[TenTec] Inrad 762

Bill Tippett btippett at alum.mit.edu
Sun Aug 15 09:20:24 EDT 2004


Hi Buck,

K4IA wrote:
 >However, with the Filter selection set to AUTO, the visual indicator - that
little line above the filter bandwidth display still shortens at 360hz as if
the 500hz filter is cutting in.  It also shortens again at 190hz as if  the
250 filter was in although I have never owned one and neither is activated  in
the menu.   If a specific filter is selected, the line shortens to  match that
filter even if one is not installed.

         Which firmware version are you using?

 >Is this normal?  Does your display show a narrowing of the roofing  filter
even though there is no such filter?

         No, it's not normal.  It does not appear in the latest beta,
so it could be an older bug that has been corrected.  FYI to all,
you can tell whether filters are actually being switched in by either:

1.  Listening to a steady carrier (WWV or sig gen) as you rotate DSP with
Filter>Auto engaged.  You'll hear a small "pop" as the filter switches.
This should be coincident with a change in the roofing filter bar length
over the left BW display.

2.  If your ears are REALLY good, listen on a dummy load as you rotate
DSP.  You should detect a distinct change in the background noise character
when the filters switch.

 >My overall impression is that the 600hz filter works just fine but since it
is solving a pretty rare problem, it is hard to hear if it is any better that
the 500hz TT filter.

         This is normal.  You will not typically hear IMD products (sounds
like phantom bleeps and bloops on CW) unless the band is extremely crowded
with strong closely-spaced signals.  These are typically found only in major
contests or large DX pileups on the low-bands.  If you don't use gain antennas
on the high bands or don't operate much on 80/160, you will probably never
hear any difference.  This is why I previously said that, for average
operating, Orion does not need ANY optional filters.  In fact, the stock 1000
is much better than either the 500 or 250 for IMD suppression.

         Just to update the info I posted previously, Sherwood did
publish a measurement comparing the 500 Hz to the 1000 Hz for 1 kHz
interfering signal spacings.  IMD measurements at 1 kHz spacings:

1000 Hz filter - 85 dB
500 Hz filter - 72 dB

Source: http://www.sherweng.com/Dayton_2004/Dynamic_Range_Data.pdf

Sherwood later stated that the improvement using the 600 Hz is
"closer to 10 dB" over the stock 1000 Hz filter, which would put
it in the ballpark of 95 dB based on his measurements above.  23 dB
difference (95 vs 72) between the 600 and 500 is very significant
under severe contest or low-band interference conditions, but you
may never need it for average operating.

                                                 73,  Bill  W4ZV 



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