Review of inexpensive Chinese LAN cable tester

I bought an inexpensive LAN cable tester to give to my daughter.

Above is the sellers pic, the specifications states that it checks data wires 1-8 and the shield / ground connection of STP cables.

On test, it failed to show the shield connection on an STP cable, the LED did not light on either the master or the slave unit.

I tore it apart to see if it was worth getting a replacement. Continue reading Review of inexpensive Chinese LAN cable tester

Speaker tick generator (for polarity testing)

A recent purchase of an inexpensive ($6) speaker polarity tester prompted a need for a stand alone driver for speakers.

Above, the tester has a microphone that senses the polarity of the pressure wave and indicates with one of two LEDs.

The tester comes with a CD containing a file that can be used to provide the test signal on a complete system with CD player, but there is a need for a stand alone driver for testing bare speakers or speaker units. Continue reading Speaker tick generator (for polarity testing)

IMD associated with colorbond sheet steel cladding

A recent experiment exposed significant IMD of a 7MHz locally radiated signal.

The source used for these tests is a battery powered low power transmitter  driving a 0.6m square loop. Radiated power is very low (of the order of -60dBm EIRP), and received signal on the station receiver is less than -73dBm.

Loop near steel shed #1

The loop was leaned against the colorbond sheet steel wall of shed #1. Shed #1 is about 40m from the receiving antenna, and is connected to the power mains. The building has colorbond sheet steel screwed to a steel frame. Colorbond is painted Zinc/Aluminium coated steel.

For this test, the submain was turned off at the main switchboard, so there is not equipment in the shed powered up, but the supply neutral is still connected and bonded to the shed steel work and shed PES ground electrode.

Above, the spectral response of receiver output, there are a number of side products at 100Hz intervals, presumably some form of IMD. At higher radiated power, products at odd 50Hz intervals become visible, though at much lower level than the 100Hz products. Continue reading IMD associated with colorbond sheet steel cladding

Baselining an antenna system with an analyser

I often receive emails from folk trying to validate continued performance of an installed antenna system using their analyser.

With foresight they have swept the antenna system from the tx end and saved the data to serve as a baseline.

The following are example sweeps from one of my own antennas, a Diamond X50N with 10m of LDF4-50A feed line.

Now I have plotted Return Loss rather than VSWR for several reasons:

  • Return Loss is more sensitive to the problems that we might want to identify;
  • Rigexpert in this case decided that the Antscope user could not be interested in plotting VSWR>5 (Return Loss<3.5dB).

Now a hazard in working with Return Loss is that many authors of articles and software don’t use the industry standard meaning.

Return Loss

Lets just remind ourselves of the meaning of the term Return Loss. (IEEE 1988) defines Return Loss as:

(1) (data transmission) (A) At a discontinuity in a transmission system the difference between the power incident upon the discontinuity. (B) The ratio in decibels of the power incident upon the discontinuity to the power reflected from the discontinuity. Note: This ratio is also the square of the reciprocal to the magnitude of the reflection coefficient. (C) More broadly, the return loss is a measure of the dissimilarity between two impedances, being equal to the number of decibels that corresponds to the scalar value of the reciprocal of the reflection coefficient, and hence being expressed by the following formula:

20*log10|(Z1+Z2)/(Z1-Z2)| decibel

where Z1 and Z2 = the two impedances.

(2) (or gain) (waveguide). The ratio of incident to reflected power at a reference plane of a network.

Return Loss expressed in dB will ALWAYS be a positive number in passive networks.

The relationship between ReturnLoss in dB and VSWR is given by the equations:

  • ReturnLoss=-20*log((VSWR-1)/(VSWR+1))
  • VSWR=(1+10^(-ReturnLoss/20))/(1-10^(-ReturnLoss/20))

Diamond X50N on 2m

So now that we are on the same page about Return Loss, lets look at my 2m plot.

The X50N does not have VSWR or Return Loss specs, but we might expect that at the antenna itself, VSWR<1.5 which implies Return Loss>25dB. Measuring into feed line, you can add twice the matched line loss to the Return Loss target (see why Return Loss is a better measure).
Continue reading Baselining an antenna system with an analyser

Thompson’s coax common mode explanation

A recent online discussion on common mode feed line current was directed to Thompson’s article with the recommendation that is ALL basically needed to discuss the common mode current.

Above is Thompson’s diagram of currents in a feed coax, and it contains two significant errors that could / would lead to formation of the wrong concepts in a learner’s mind. Continue reading Thompson’s coax common mode explanation

An RF choke for a 1.8-30MHz coax power injector – LF1260 core

This article describes a prototype RF choke (RFC) for use in a power injector for 50Ω coax over range 1.8-30MHz. Power injector / extractors are often used to connect power and / or signalling on a shared common RF coax feed line to accessories such as remote antenna switches and ATUs.

Design criteria are:

  • Insertion VSWR of the RFC in shunt with 50+j0Ω < 1.1;
  • Dissipation < 2% of a 100W transmitter.

The core chose is a LF1260 ferrite suppression bead from Jaycar. It is a medium / high µ core readily available in Australia at $7.50 / 6.

 

Above is the prototype RFC wound with data cable wire for the purpose of measurement. In application it could be wound with 1mm enamelled copper or PTFE insulated wire (Curie point is lowish at 120°+, but it still benefits from higher temperature insulation). Continue reading An RF choke for a 1.8-30MHz coax power injector – LF1260 core

Riding the RF Gain control – part 4

This article continues on from Riding the RF Gain control – part 3 and explores the operating advice when applied to the next generation of receivers.

Direct sampling SDR

Lets jump a generation to the direct sampling SDR configuration.

In this category, I am covering receivers that do not convert the receive signal to an intermediate frequency, the ADC samples the signal at its off-air frequency.

The receivers may or may not have the following elements ahead of the ADC:

  • preamplifiers;
  • bandpass filters;
  • attenuators.

Because there may be no amplification prior to the ADC in some operating configurations, a voltage controlled attenuator may be used to prevent overflow of the ADC, this is the ‘analogue’ part of the AGC system. Continue reading Riding the RF Gain control – part 4

Riding the RF Gain control – part 3

This article continues on from Riding the RF Gain control – part 2 and explores the operating advice when applied to the next generation of receivers.

Conventional superheterodyne communications receiver with DSP demodulation.

The next generation of receivers was a conventional superheterodyne with a DSP based demodulation stage (initially at quite low Intermediate Frequency to suit the power of the available DSP chips).

Communications receivers were enhanced by replacement of the demodulators with a DSP performing demodulation digitally. The DSP sampled the IF signal and digitised it, and channel filtering and demodulation was performed ‘mathematically’ using the digital data stream as input.

There are two significant differences with this change:

  • receiver bandwidth can be determined by digitally synthesised passband filters in the DSP; and
  • first step in the DSP process is conversion of the IF signal to a digital stream in an analogue to digital converter (ADC).

Critically, the Analogue to Digital Converter (ADC) had an overflow point, and overflow of the ADC creates serious IMD and major degradation of received signal, overflow has to be prevented at all cost. To limit the power delivered to the ADC, a narrow ‘roofing’ filter usually preceded it, and the channel filter was digitally synthesised. Continue reading Riding the RF Gain control – part 3

Riding the RF Gain control – part 2

This article continues on from Riding the RF Gain control – part 1 and explores the operating advice when applied to the next generation of receivers.

Conventional superheterodyne communications receiver designed for SSB telephony.

Move on to the generation of superheterodyne communications receivers that incorporated a demodulator designed for SSB, and an AGC system that protected the receiver intermediate and later stages (including the demodulator) from overload for even very strong signals, and you have a receiver that broadly, worked automatically from the weakest to very strong signals with RF Gain control set to maximum and little need to adjust AF gain.

The AGC system’s most important function is to protect the receiver intermediate and later stages (including the demodulator) from overload. Levelling the AF output is a lesser priority.

These receivers were not perfect, the AGC was derived from the signal, and SSB suppressed carrier does not contain a consistent component from which AGC can be derived, so it is derived from the signal using a pseudo peak detector. The dynamics means that the AGC leaks away between syllables, and there is small overload during the attack time of the next syllable as it charges again, but with appropriate time constants, the distortion is small. (Some commercial HF links solved this problem by transmitting a reduced carrier typically at -26dBc from which AGC was derived.)

These receivers were subject to IMD in the front end, and operating them with no more preamplification than necessary improved handling of strong out of band signals, and in extreme interference cases inserting a front end attenuator improved IMD performance. The working configuration on low HF where external noise dominates the receiver is to improve IMD response even at the expense of Noise Figure.

These receivers were not perfect, but by and large, good implementations worked very well hands off most of the time.

Delayed AGC

AGC reduces gain ahead of the demodulator, typically in both RF and IF stages, and has the effect of increasing the receiver Noise Figure. Increasing Noise Figure degrades S/N ratio at the receiver output.

The diagram above from (Terman 1955) illustrates the behaviour for AVC, the term that was used with AM receivers at the time and superseded by AGC for SSB receivers. Continue reading Riding the RF Gain control – part 2

Riding the RF Gain control – part 1

Every so often one sees advice from experts on how to operate a communications receiver or transceiver for SSB reception on the HF bands.

Very often that advice is to adjust AF Gain to max, and adjust RF Gain for a comfortable listening level. This is argued today to deliver the best S/N ratio, partly due to delivering the lowest distortion due to IMD in the receiver front end.

The downside of this is that it has prevented normal AGC operation, so the operator must continuously adjust the RF Gain to compensate for fading etc, and the settings may be quite different for each station in a n-way QSO.

I cannot recall ever seeing quantitative support for the claimed improvement in  S/N or ‘quality’, so it seems that it is based on subjective assessment and there may not be quantitative evidence.

Now I was taught this method by my mentor 50+ years ago to operate a receiver he had loaned me… and it DID work in the specific case. It worth exploring why it did work, since this may be the root of the advice that is offered generally, whether appropriate or not.

Further articles will critically examine the advice applied to newer technologies.

Once upon a time…

There was a time when receivers had AGC systems that performed poorly, most commonly because they used an envelope detector with BFO injection for SSB and CW reception, and had AGC time constants quite unsuited to SSB Telephony.

In fact, my mentor’s instruction was for such a receiver, an AR8 receiver of WWII vintage which was essentially an AM receiver (including MCW) with BFO for A1 Morse code (CW) reception, that was its intended purpose. The AGC characteristic was tolerable for AM and MCW, and less suited to CW, but quite usable on mid range signals.

It could also receive SSB Telephony using the BFO, but BFO injection level was not adjustable and insufficient for strong signals so it was necessary to reduce RF Gain on strong signals to ensure reasonably good demodulation. The same was required for strong CW signals.

So, the instruction to set AF Gain to maximum and adjust RF Gain to comfortable listening level was a circumvention for the deficient means of demodulation of SSB Telephony, and poorly performing AGC system.

Next part

In the next part, we will explore a basic ‘conventional’ superheterodyne receiver with demodulator designed for SSB telephony.