owenduffy.net – Page 103 – Mostly electronics…

Backup for NBN VDSL access

The change from Telstra ADSL to NBN VDSL drops the POTS line that gave telephone access independent of premises power.

Telephony is now provided by an ATA integrated into the VDSL modem, and dependent on mains power.

NBN makes no definitive statements about battery backup to the node, or endurance of any battery backup. Nevertheless it appears from pics people have taken of the node cabinets that the bottom layer is batteries and it is likely that they have endurance of more than 10 hours.

Above, a pic of an NBN FTTN cabinet from NBN’s website, and showing batteries in the bottom layer.

NBN is not customer facing, and it seems they will provide are reluctant to provide all manner of information useful to end users, perhaps guided by lawyers who do not want to create any obligation or liability for the company… the way of modern telecommunications.

In that context, it would seem worth the risk to provide battery backup to the VDSL modem to provide broadband access to battery devices such a tablets, laptops and smartphones, and access for a POTS handset plugged into the ATA port.

The supplied modem includes a 12V DC power pack rated at 2A.

Above, measurement of the current drawn averages around 0.5A with all features running (though it may draw more current during phone ringing). Continue reading Backup for NBN VDSL access

Last update: 23rd October, 2016, 11:17 AM

Small signal diode characteristics

We often use diode detectors at microamp currents, and the question arises as to the type of diode best suited to sensitive detectors.

Setting aside zero bias Schottky diodes which are a topic in themselves, the choice is typically between commonly available germanium, silicon and Schottky signal diodes.

Above is a plot of the I,V characteristic of four common signal diodes at currents up to 1mA. It can be seen that at currents below 600µA, the forward voltage drop of the humble 1N34A germanium diode is lower than the others. The 1N270 is an alternative if you really need its higher breakdown voltage. Both of these diodes are reasonably easy to obtain, and cheap at that.

References

Duffy, O. May 2011. Measuring common mode current. https://owenduffy.net/module/icm/index.htm (accessed 22/10/16).

Last update: 22nd October, 2016, 7:22 PM

Queueing in data networks

Modern data networks route or switch relatively small ‘packets’ of data across shared links that along with the switching nodes, form the wider data network.

One of the roles of the switches is to receive packets on one link, and send them onwards on the most appropriate link. Since links may be at different speeds, and many links may source packets to be sent on any link, there exists a mechanism in the switch to store packets pending transmission, in the simplest case it is a first come first served link queue.

The function of the queue then is to hold packets until they can be sent on the link, and to offer them in first come first served order. That raises two important questions:

how long will packets be delayed;
how many slots does the queue need.

Queuing theory gives us a method of estimating these quantities.

Lets make some assumptions about the traffic:

service requests arrive randomly in time; and
service time is exponentially distributed with an average time of 1.

Above is a plot of normalised average response time (service time + queue wait) vs resource (link) utilisation (pu means per unit). It can be seen that when the link utilisation is 0.5pu (50% busy), that response time is 2pu (ie twice the average service time), twice that needed to send an average packet at very low utilisation. Response time rapidly degrades:

at 70% link busy, response time is 3.3 times packet transmission time; and
at 90% link busy, response time is 10 times packet transmission time.

To ensure packets are not discarded, the queue need sufficient slots to hold packets even in most peak bursts. Whilst at 70% link utilisation, the average queue size is 2.3 slots (3.3-1), a larger queue size accommodates bursts better. Discarded packets can severely affect performance, not only are they likely to be resent after some delay and network overhead, they can break a higher level protocol unit in simple systems and waste the link capacity and other links used to send the rest of that protocol unit. Continue reading Queueing in data networks

Last update: 22nd October, 2016, 5:06 PM

Android and local IPv4 DNS name resolution

Much has been written of problems with local IPv4 DNS name resolution over quite some years, and some of it may have been software defects, but some of it seems to be a side effect of IPv6.

I have run a home network with local DNS and DHCP (doing dynamic DNS updates) for close on 20 years mostly without problems. This is a similar configuration to many corporate networks with an Intranet and gateway to the Internet.

Recent installation of a iiNet TG-1 VDSL gateway changed all that.

Try as I might, the Android tablets (6.01) would not resolve names defined in the local DNS even though they showed that they had acquired the correct IPv4 DNS and search path.

The Android tablets certainly try to discover available routers (and DNS).

The packet trace above shows the Router Solicitation message from one of the Android tablets. Continue reading Android and local IPv4 DNS name resolution

Last update: 18th October, 2016, 11:53 AM

First week of iiNet FTTN broadband Internet access

Our iiNet / FTTN NBN 12Mb/s service has been connected for just over a week now, and the first full week of download test results (ISP performance graphs) bears examination.

Above is a plot of the measured transfer rate of the test file from iiNet’s site. Continue reading First week of iiNet FTTN broadband Internet access

Last update: 17th October, 2016, 1:24 PM

Backup options for NBN VDSL access

The change from Telstra ADSL to NBN VDSL drops the POTS line that gave telephone access independent of premises power.

Telephony is now provided by an ATA integrated into the VDSL modem, and dependent on mains power.

Above, a pic of an NBN FTTN cabinet from NBN’s website, and showing batteries in the bottom layer.

The supplied modem includes a 12V DC power pack rated at 2A.

Above, measurement of the current drawn averages around 0.5A with all features running (though it may draw more current during phone ringing). Continue reading Backup options for NBN VDSL access

Last update: 18th October, 2016, 12:53 PM

mpd on a RPi, streamripper and sox

For some years I have run mpd on an early RPi as a media player, and it has worked very well… better as Raspbian improved in robustness. Playback is controlled mostly from mpdroid on our Android tablets or phones. Playlists switches can be automated using cron on the RPi.

Whilst using Telstra Bigpond ADSL for broadband access we have been unable to stream Shoutcasts (“Internet Radio”) without frequent stops and starts rendering it unusable. The 8Mb/s broadband service performed so badly it would not sustain near real time traffic at 200kbps.

With the experience that Telstra does not maintain sufficient capacity to give its customers good service, we chose another provider once NBN access became available. Continue reading mpd on a RPi, streamripper and sox

Last update: 11th October, 2016, 9:07 PM

A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use

Discussion at A method for design of small broadband RF transformers using medium µ ferrite core for receiving use was around a 9:1 transformer on a BN43-2402 core. In that design, 4t was proposed as a suitable winding for a nominal 50Ω primary.

This article describes a 4:1 transformer needed for a project and based on the same 4t primary design, and using a separate 8t secondary.

First, lets find the largest wire that will fit 12t in the core aperture.

Ok, so allowing a bit or working room, lets use 0.25mm enamelled wire (~0.28mm dia). Continue reading A prototype small 4:1 broadband RF transformer using medium µ ferrite core for receiving use

Last update: 9th March, 2022, 2:39 PM

A method for design of small broadband RF transformers using medium µ ferrite core for receiving use

A simplified design for small broadband RF transformers using medium µ ferrite core for receiving use.

The characteristic of typical medium µ ferrite mixes, particularly NiZn, are well suited to this application.

This article continues with the design discussed at BN43-2402 balun example, but using a 4t primary and 12t secondary for a nominal 1:9 50:450Ω transformer.

Lets consider a couple of simple starting points for low end and high end rolloff.

Low end roll off

A simple model for these devices with low flux leakage is an ideal transformer with primary shunted by the magnetising impedance. To obtain low InsertionVSWR, we want the magnetising impedance in shunt with 50+j0Ω to have a low equivalent VSWR.

Typically complex permeability changes in-band, and although it tends to decrease, increasing frequency means that the critical point for magnetising impedance is the low end.

High end roll off

At the high end, transformation departs from ideal usually when the length of wire in a winding exceeds about 15°.

Going forward

A small core makes for short windings to obtain high frequency performance, and sufficient turns are needed for low end… but not too many as it restricts the high end.

There are lots of rules of thumb for minimum magnetising impedance, most treat the inductor as an ideal inductor and these ferrites are not that.

A quick analysis using the method in BN43-2402 balun example hints that a 4t primary is probably good enough down to 1.8MHz, depending on one’s limit for InsertionVSWR. We are not being too fussy here… this is not an application that demands InsertionVSWR < 1.2.

Above is a plot of expected R and X for a 4t winding using my common mode choke design tool. Z at 1.8MHz is 49+j199Ω, or Y=0.00117-j0.00474S. (If your design tools are not giving you similar values, you might consider validating them.) Adding the shunt 50Ω (Y=0.02), we get Yt=0.02117-j0.00474S, and plugging that in to calculate VSWR… Continue reading A method for design of small broadband RF transformers using medium µ ferrite core for receiving use

Last update: 30th July, 2018, 11:36 AM

Turning 1kW into QRP

Effective Isotropically Radiated Power (EIRP) is one means of comparing the performance of a transmitting station.

An inefficient antenna can lead to very low EIRP, perhaps surprisingly low. Consider these four examples at 3.6MHz,

The following NEC-4.2 models give some insight.

QW vertical with 120 buried radials

Considered by so many experts to be the benchmark for a grounded monopole, here is a quarter wave vertical with 120 buried radials.

Above, 120 buried radials: GAIN=-1.8dBi, radiation efficiency=20.7%.

At 1kW RF input, EIRP=661W. Continue reading Turning 1kW into QRP

Last update: 5th October, 2016, 1:04 AM