Friday, 15 April 2016

Airspy, Spectrumspy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer?

Airspy, SpectrumSpy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer?


A basic spectrum analyzer/tracking generator for less than $250? Yes. Can it be used to do a demanding task like tuning a UHF cavity filter from a repeater? Seems so.

The "proof of concept", spectrum analyzer software, SpectrumSpy, can be used with the Airspy SDR and a noise source to show the characteristics of a pass-reject UHF cavity filter.

SpectrumSpy and Airspy

SpectrumSpy, "proof of concept", spectrum analyzer software is a new addition to the SDR# download for use with the Airspy SDR. It has the potential for a new direction with low cost SDRs, spectrum analyzers.  Spectrum analyzers are expensive; $1500 then skyward. SpectrumSpy: (separate executable in SDR# folder).

Airspy has a 24 – 1800 MHz native RX range, but down to DC with the SpyVerter option. $199  and US$59

An earlier post is of SpectrumSpy used as a spectrum analyser:

However, a spectrum analyzer needs a tracking generator to be really useful and to test radio filters. A noise source can be used to achieve much the same purpose.

Noise source

A low cost  Zenar diode based noise source is available for about $20. It uses three Mini-Circuits ERA-5+ wideband amplifiers (DC-4 GHz) to get the noise to a usable level.

I did a quick check of the white noise with SpectrumSpy right up to 1 GHz. The level dropped a little with frequency, but that could be either the noise source or Airspy. and many others.

Noise source output

Cavity filter characteristics 

A cavity filter is part of a duplexer that allows a radio repeater to simultaneously transmit and receive with the same antenna, an amazing feat in itself and one of my other interests. Duplexers are fairly complex in design and setup.

Pass/reject is one type of cavity filter that passes the receive signal on one frequency, but rejects the re-transmit of the repeater on another frequency with a notch for the receive side of the duplexer and the opposite for the transmit side.

An expensive spectrum analyzer/tracking generator is needed to adjust the pass and reject frequencies and to minimize losses.

However, SpectrumSpy with a noise source does a pretty good job. The shape, frequencies and depth of notch (>30 dB) are about right in this very preliminary test.

Characteristics of single cavity filter using SpectrumSpy and noise source

Spectrum analyzer/tracking generator plot of similar but different filter.

Typical characteristics from half a duplexer (two cavities) with my old HP spectrum analyzer and tracking generator ($12K in 1990s and about $1000 secondhand now). (I will do it for the same filter another time and edit.) It has different tuning to the one in this post, hence the mirrored shape.


Airspy with SpectrumSpy does a good job in this demanding task. It probably could be used to tune a duplexer; amazing for the very low cost.

Two immediate issues however. First, SpectrumSpy is not calibrated or necessarily linear. Second, it does not have the bells and whistles of a spectrum analyzer such as digital analysis and data, or amplitude/frequency markers.

SpectrumSpy is proof of concept of an SDR used as a spectrum analyzer. This is a very good additional application for low cost SDRs, normally used as receivers. Similar software could be developed for other SDRs, as has been done, but less successfully, for the RTL-SDRs.

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