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Softrock Lite 6.2
Adventures in Electronics and Radio
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K2 Pre-Selector Filter
To provide improved dynamic range, some spectrum
analyzers offer an optional "tracking pre-selector" input filter. The Z90
will normally be used with one or two receivers with a fixed IF frequency,
so it isn't necessary to provide a tracking pre-selector, but a pre-selector
filter between the IF output and the Z90's input can significantly improve
the Z90's ability to display weak amateur band signals by rejecting
extremely strong out-of-band shortwave broadcast signals.
I've decided to provide a 4915 KHz pre-selector filter
along with the Z90 (and Z91) if the purchaser will use the panadapter with
an Elecraft K2 receiver.
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| The filter design is a conventional
Butterworth coupled resonator, with matching to 50 ohms input/output
provided by C1 and C7. |
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Here is the assembled filter printed circuit board.
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The filter is mounted in a Hammond 1590A die-cast box.
This view shows the filter PCB mounted to the die-cast box case, ready for
the lower part of the case to be installed.
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A SPICE simulation of the filter. Small changes in
tuning capacitance will alter the bandpass response from flat to peaked to
rounded.
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The first printed circuit board filter, after tuning.
This filter is on its way to Stan, W5EWA, for user testing with his
K2/Z90 combination.
By tweaking the four trimmer capacitors I've been
able to flatten the bandpass and make it more symmetrical. Marker 1
is the nominal center, 4915 KHz, whilst markers 2 and 3 are -100 KHz and
+100 KHz from center, respectively.
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A wider view of the filter's bandpass shows the
ultimate attenuation is around 60 dB, not the 100 dB shown in the SPICE
simulation. The difference is leakage due to stray coupling. For its
intended purpose, however, the ultimate attenuation shown in the
measurement is more than adequate.
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| What does this filter mean in
practice? If, for example, your K2 is tuned to, say, 7200 KHz, the very
strong (at least in my location, near Washington DC) shortwave broadcast
station WWCR in Nashville TN at 7465 KHz will be attenuated nearly 20 dB
beyond the attenuation provided by the K2's input bandpass filter. This
extra 20 dB reduction will make a noticeable difference in the Z90's usable
dynamic range when looking at signals near 7200 KHz. |
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