Eric Scace 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 made a mistake by blaming additional amplification
for degrading IM performance, without any detailed insight.
Just opened schematic and the box itself and verified that signal path is as
follows:
preamp Q27 (if engaged)
first mixer Q1/Q2/Q3/Q4
post mixer amp Q24
2-pole filter X1/X2
hardware noise blanker gate D1/D2/D3/D4 (if engaged)
=> one of 1/1.8/2.4/6/15 kHz filters (15 kHz if 500/250 Hz selected)
=> postfilter amp Q9
=> through or (250/500 Hz filter + postfilter amp Q10)
hardware AGC PIN diode attenuator D4/D5/D6/D7
second mixer Q9/Q10
cascaded 455 kHz filters FL5/FL1, etc.
Additional amplification (Q10) appears AFTER 500/250 Hz filters and
therefore cannot be blamed for degrading IM performance,
nor it can help noise figure/sensitivity, but of course
it can maintain proper signal level at ADC input.
I performed the following test:
two signals -43 dBm each (S9 + 30 dB) at 7001.0 and 7001.7 kHz to ANT1 input
preamp off, attenuator off, 1 kHz roofing filter selected
main RX to ANT1, sub RX to ANT2
At 1 kHz filter input IM level was undetectable.
The 6 kHz filter was then inserted into 1 kHz slot and at RFIC output
IM was -68 dBc, translating into input-referred IP3 of only -9 dBm.
Therefore, your assumption is correct: Q9 seems to be creating IM
(not claiming it's the transistor itself, but it's clearly that stage).
This is an instance of a long-standing mistake of building weak stages
after the first mixer. Unfortunately, Orion is no exception,
it can mask the problem to some degree by selecting 1-6 kHz filters,
but the emperor appears completely naked with 500/250 Hz filters.
> 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.
None of these can correct the real problem - overload of Q9,
although some IM products may be masked out by using 1 kHz filter
in front of 500/250 Hz, which doesn't appear as a significant
improvement over 1 kHz filter alone.
500/250 Hz filters can be promoted to real IM-fighting devices only
by removing the essential cause, which can be done in two ways:
a) insert them before Q9, that is where other filters are, or
b) improve Q9's large signal handling capabilities.
73,
Sinisa YT1NT, VA3TTN
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