In the SAWBird line of LNAs (e.g., for 1420 MHz, GPS, L-band), the SAW filter is the component that provides the ultra-narrow preselection that makes these modules so effective for weak-signal radio astronomy and RF work.

Here’s how it functions in the circuit:

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1. What a SAW filter does

A Surface Acoustic Wave (SAW) filter converts the incoming RF signal into a tiny acoustic vibration on a piezoelectric substrate. Because the acoustic wavelength is thousands of times shorter than the electromagnetic wavelength, very sharp filtering can be achieved in a small package.

The acoustic path includes tuned mechanical resonators that determine:

• Center frequency (e.g., 1420.4 MHz for hydrogen line version)
• Bandwidth (tens of MHz for most SAWBirds)
• Shape factor (very steep skirts)

It then reconverts the acoustic wave back into RF.

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2. Where it sits in the SAWBird

In the SAWBird 1420:

Antenna → Input LNA → SAW filter → Output LNA → SDR

This placement (filter between two LNAs) has important consequences:

✔ The first LNA provides low-noise gain

This ensures the signal is amplified before the filter’s insertion loss (typically 2–4 dB).
This protects system noise figure.

✔ The SAW filter then removes out-of-band interference

It strongly rejects:

• Cellular bands
• Broadcast TV
• 900 MHz ISM
• 1800/1900 MHz LTE bleed

✔ The second LNA restores gain to feed the receiver

Brings overall gain to ~34–40 dB depending on model.

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3. Why SAW filtering is ideal for radio astronomy

Narrow and stable

The center frequency doesn’t drift with temperature the way LC filters can.

Steep skirts

This prevents RFI from desensitising your ADC or SDR front end.

Low group-delay ripple

Better for spectral line work—important if you’re measuring the 1420 MHz hydrogen line structure.

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4. Practical effect in a hydrogen-line system

Using the 1420 MHz SAWBird:

• Your SDR sees a cleaner band, reducing strong out-of-band signals that would otherwise raise your noise floor through mixer overload or internal IM products.
• The effective system temperature drops because RFI-induced gain compression is avoided.
• Spectral baselines become much smoother (less ripple, fewer “shoulders”).

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5. Key specifications (typical)

• Center frequency: ~1420.4 MHz
• Filter bandwidth: ~25–40 MHz (model dependent)
• Insertion loss: ~3 dB
• Rejection: >40–50 dB out of band
• Noise figure (whole module): ~1 dB typical

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