[NLRS] Diode detector questions

[Mike KBØozn]

   Hi all,
   I  have a question about diode detectors.  I was checking out a few of
   them  that  I've collected over the years and they behaved differently
   in  a  variety of ways.  All units were N style except for 1 which was
   SMA.   I  was using a Wavetek frequency generator at 550MHz 1000 Hz AM
   mod 100%.
   I  forget  the  model  number, but it has a digital display and allows
   adjusting the output in tenths of a db.
   I  was  measuring  them  with my HP415D as the 415E was buried under a
   bunch of stuff.  I would start at about -55dbm for a level and set the
   415 to 50db.  I'd then ramp up the signal level until I could zero the
   415  and I would then decrease the signal until I had adjusted it down
   10  db  and would watch the meter from step to step and note the final
   reading.
   Here's what I found:
   1    Aerotech  tunnel  detector      No output up to 0dbm, either it's
   not a diode detector or it's broken.....
   2    Hp420  #1                              Behaves in a smooth normal
   way with moderate sensitivity -37 to -47dbm
   3   Hp420 #2                              Works over the same range as
   #1 but the needle jitters (why is this????????)
   4    Hp420  #3                                Needle jitters, it's non
   linear,  and  needs about 15db more signal -22 to -32 Is this unit out
   of the sq law region?
   5    Hp423                                      Needle jitters, but is
   more   sensitive.    -40dbm   to   -50dbm  (again,  what's  with  this
   jittering????)
   6    Unknown  brand                      Behaves like a 420 #1 but the
   needle jitters (why the jitter?)
   7    Wavetek unit                          Sensitive, no jitter, works
   great.
   8    Litton  uW SMA unit               This is new old stock and works
   great, no jitter, and very sensitive
   So,  given  all  of  that, my biggest questions are, WHY the jitter???
   Whats  that  all about?  How or why can a diode jitter???  Jitter here
   means that the needle shakes back and forth by a small amount.
   Is  the  number 1 unit even a diode detector?  If not, what is it?, if
   it  is,  are  these  things repairable by putting a new diode in them?
   Can new diodes even be purchased these days?
   On number 4 the the 420 #3, I'm assuming that the diode is 'partially'
   broken  and thus the need for more signal.  Again, is this something I
   can fix?
   Thanks for all your thoughts guys.
   Mike
   KB0ozn