[TenTec] guide to adjusting Orion receiver; filters and performance

[Bill Tippett <btippett@alum.mit.edu>]

K3NA wrote:
>Your guide also mentions the problem of 500/250 Hz roofing filters coming 
after an amplifier stage.  According to my
understanding, when the 500 or 250 Hz roofing filters are employed, the 
signal is routed first through the wide L-C filter (I think
it's 12 kHz?)... then through this amplifier that causes the problems... 
and then into the 500 or 250 Hz filter... and finally
through another amp to make up for the filter loss.

>   I have just received my Orion recently, and haven't taken it apart to 
examine the PIN diode control signals to confirm this
report.  But, if it's true, then the following two potential modification 
may correct the problem:

>   1) Change the routing of the PIN diode control signal, so that the 1 
kHz filter position is selected instead of the L-C filter
position in the first stage.  This could be done in firmware.

>   2) Replace the L-C filter stage with a 600 Hz bandwidth 4-pole filter 
centered on 9000.75 kHz.  One may surrender the ability to
transmit wideband modes such as AM.  For many of us, that is no sacrifice.

        Eric, we've discussed this before but I don't think you asked what you
really meant to ask, at least if you did it's different than what we've 
discussed.
What I believe you meant to ask Sinisa is, what would be the effect of 
cascading
500 Hz filters, with the first being the one you described in place of the wide
filter (which in some places is called 15 kHz and other places 20 kHz!) and the
second being one in the optional 500 Hz slot.  I'm sure this is what you meant
because both of these must be at 9000.75 for your scheme to work correctly.

        If you meant you really only wanted one 500 Hz filter and one 
amplifier
contributing to IMD, then the Inrad #352 is the answer since it is at 
9001.5 MHz
and uses only the first amplifier.  BTW, it is most likely the SECOND amplifier
is causing the problems (not the first as you stated above) since Orion's 
excellent
IMD is achieved with only the stock 1000 Hz filter and first 
amplifier.  Actually
the problem may really be the combination of 2 filters and 2 amplifiers instead
of one.

        The problem I see with cascading as (I think) you propose is the 
following:

1.  2 filters and 2 amplifiers are used.  All 4 of these components can
contribute to IMD.  A better approach is 1 filter and 1 amplifier as
the 1000 Hz uses or the Inrad #352 600 Hz will use.  The reason the
#352 does not need another amplifier is that it is 4-pole, like the
1000 Hz, and therefore does not have high insertion loss which requires
another amplifier stage like the Ten-Tec or Inrad 8-pole filters, yet
it has sufficient rejection since shape factor is not a critical issue
for roofing filters (unlike the application of standard Ten-Tec or Inrad
500/250 8-pole filters in the Omni series).

2.  You are also cascading group delay for 2 filters.  If you look at
Sinisa's measurements on page 5 of his document, you can see that two
500 Hz filters might have fairly high group delay.  I'm no expert on
the effects of group delay, but I believe it can cause serious problems
when impulse noise hits the filters.  Maybe Sinisa will comment on the
effects of group delay, noise, etc.

        Anyhow, correct me if I'm wrong, but I believe you meant to ask
Sinisa his opinion of two cascaded 500 Hz filters, one in the 20 kHz
position (which is on the board and not a plug-in) and one in the
optional 500 Hz slot.

                                                        73,  Bill  W4ZV

P.S.  One last thought...I would not count on Ten-Tec being willing
to do a firmware mod for your application, since it would then not
allow users to use FM.  I suppose they could do a custom mod for you
but be prepared to pull out your checkbook, and be prepared to wait
awhile since they have their firmware dance card fully booked at
the moment. 
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