(tr)uSDX IMD performance

(tr)uSDX uses less common techniques for generation of an SSB telephony signal at modest power (a few watts).

There are aspects of the techniques that might raise questions as to how well they work, questions that do not seem to be addressed by the developers.

Wide signal / distortion reports

Whilst there are lots of videos where users demonstrate making QSOs with the (tr)uSDX, credible evaluation of common reports of wide transmit bandwidth is scarce, though spectrum displays of excessively wide (tr)uSDX signals on air are not so rare.

One simple objective measure of IMD is that of a two tone test with spectrum analysis.

Two tone test with spectrum analysis

A two tone test calls for combining two equal amplitude non-harmonically related pure sine waves and feeding that to the transmitter input, and observing the spectral distribution on a Spectrum Analyser.

Ideally, the output should be just the two input components frequency shifted by the virtual carrier frequency (in the case of USB). Practical transmitters are not perfect, so there will also distortion products, and a common measure is that of the 3rd order mix at  If F1 and F2 are the frequencies of the two tones (at RF), the third-order distortion products occur on both sides of these tones at 2F2 – F1 and 2F1 – F2.

Above is a figure from an Anritsu application note showing a wider scope of IMD. Distortion products are measured wrt to each of the desired signals or ‘carriers’, and this value is often given relative to those signals as dBc. (The ARRL uses a different method, referenced to two tone PEP.) Continue reading (tr)uSDX IMD performance

Can a diode be used to rectify signals smaller than its ‘threshold’ voltage?

Several articles on this site have used diode half wave detectors down to very low signal levels, well below the commonly perceived ‘threshold’ of the diodes, and it has prompted comments to the effect that this cannot work.

Really simple PN junction diode model

An ideal diode is a device that conducts in one direction with zero voltage drop, and does not conduct in the other direction.

Practical diodes typically have an IV characteristic with a knee at some small forward bias from about 0.2V to 0.6V depending on the nature of the PN junction.

An often used simple model of a practical diode is an ideal diode with a series battery of voltage equal to the offset of that knee, the ‘threshold’ if you like.

This model may be quite adequate when the applied voltage is much larger than the knee voltage, eg if you were rectifying 24V AC.

Practical diodes

Shockley’s diode equation

William Shockley modelled the IV characteristic of a diode as \(I_D=I_S(e^{\frac{V_D q}{n k_B T }}-1)\) where ID is the diode current, IS is the reverse-bias saturation current (or scale current), VD is the voltage across the diode, kB is Boltzman’s constant, T is absolute temperature, q is an elementary charge, and n is the ideality factor, also known as the quality factor or emission coefficient.

\(\frac{k_B T }{q}\) is often known as VT.

Shockley’s equation with n=1 is often known as Shockley’s ideal PN diode.


Let’s look at the BAT46 Schottky diode, it has PIV=100V and is very suited to a lot of these higher voltage RF signal projects.

Above is the IV characteristic from a datasheet. They are often not very helpful at really low currents as used in some of these applications, but note the  great temperature sensitivity. Continue reading Can a diode be used to rectify signals smaller than its ‘threshold’ voltage?

R2009D oscilloscope input impedance

The Motorola 2009D incorporates a basic oscilloscope function, specified with bandwidth of 0.5MHz and input impedance of 1MΩ. The input impedance specification is naively incomplete, there will be some parallel equivalent capacitance that is very important to selecting a compatible probe.

Above is the schematic of a test circuit to find that input capacitance Cin. Continue reading R2009D oscilloscope input impedance

Refurbishing aluminium antenna parts affected by weather / corrosion

Weather is not kind to aluminium antenna parts, often giving rise to corrosion that may result in high resistance joints that then reduce radiation efficiency.

It is good practice to document antenna behavior at installation, and through life, measurements can be compared to that benchmark to possibly reveal changes resulting from corrosion. See Diagnosing a possible antenna problem by comparison with a baseline for more discussion.

WARNING: wear eye protection when using any of the appliances mentioned in this article.

Corrosion preventative compounds

Above is the kind of structure that often develops corrosion in the overlapping / clamped tube sections. They can be recovered by cleaning the corrosion products out, and reassembling them with purpose specific conductive waterproof grease containing irregularly shaped shavings of zinc. The way in which it works is that the sharp particles of zinc penetrate the insulating oxide layer on clean aluminium providing a low R conducting path, and the grease prevents ingress of water and oxygen, so preventing corrosion.

Some forms use a silicon grease base for applications where mineral grease is incompatible with wire insulation, but they are more expensive. Continue reading Refurbishing aluminium antenna parts affected by weather / corrosion