[TenTec] Omni Vi+ 1.8 kHz filter response (2nd IF @ 6.3 MHz)

[Phil Erickson]

Hi all,

  If this list will indulge a further curiosity:

  With my VI+ back together, I measured the filter response of the two SSB
filters @ 2nd IF : the default one (2.4 kHz nominal) and the one that I
thought was 1.8 kHz (supposed to be Ten-Tec #288).

  Procedure for each filter:

- Injected a test tone from signal generator at a known frequency
- set the radio for LSB
- AGC off
- slowly move the tuning dial upwards 100 Hz at a time away from the
frequency
- record freq offset and approximate tone amplitude in V using a Tek scope
(1 Mohm input) clipped to the External Speaker leads
- plotted 20*log10(V) as a function of frequency

  To my surprise, the 1.8 kHz has almost exactly the same response width as
the 2.4 kHz.  It makes me think that someone accidentally installed a 2.4
kHz in that position, as I definitely expected the 1.8 to be narrower.

  Is there any marking on the standard TT filters that allows you to tell
them apart?  I couldn't find any, but I didn't lift the filters off the PBT
board to see the underside.  (I note with some interest as well that Inrad
didn't bother to make anything narrower than 2.1 kHz for the 2nd IF.  Was
SSB somewhat of an afterthought on this radio?)

73
Phil W1PJE