RF transformer design with ferrite cores – saturation calcs

Ferrite cored inductors and transformers saturate at relatively low magnetising force.

#61 material example

Lets work through an example of a FT50-61 core with 10t primary at 3.5MHz.

Magnetic saturation is one limit on power handling capacity of such a transformer, and likely the most significant one for very low loss cores (#61 material losses are very low at 3.5MHz).

Let’s calculate the expected magnetising impedance @ 3.5MHz.

Zm=0.966+j144Ω, |Zm|=144Ω. Continue reading RF transformer design with ferrite cores – saturation calcs

RF transformer design with ferrite cores – initial steps

A review of transformer design

In a process of designing a transformer, we often start with an approximate low frequency equivalent circuit. “Low frequency” is a relative term, it means at frequencies where each winding current phase is uniform, and the effects of distributed capacitance are insignificant.

Above is a commonly used low frequency equivalent of a transformer. Z1 and Z2 represent leakage impedances (ie the effect of magnetic flux leakage) and winding conductor loss. Zm is the magnetising impedance and represents the self inductance of the primary winding and core losses (hysteresis and eddy current losses). Continue reading RF transformer design with ferrite cores – initial steps

An online expert on the unsuitability of #43 for HF UNUNs

An online expert recently advised:

…The spec for type 43 makes it clear that it should never be used for HF unun construction. It is specifically engineered with a complex permeability that makes the core lossy on most HF frequencies. Since an unun is not a TLT (transmission line transformer) but rather an autotransformer, a low loss core is essential for efficient operation….

Now it contains the very common FUD (fear, uncertainty and doubt) that masquerades as science in ham radio, but without being specific enough to prove it categorically wrong. To a certain extent, the discussion goes to the meaning of efficient operation. Continue reading An online expert on the unsuitability of #43 for HF UNUNs

Speaker tick generator #2 (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.

Speaker tick generator (for polarity testing) described a stand alone test pulse generator based on re-purposing a brushless DC motor controller (ESC, used for RC models).

This article describes  a simple tick generator using a inexpensive 8051 type MCU (STC15F104E) and a H-bridge IC (TC427).

Above, the prototype was build on a small piece of Veroboard. DC input of 6-15V is applied to the 2.1×5.5mm DC jack, and speaker output is on the screw terminals (nearest to DC jack is -ve). Continue reading Speaker tick generator #2 (for polarity testing)

Fox flasher MkII update 7/2019

Fox Flasher MkII and several follow on articles described an animal deterrent based on a Chinese 8051 architecture microcontroller, the STC15F104E.

This is an update after several years operation outside, and some in service modifications to improve performance.

ff201Above is the original basic schematic.

Above is the revised schematic. One only high current LED driver is shown, use as many as needed. The battery charger / protection module is based on TP4056 and DW01 chips and modules sell on eBay for $1 or so. Continue reading Fox flasher MkII update 7/2019

Basic programming jig for STC15F104E and STC15F204E chips

The STC15Fx chips use a simple TTL/CMOS async programming interface that is suited to the common USB-RS232(TTL) adapters, some of which are less than A$2 on eBay (CH341 chip).

Above, the completed adapter. DIP-28 are located carefully so that the pins 10-18 are in the socket, the same connections are used for both chip sizes for STC15F104E and STC15F204E. Continue reading Basic programming jig for STC15F104E and STC15F204E chips

From lossless transmission line to practical – Zo and γ

On the concept of that P=Pfwd-Prev discussed the expression for power at a point on a line in terms of the travelling wave voltage and current components.

The expansion of P=real((Vf+Vr)*conjugate(If+Ir)) gives rise to four terms.

This article looks at the components of that expansion for a mismatched line for a range of scenarios.

The scenarios

  • Lossless Line;
  • Distortionless Line; and
  • practical line.

We will override the imaginary part of Zo and the real part of γ (the complex propagation coefficient) to create those scenarios. The practical line is nominally 50Ω and has a load of 10+j0Ω, and models are at 100kHz.

Lossless Line

A Lossless Line is a special case of a Distortionless Line, we will deal with it first.

A Lossless Line has imaginary part of Zo equal to zero and the real part of γ equal to zero.

Above is a plot of the four components of power and their sum at distances along the line (+ve towards the load). Continue reading From lossless transmission line to practical – Zo and γ

SimSmith example of VSWR assessment

A reader of On the concept of that P=Pfwd-Prev asked if / how the scenario discussed could be modelled in SimSmith.

SimSmith uses different transmission line modelling to what was used in that article, but a SimSmith model of RG58A/U allows illustration of the principles and it will deliver similar results.

Let’s explore the voltage maximum and minimum nearest the load to show that VSWR calculated from the magnitude of reflection coefficient is pretty meaningless in this scenario.

Above is the basic model. I have created two line sections, one from the load to the first voltage maximum, and another to the first voltage minimum where I have placed the source. I have set Zo to the actual Zo of the line as calculated by SimSmith (56.952373-j8.8572664Ω), effZ as SimSmith calls it, so the Smith chart relates to the real transmission line. Continue reading SimSmith example of VSWR assessment