Field Guide · hardware

Also known as: upconverter

An upconverter is an external device that shifts HF signals up into the tuning range of a VHF/UHF SDR such as an RTL-SDR, letting radios that cannot tune HF directly receive shortwave.1 It is essentially a single mixer stage in a box: feed in a low-frequency antenna signal, and it comes out translated to a higher band the dongle can reach.

HF 7 MHz + LO 125 MHz 132 MHz RTL-SDR range frequency →
An upconverter mixes HF up by a fixed offset into the RTL-SDR's tunable range, since the dongle can't reach HF directly.

How it works

An upconverter is a heterodyne frequency translator. It mixes the incoming HF signal with a fixed local oscillator — commonly 100 MHz or 125 MHz — in a mixer, producing sum and difference products.2 The sum product (input + LO) is kept: a 7 MHz signal with a 125 MHz LO appears at 132 MHz, comfortably inside the dongle’s tuning window. The whole HF spectrum, 0–30 MHz, translates as a block to 125–155 MHz. In the SDR software you simply tell the app the LO offset and it subtracts 125 MHz so the display and logs read the true HF frequency.

Because the LO is quartz-referenced, its stability and phase noise add directly to whatever the dongle contributes — a poor upconverter LO smears weak signals. Mixing also creates an image: a signal at LO + 7 MHz and one at LO − 7 MHz both land at the same output if the front end is not filtered, so a good upconverter includes a low-pass filter on its HF input to reject anything above ~30 MHz before mixing. Cheaper units also let the strong FM broadcast band leak through as spurs.

Variants

  • Passive vs powered LO — most hobby upconverters (the classic Ham It Up, the SpyVerter) use a crystal oscillator and are USB- or bias-tee-powered.
  • Switchable / bypass — better boards let you route VHF/UHF straight through, so you can leave the converter inline and only engage it for HF.
  • Different LO choices — 100 MHz keeps the arithmetic round (subtract 100); 125 MHz pushes the FM broadcast image further from the HF output.

In practice

An upconverter is the budget route to HF on an RTL-SDR and works well for casual shortwave, ham bands, and utility listening. Its limits are the ones you would expect from adding a mixer ahead of an already-noisy tuner: modest dynamic range, LO phase noise, and possible images from strong signals. For serious HF work a direct-sampling receiver or a dedicated Airspy HF+ — which digitises HF natively with far better dynamic range — is the higher-performance alternative.

Relevance to SDR

Upconverters are a staple of the RTL-SDR-plus-HF hobby, opening shortwave broadcast, ham SSB/CW, and digital HF modes to a dongle that otherwise stops around 24 MHz. GopherTrunk targets trunked land-mobile systems in VHF/UHF and does not itself do HF work, so an upconverter is not part of a typical GopherTrunk setup; it is included here as core front-end vocabulary for SDR users whose interests reach below the dongle’s native range.

Sources

  1. Frequency mixer — Wikipedia, on the mixing principle an upconverter uses to shift HF up into a VHF/UHF tuning range. ↩

  2. Heterodyne — Wikipedia, on producing sum and difference frequencies by mixing with a local oscillator, the basis of frequency translation. ↩

See also

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