Was Tohtsu CX-520D a good choice?
Question: I am concerned that I have made a bad choice by buying Tohtsu CX-520D coaxial relay from ebay to use to switch reference signal in amd out of my 1420 MHz radio telescope and should have pin diode or other alternative. Other people have suggested the coaxial relay will introduce too much insertion loss.
Answer: the CX-520D will “work,” but if it sits in front of your first LNA, it’s not an ideal part for a hydrogen-line receiver.
Why:
- The CX-520D’s spec at ~1.5 GHz is insertion loss ≤0.20 dB and isolation ≥42 dB, VSWR ~1.17. (funkelektronik.eu)
- A 0.20 dB passive loss before the LNA raises system noise by about (10^(0.2/10)−1)×290 K ≈ 13–14 K, which is a very noticeable hit for 1420 MHz work.
- 20 ms switching, mechanical contacts, and finite life make it poor for any chopped/cal-switched scheme. (wimo.com)
What to do instead (practical options):
- Keep the antenna→LNA path “sacred.” If you’re switching a reference/noise source, do it after the first low-noise gain stage, or inject with a directional coupler ahead of the LNA and accept the coupler’s tiny, fixed loss rather than relay loss in series with the sky path. This is standard practice in 21-cm receivers. (DSpace)
- For fast cal switching (Hz–kHz), use a solid-state absorptive SPDT/MMIC or PIN-diode switch. Typical parts around 1–2 GHz have ~0.3–0.8 dB IL and 55–70 dB isolation, switch in microseconds, and won’t wear out. Place them after gain so the added NF is negligible. Examples: ADI/Hittite HMC349 family, Skyworks PE/SKY/AS series, Mini-Circuits MMIC SPDTs. (DigiKey)
- If you only need the relay to occasionally select between inputs downstream (not before the LNA), the CX-520D is fine. Its IL at 1.0–1.5 GHz (≤0.20 dB) won’t matter after you’ve got 20–40 dB of low-noise gain in front, and isolation in the 40–60 dB range is usually adequate. Still, gate/mute the IF/ADC during switching to avoid transients. (funkelektronik.eu)
Quick rule of thumb you can use right now:
- If the relay is before the first LNA: replace it. You’re probably paying ~10–14 K in extra Tsys and adding slow, clicky mechanics where you don’t want them. (funkelektronik.eu)
- If it’s after the first LNA and you switch rarely: keep it.
- If you need rapid reference chopping: go PIN/MMIC switch after gain.