[TenTec] Orion - TT #218 1.8 Filter

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

N0KHQ wrote:
>I run 100% SSB and spend a lot of time on 75m. The use of the filter is
absolutely remarkable. Man, what a difference. If there is any
interfering stations that are close, engaging this filter wipes them out
100%.

        Inrad's entire article on Roofing Filters is worth reading,
but here are some excerpts which apply to what you observed:

Is an 8 pole filter necessary? How does a 4 pole filter compare? One difference
between the two filters is insertion loss. For a 500 Hz filter this 
difference can be
a difference of about 5 dB for a 9 MHz filter. The receiver overall gain 
should be
kept fairly constant as filter bandwidths change to preserve the agc 
characteristics
and to keep the S meter reading constant. Also, the receiver
noise floor can suffer if there is a gain reduction close to the front end. 
We need
to insert an amplifier or otherwise change the gain to make up for the 
extra filter
loss when a narrow 8 pole filter is selected. This can reduce the dynamic range
of the radio, as has been seen in the Orion performance numbers in the ARRL
review. So 4 pole filters have an advantage, particularly for narrow 
bandwidths,
even though the selectivity is not as good for signals falling down the skirts.
There is less advantage in going to a wider filter such as a 2400 Hz bandwidth.
For example, Inrad's 10 pole, 2400 Hz filter has an insertion loss of about 
2.2 dB,
while the 4 pole filter with the same bandwidth has a loss of 1 dB. The 
difference
of 1.2 dB is small enough that it could be ignored and the 10 pole filter would
provide better off-channel rejection. Thus for the SSB bandwidths a good 8 
or 10
pole filter will outperform a 4 pole filter, but for the narrow bandwidths 
the simpler
filter is best.

http://www.qth.com/inrad/roofing-filters.pdf (pages 2 & 3)

        Bottom line for Orion roofing filters,  8 or 10 poles for SSB are 
best and 4 poles
for CW are best, given the IMD problems introduced by the 500/250 
configuration.
George comments elsewhere in the article that most of the IMD rejection 
benefits from
CW roofing filters are achieved at -30 dB and above on the skirts due to 
the fact that
the following 3rd IF DSP filter can then handle any necessary remaining 
rejection.  Of
course if we could have a low insertion loss 8-pole 500 Hz filter, that 
would be ideal!
Unfortunately such a filter violates the laws of physics.

                                        73,  Bill  W4ZV




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