owenduffy.net – Page 112 – Mostly electronics…

Optimal Zo for TLT sections challenge – a solution

Optimal Zo for Guanella balun sections left the reader with a though exercise, a transmission line transformer used by PA0V in a 144MHz power amplifier output network.

The pair of tabs to the left are driven by FET drains, the upper pink centre conductor is grounded, the lower end connecting to C1 is the output to a nominal 50R load. The network shown near OUT is for fine load adjustment. There are two coax sections making this TLT, shields bonded all the way around and the centre conductors connected as shown. What is the optimal value of Zo for each the coax sections?

Above is a pic of the PA, and we are looking at the network to the right of the dual FET. Continue reading Optimal Zo for TLT sections challenge – a solution

Last update: 11th July, 2016, 11:37 AM

Optimal Zo for Guanella balun sections

A Guanella balun may have several sections, and they may be connected in parallel on one side and series on the other side so as to achieve nominal impedance transformation ratios other than 1.

The question is often asked, what is the optimal Zo for these line sections?

Several answers exist in ham lore, but the answer is relatively simple and revealed by the most basic understanding of transmission lines.

If you do not want standing waves on a line section and its associated impedance transformation, then make sure that Zo=V/I… easy as that.

(Guanella 1944) explains it with examples:

Note above that he refers to coil systems. He did not describe for instance (b) on a single core, a shared magnetic circuit which would be a single core system, but he states clearly two coil systems. (Sevick 2001) and lots of other hams say otherwise, but they are wrong. Continue reading Optimal Zo for Guanella balun sections

Last update: 9th July, 2016, 5:45 PM

Comparison of SimonK and BLHeli 14.6 on an Afro30 ESC

Plans are being made to build another copter using a Seriously Pro F3 Flight Controller, and drive options are being considered.

Since this is a high performance flight controller with faster loop times, and Oneshot125 is used on the ESCs for faster response, an ESC that exhibits fast response is warranted to fully explore the benefits of faster loop times.

Most stock ESCs are quite slow, though with replacement firmware they can perform much better.

Though I have dismissed BLHeli for several reasons in the past, I have a set of Hobbywing Skywalker 40A Silabs based ESCs which will run BLHeli and not SimonK, so review of the current version (14.6) of BLHeli is warranted.

Dowloading 55MB of bloatware and updating the firmware using a Silabs Sticktool / C2 interface (which itself is quite fast and convenient as I have added permanent cables for the C2 interface), reminded me of most of the bad things of BLHeliSuite. Despite falling foul of the traps too many times, I achieved the desired result

This article documents some comparison tests of:

SimonK commit 1f75da384e5c83f83916aa752819ca6eed712565 afro_nfet.hex; and
BLHeli v14.6 for Hobbywing Skywalker 40A.

Continue reading Comparison of SimonK and BLHeli 14.6 on an Afro30 ESC

Last update: 12th July, 2016, 8:02 AM

Can a hand held analyser be used to evaluate Insertion VSWR of a VSWR meter?

Some recent articles here used a two port analyser to evaluate Insertion VSWR of some coax switches, and it raises the question about application of a hand held analyser and Insertion VSWR of a VSWR meter.

(Duffy 2007) listed tests for evaluation of a VSWR meter:

Testing a VSWR meter

The tests here need to be interpreted in the context of whether the device under test (DUT) has only calibrated power scales, or a VSWR Set/Reflected mode of measurement, and whether directional coupler scales are identical for both directions.

Connect a calibrated dummy load of the nominal impedance on the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be 1. (Checks nominal calibration impedance);

Repeat Test 1 at a selection of test frequencies and for each test, without changing transmitter power, reverse the DUT and verify that repeat the forward/set and reflected readings swap, but are of the same amplitude (checks the symmetry / balance of the detectors under matched line conditions).

Connect a s/c to the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be infinite. (Discloses averaging due to excessive sampler length);

Connect an o/c to the instrument output and measure the VSWR at upper and lower limit frequencies and some in between frequencies. The VSWR should be infinite. (Discloses averaging due to excessive sampler length);

Connect a calibrated wattmeter / dummy load of the nominal impedance on the instrument output and measure calibration accuracy of power / ρ / VSWR scales at a range of power levels in both forward and reflected directions (Checks scale shape and absolute power calibration accuracy).

Repeating Test 1 additionally with a calibrated VSWR meter connected to the input to the DUT, and measure the VSWR caused by the DUT at a range of test frequencies (Checks Insertion VSWR).

It is not unusual for low grade instruments to pass Test 1, but to fail Test 6 (and some others, especially Test 3 and Test 4) towards the higher end of their specified frequency range.

Item 6 in the list was to evaluate the Insertion VSWR. Continue reading Can a hand held analyser be used to evaluate Insertion VSWR of a VSWR meter?

Last update: 8th July, 2016, 9:15 AM

Dataflash on NAZE32

The Acro NAZE32 flight controller ships without a dataflash chip, which is so shortsighted of the designer / manufacturer. Many users will not be at all interested in Baro or Magnetometer which come on the deluxe version, but the dataflash is so useful in tuning a copter.

Unfortunately, the designer put pads on the rev5 board for 150mil SOIC-8 chips (SOICN), but lots of dataflash chips are in 208mil packages (SOICW) and all the large capacity chips are SOICW. Rev6 boards appear to have pads that will suit both sizes… but then the Acro comes with dataflash, even if small.

If you are happy to install a 2Mb dataflash chip, buy a SOICN package and your job is easy.

I installed a 64Mb Winbond chip (W25Q64FVSIG), they are easy to find on eBay for a dollar or so. (You will also need a 10nF ceramic 0603 cap).

If you want to install a SOICW package… Continue reading Dataflash on NAZE32

Last update: 26th July, 2016, 6:06 PM

U-BLOX LEA-6T GPS module – for experiments

This article documents a LEA-6T module build for general experiments.

The LEA-6T is an inexpensive GPS module (~$40 at time of purchase, but getting cheaper) that can supply raw pseudo range data.

The module above is supplied for use on UAVs of various kinds, and came complete with a plastic radome and cables to suit an APM copter. The module also contains a 3D compass (magnetometer) which is not used here.

Above is the internals of the module with a custom cable to pick up just the RS232-TTL signals from the GPS (and supply 5V). The connector is a 8pin Hirose DF13. Continue reading U-BLOX LEA-6T GPS module – for experiments

Last update: 6th July, 2016, 11:46 AM

Coax switches – is shorting unused port necessary for isolation?

At Ratings of coax antenna switches I showed characteristics of a home made switch which has very low InsertionVSWR, but poor isolation.

A couple of correspondents have offered an explanation that the unused port must be shorted to get good isolation.

If that was the case, then we would expect all coax switches that leave the unused port open to have poor isolation.

Let us look at a very good coaxial relay

Above is a Dowkey 402 series relay which has good performance to GHz. It does not short the unused port.
Continue reading Coax switches – is shorting unused port necessary for isolation?

Last update: 6th July, 2016, 9:19 AM

Ratings of coax antenna switches

In a recent article I discussed how InsertionLoss implies InsertionVSWR in lossless devices.

This article looks at measurements of a few antenna switches at hand.

Daiwa CS-201G II

It is difficult to find comprehensive data on the very popular Daiwa CS-201 series switches.

Above is the data from the packet of one of these switches, a CS-201G II. The specifications are pretty loose, and one must depend on one’s own measurements.

Above, the CS-201G II, a basic CS-201 series switch with N connectors, advertised as useful to 2000MHz where InsertionLoss is given as 1.2dB (or better?). If there were no TransmissionLoss in the switch, that would imply InsertionVSWR=3.6, but there is probably some significant TransmissionLoss and InsertionVSWR would be somewhat less. Nevertheless, IMHO InsertionLoss=1.2dB indicates it as unsuitable such frequencies. Continue reading Ratings of coax antenna switches

Last update: 5th July, 2016, 7:56 PM

InsertionLoss implies InsertionVSWR in lossless devices

Devices inserted in transmission lines often characterised by one or more of:

Insertion VSWR (the input VSWR when terminated with a matched load);
Return Loss (RL) in dB (20 times the log of the magnitude of the complex reflection coefficient); and
Insertion Loss.

Practitioners often find Insertion VSWR (1) of most use as it indicates whether the device is worse than other system devices, the weak link in the chain if you like. You might see a coax antenna switch specified to have InsertionVSWR<1.2 to 60MHz.

Return Loss (2) is a function of VSWR and vice versa, so it appeals when the designer thinks in terms of Return Loss rather than VSWR (and it is a better metric for VSWR<1.2). You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz.

Insertion Loss (3) is not so readily compared to the other two which are measures of input reflection with a matched termination. It often yields some numbers that appear very acceptable, but might be deceptively so. You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz. You might see a coax antenna switch specified to have InsertionLoss<0.2dB to 100MHz. Continue reading InsertionLoss implies InsertionVSWR in lossless devices

Last update: 5th July, 2016, 12:07 PM

Seriously Pro F3 flight controller gyro/acc failure

I purchased an Acro Seriously Pro F3 flight controller (FC) recently and having soldered the connectors on and loaded the current firmware, it was time for a pre-installation checkout.

The article outlines a basic pre-installation test that revealed problems that would prevent effective use of the FC.

Acc/gyro checkout

Doing no more than an acc calibration and allowing initialisation with a stabilised FC, the FC on the end of a USB cable should deliver acceptable sensor responses in Cleanflight Configurator (CC).

This flight controller has two problems:

offset in the gyro z axis (yaw);
low acc output in the Y axis (roll).

Continue reading Seriously Pro F3 flight controller gyro/acc failure

Last update: 4th July, 2016, 7:21 AM