Common mode choke for DSL line

Having decided to sack iiNet broadband because of recurrent under performance, I need to change VDSL2 modem as the one they supplied was locked to their SIP server (despite their assurances that there was no equipment lock-in).

I replaced it with a TP-Link Archer VR200v which seems to work ok except it is susceptible to disruption when I transmit on HF. The disruption is severe, it causes the VDSL2 modem to disconnect, and it takes around 5 minutes to reconnect.

Several different common mode chokes were tried, all of measured performance, and they all worked in that they eliminated the disconnection problem though they all resulted in small but acceptable uncorrected upstream errors. (Upstream errors are interesting since the upstream modem is 1000m distant.)

The ‘final’ design was chose as the core was just large enough to wind ordinary 4W modular cable through it. So the choke comprises a 2m length of 4W flat modular cable, one end wound 6 turns through a Fair-rite 2643102002 (FB43-1020) suppression sleeve, and RJ12 connectors crunched on in straight through pin wiring (ie reverse the plugs). I found the line jack in the modem would not accept RJ11 (4P4C) plugs readily (common with RJ45 sockets), it required an RJ12 plug. Continue reading Common mode choke for DSL line

Vacuum capacitors – construction implications for SRF

Vacuum capacitors are used for high end applications that require high voltage withstand and low loss.

Though they are called capacitors, and simple analyses treat them as a capacitance with some small equivalent series resistance (ESR), there is more to it.

Above is a view (courtesy of N4MQ) looking into one side of a vacuum capacitor. It consists of an outer cup, and a series of 5 inner cups progressively smaller in diameter. The other side of the capacitor has a similar structure but the cups site in the middle of the spaces between cups in the first side.
Continue reading Vacuum capacitors – construction implications for SRF

Vacuum capacitors – construction implications for Q

Vacuum capacitors are used for high end applications that require high voltage withstand and low loss.

Though they are called capacitors, and simple analyses treat them as a capacitance with some small equivalent series resistance (ESR), there is more to it.

Above is a view (courtesy of N4MQ) looking into one side of a vacuum capacitor. It consists of an outer cup, and a series of 5 inner cups progressively smaller in diameter. The other side of the capacitor has a similar structure but the cups site in the middle of the spaces between cups in the first side.
Continue reading Vacuum capacitors – construction implications for Q

A comparo of two bare light dimmer modules

Two bare dimmer modules sold on eBay with identical specification and similar price are compared.

Both claim to have zero hysteresis. Zero hints a lie!

Hysteresis is caused in simple phase control dimmer circuits at low settings because in each half cycle the trigger capacitor starts at a different voltage depending on whether the diac fired on the previous half cycle.

A serious issue with this snap-on effect is that if power is turned off at low power setting and re-applied, the controller may not switch on.

Above is type 1, a very triac basic phase control circuit. The red capacitor and resistor to its left are snubber components, the yellow capacitor, 4.7kΩ resistor to its left and the 500k pot are the phase delay circuit, the diac is just visible above the red capacitor. Continue reading A comparo of two bare light dimmer modules

Comparing toroidal inductors of different core dimensions

I often see comparisons of toroidal inductors of different core dimensions with all other characteristics (eg turns, core type, frequency) held the same.

There seems an implicit assumption by many that the bigger the core, the larger the inductance. There are several failure in that thinking.

The ‘inductance’ of a toroidal inductor is µ*n^2*a/l where:

  • µ is complex permeability, µ0+µr;
  • n is the number of turns;
  • a is the cross section area; and
  • l is the effective magnetic path length.

Note that at RF, permeability may be a complex frequency dependent value, and therefore ‘inductance’ will be a complex value.

Many online calculators incorrectly calculate l from core dimensions using a simplistic formula.

Many online calculators treat permeability as a real number that is not frequency dependent, they use initial permeability (µi). Continue reading Comparing toroidal inductors of different core dimensions