1415-1425MHz Cavity Filter
Alex P
Robert Hamers
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Andrew Thornett
Sent: Saturday, March 7, 2026 2:09:25 PM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: [SARA] Re: 1415-1425MHz Cavity Filter
Andrew Thornett
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: [SARA] Re: 1415-1425MHz Cavity Filter
b alex pettit jr
Andrew Thornett
An anti-aliasing filter is a filter placed before a signal is sampled by an analog-to-digital converter (ADC) to remove frequencies that are too high to be represented correctly after sampling.
Why it is needed
When converting a continuous signal into digital samples, the sampling rate limits the highest frequency that can be represented. This limit is defined by the Nyquist–Shannon Sampling Theorem.
- If the sampling rate is Fs, the highest recoverable frequency is Fs / 2 (the Nyquist frequency).
- Any signal component above Fs/2 will appear falsely as a lower frequency after sampling.
- This distortion is called Aliasing.
The anti-aliasing filter removes those high-frequency components before sampling, preventing them from folding into the measured spectrum.
What the filter usually is
Most commonly it is a low-pass filter:
- Passband: desired signal frequencies
- Stopband: frequencies above the Nyquist frequency
Example:
| Sampling Rate | Nyquist Frequency | Anti-Aliasing Filter Cutoff |
|---|---|---|
| 10 kHz | 5 kHz | ~4–4.5 kHz |
The cutoff is set slightly below Nyquist to allow the filter to roll off.
Simple diagram
Analog signal │ ▼ Anti-Aliasing Filter (Low-Pass) │ ▼ ADC (Sampling) │ ▼ Digital Signal
Real-world examples
Anti-aliasing filters are used in:
- Digital audio recording (before ADC in microphones or audio interfaces)
- Software defined radio receivers
- Oscilloscopes
- Image sensors (optical low-pass filters in cameras)
- Scientific instruments
Example in radio astronomy
If a receiver samples at 2 MHz, any signals above 1 MHz baseband must be suppressed. Otherwise strong RFI above that frequency could alias into the band of interest and appear as false spectral lines.
ANTI-ALIASING FILTER
An anti-aliasing filter is a filter placed before an Analog-to-Digital Converter (ADC).
Its purpose is to remove signal frequencies that are too high to be correctly represented after sampling.
If those high frequencies are not removed, they appear as incorrect lower frequencies in the digital data.
This effect is called aliasing.
- Why aliasing happens
When a signal is sampled, the sampling rate limits the highest frequency that can be measured correctly.
Nyquist frequency = Fs / 2
where
Fs = sampling rate
Example
Sampling rate Fs = 2 MHz
Nyquist frequency = 1 MHz
Any signal above 1 MHz will appear at the wrong frequency after sampling.
Example of aliasing
Sampling rate = 2 MHz
Nyquist frequency = 1 MHz
Suppose a signal exists at 1.3 MHz.
Aliased frequency can be calculated as
f_alias = | f_signal - n * Fs |
Choose n = 1
f_alias = | 1.3 - 2.0 |
f_alias = 0.7 MHz
So a signal at 1.3 MHz appears in the spectrum at 700 kHz.
What the spectrum would look like
Actual signal:
1.3 MHz (outside the observable band)
Observed spectrum after sampling:
0 Hz ---------------------- 1 MHz * 700 kHz (false signal)
This false signal is the alias.
- Purpose of the anti-aliasing filter
The anti-aliasing filter removes frequencies above the Nyquist frequency before the ADC samples the signal.
Signal chain:
Analog Signal | V Anti-Aliasing Filter (Low Pass) | V ADC | V Digital Signal
- Example for an SDR receiver
Suppose:
Sample rate = 2 MHz
Nyquist frequency = 1 MHz
We want to observe frequencies from
0 Hz to 900 kHz
The anti-aliasing filter should therefore suppress frequencies above about 1 MHz.
Target:
0 - 900 kHz pass band
1 MHz strongly attenuated
- Simple RC anti-alias filter
Cutoff frequency formula
fc = 1 / (2 * pi * R * C)
Example
R = 180 ohms
C = 1 nF
fc approx 884 kHz
However, a single RC filter only rolls off slowly (6 dB per octave), so strong interference may still leak through.
- Better solution
Most receivers use higher order filters such as
Butterworth low-pass filter
Typical design
Order: 4 to 6
Cutoff: about 0.9 * Nyquist frequency
Advantages
- flat passband
- steep attenuation above cutoff
- much better rejection of unwanted signals
- Typical receiver chain in radio astronomy
Antenna | LNA (low noise amplifier) | RF bandpass filter | Mixer or downconverter | Anti-alias low-pass filter | ADC or SDR
- Why this matters in radio astronomy
Without an anti-aliasing filter:
- strong transmitters outside the band
- can fold into the observed spectrum
- appear as false spectral lines
- corrupt long integrations
This is particularly important when observing weak signals such as the 1420 MHz hydrogen line.
ANTI-ALIAS FILTER DESIGN FOR A 1420 MHz SDR RECEIVER
In hydrogen line radio astronomy the receiver usually converts
1420.405 MHz down to baseband before sampling.
Example receiver parameters:
RF frequency = 1420.405 MHz
Sample rate (Fs) = 2 MHz
Nyquist frequency = Fs / 2 = 1 MHz
Therefore only frequencies between
0 Hz and 1 MHz
can be represented correctly.
To avoid aliasing we normally design the anti-alias filter slightly below Nyquist.
Typical design target:
Passband : 0 to 900 kHz
Stopband : above 1 MHz
Example simple LC low-pass filter (3 pole)
L1 L2
Input ----coil----+----coil----+---- Output | | C1 C2 | | GND GND
Example component values for about 900 kHz cutoff:
L1 = 22 uH
L2 = 22 uH
C1 = 1.2 nF
C2 = 1.2 nF
Characteristics:
Passband: flat to about 800 to 900 kHz
Rolloff : about 18 dB per octave
For stronger RFI rejection, a 5 pole filter is better.
Example 5 pole low-pass filter
Input --L1--+--L2--+--L3-- Output | | | C1 C2 C3 | | | GND GND GND
Typical values for about 900 kHz cutoff:
L1 = 18 uH
L2 = 27 uH
L3 = 18 uH
C1 = 1.5 nF
C2 = 2.2 nF
C3 = 1.5 nF
This gives much better suppression near Nyquist.
Typical SDR receiver chain
Antenna | V LNA | V RF Bandpass Filter (around 1420 MHz) | V Mixer / Downconverter | V Anti-Alias Low Pass Filter (about 900 kHz cutoff) | V ADC / SDR
TABLE OF ALIAS FREQUENCIES
This table shows where signals above Nyquist appear in the spectrum.
Example sample rate:
Fs = 2 MHz
Nyquist = 1 MHz
Real Frequency Observed Frequency
1.1 MHz 0.9 MHz
1.2 MHz 0.8 MHz
1.3 MHz 0.7 MHz
1.4 MHz 0.6 MHz
1.5 MHz 0.5 MHz
1.6 MHz 0.4 MHz
1.7 MHz 0.3 MHz
1.8 MHz 0.2 MHz
1.9 MHz 0.1 MHz
ASCII visualization
Real spectrum (before sampling)
0 ----1 MHz----2 MHz | real signal 1.3 MHz
Observed spectrum (after sampling)
0 -----------1 MHz * 700 kHz
The signal has "folded" around the Nyquist frequency.
Quick rule for alias frequency
alias = abs( signal_frequency - n * sample_rate )
where n is an integer chosen so the result falls inside the sampled band.
Why this matters for hydrogen line work
If strong signals exist near the receiver, for example
mobile transmitters
satellites
radar
they may enter the receiver above the Nyquist frequency and appear as false narrow lines in the spectrum.
Without an anti-alias filter these signals can look very similar to real astronomical spectral features.
Sent: Saturday, March 7, 2026 10:53:20 PM
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Re: 1415-1425MHz Cavity Filter
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b alex pettit jr
Marko Cebokli
This filter has much better far away attenuation than a SAW filter. Saw filters can be made narrow, with steep flanks, but suck at far away attenuation. If you have a powerful (radar..) interference 100 or 200 MHz away, this will attenuate it much better than a SAW.
Marko Cebokli
08.03.2026 01:11, je 'b alex pettit jr' via Society of Amateur Radio Astronomers napisal
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Andrew Thornett
Sent: Sunday, March 8, 2026 5:34:32 AM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] Re: 1415-1425MHz Cavity Filter
This filter has much better far away attenuation than a SAW filter. Saw filters can be made narrow, with steep flanks, but suck at far away attenuation. If you have a powerful (radar..) interference 100 or 200 MHz away, this will attenuate it much better than a SAW.
Andrew Thornett
Sent: Sunday, March 8, 2026 6:30:11 AM
b alex pettit jr
Andrew Thornett
Sent: Sunday, March 8, 2026 9:45:17 AM
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Re: 1415-1425MHz Cavity Filter
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b alex pettit jr
Andrew Thornett
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Re: 1415-1425MHz Cavity Filter
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b alex pettit jr
Andrew Thornett
To: 'Andrew Thornett' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Re: 1415-1425MHz Cavity Filter
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b alex pettit jr
Eugene Ham
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Andrew Thornett
Sent: Sunday, March 8, 2026 1:55:08 PM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] Re: 1415-1425MHz Cavity Filter
Eugene Ham
To view this discussion visit https://groups.google.com/d/msgid/sara-list/FRWP194MB2752493D7BD7EA5C97BE8480FF78A%40FRWP194MB2752.EURP194.PROD.OUTLOOK.COM.
Andrew Thornett
Eugene Ham
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Marko Cebokli
In some parameters, like passband loss and stopband attenuation, cavity filters beat SAW filters hands down, but they are huge in size, a problem for modern microstrip/SMD tech. Most SAW filters will struggle to have more than 30 or 40dB of stopband attenuation over a wide range of frequencies. Saw filters are conveniently small, just a few mm, and have steep flanks in the frequency response, but are in other respects no magic.
Marko Cebokli
09.03.2026 04:31, je 'Andrew Thornett' via Society of Amateur Radio Astronomers napisal
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