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Analog Filtering
Reinhard Keller
This is a little overview of what is done and what is planned against RFI with analogfilters at the Effelsberg Telescope.
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Analog Filters are everywhere
In the frontends:– at RF stages
– at IF stages
– at DC
In the backends– at IF level
– at DC
Besides many classical filters a lot of shielding is used to avoid RFI. That is whatindustry calls EMC, electro-magnetic compatibility. Filters are also incorporated in theDC connectors.
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RFI at 21cm in Effelsberg:
The need of RFI mitigation is obvious! Small bands are reserved for astronomy butwideband observation are wanted. Only a few interferers are known. Most of themare changing with time and also in frequency. This delimits hardware filtering
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Fixed Filters:
Frequency domain: – Bandpassfilter
– Bandstopfilter (Notch)
(Time domain)
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Fixed Frequency Filters
Before LNA:1. Avoid saturation
(„robust receiver“)
2. Select desired frequencies
→ low loss → high gain stability
After LNA:1. Select desired
frequencies
2. Avoid saturation(„robust receiver“)
→ high selectivity → high gain stability
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Fixed Frequency Filters
are
simple
Need no info about RFI
but
Seperation of RFI ⇔ signal in frequency domain
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Example 1: 1.9cm Receiver
Realized at the Effelsberg telscope
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13 GHz High Pass Filter
-80
-60
-40
-20
0
10 12 14 16 18 20
f/GHz
S/d
B
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13 GHz High Pass Filter
Insertion loss S21-Betrag
-0.5
-0.4
-0.3
-0.2
-0.1
0
10 12 14 16 18 20
f/GHz
S/d
B
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13 GHz High Pass Filter
waveguide realization:
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„Adaptive“ analog filters
Switched filters:– Realized in Effelsberg
Prooved, but after LNA!
Tunable filters: – Waveguide filters;
Mecanicaly tuned in dewar
– HTSC-filters (Uni Wuppertal);Electr. Tuned by ferro elektric – magnetic material
Piezo electric tuning
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Example 2: 21cm Receiver
Realized filter bank after the LNAFolie 13
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Example 2: 21cm Receiver
Planned implementation of High Temperature Superconducting Filters. The filtersare realized.Folie 14
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1st HTSC Filter: Notch at low end
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1st HTSC Filter:
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Example 3: 5cm Receiver
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Corrugated bandpass filter
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Corrugated bandpass filter
Fcenter=6.2GHz, RBW=21%; simulation results:
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CMT + Bandpass integrated
Currently under developmentFolie 20