any diode.

It is a good engineering practice to have all AC rectifying diodes blocked
by suitable capacitors in order to reduce inherent avalanche noise.

73 de Mario, S56A, N1YU
  ----- Original Message -----
  From: Silver Ward
  To: Reflector
  Sent: Wednesday, September 26, 2001 3:37 PM
  Subject: [CQ-Contest] Diode-Generated Crud



  Many rotor control boxes and other antenna-related accessories also have
  unbypassed rectifiers in them that can generate harmonics and mixing
  products.

  As K8CC points out, the products can also cause TVI for nearby neighbors
  because they are in-band signals to the TV and can NOT be filtered out
with
  a high-pass filter.  A spectrum analyzer sweep of your output signal will
  show it to be clean, but the products can definitely be caused by your
  transmit signal .  Unbypassed diodes in the neighbor's equipment can also
  cause the products - disconnect antenna or other cables to see if a device
  (even one that is off) is causing the problem.

  Most single-op stations do not have to be concerned (except about TVI)
  because the products are only generated when the transmitter is on (and
the
  receiver is off).  However, if you are considering SO2R or a multi-station
  operation, this can be a real problem.  Not only will a transmitted signal
  generate harmonics in the diodes, but more than one transmitted signal
will
  mix and generate all sorts of intermod that can't be cured with stubs and
tx
  output filtering.

  In my case, the culprit was a T2X control box generating crud on 160
through
  40-meters.  I live near several AM BC stations and there was enough RF on
  the control cable to generate substantial intermod products without my
  transmitter being on.  I only discovered this when I noticed that the
  products were greatly reduced when I rotated the antennas - which either
  cuts off or turns on the rectifiers enough to get them out of their
  non-linear region.  The cure was to bypass each line in the rotor control
  cable to the chassis with a .01uF/50V (anything from .001-.1uF will do)
  capacitor and then to ground the chassis.  I did this on all three control
  boxes in the shack and have also made sure that all relay kickback diodes
or
  current steering diodes are bypassed in all antenna or rotor accessories.

  Band-pass filtering of a receiver's front end may not cure in-band mixing
  products from protection diodes.  Strong local signals on the same band
will
  mix in the diodes and generate odd-order intermod products.  (This isn't a
  problem except for multi-station environments or for those with nearby
  strong stations.)  Grain-of-wheat light bulbs are usually sufficient and
are
  sufficiently linear to avoid generating nasty mixing products.

  73, Ward N0AX

  ----- Original Message -----
  From: k8cc <k8cc at mediaone.net>

  > While the circuit G4BUO described will most certainly protect the
receiver
  > front end, it will also likely have the nasty side effect of generating
  > spurious signals all over the spectrum.  When the diodes conduct from
the
  > RF coming in the antenna, the spurious signals are re-radiated out the
  > antenna.
  >
  > Back when I first got going at this QTH, I had a 2L 40M quad switchable
  > NE/SW using a relay on the tower between the loops.  When the quad was
in
  > the default (relay relaxed) position, there was a S9+20dB harmonic on
  > 14.200 (not even the band of the quad!) which absolutely disappeared
when
  > the quad was switched SW (relay actuated).  The problem turned out to be
a
  > diode across the relay coil to protect against "flyback" transients when
  > the relay was de-energized


 


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