R2009D oscilloscope input impedance

The Motorola 2009D incorporates a basic oscilloscope function, specified with bandwidth of 0.5MHz and input impedance of 1MΩ. The input impedance specification is naively incomplete, there will be some parallel equivalent capacitance that is very important to selecting a compatible probe.

Above is the schematic of a test circuit to find that input capacitance Cin. Continue reading R2009D oscilloscope input impedance

Refurbishing aluminium antenna parts affected by weather / corrosion

Weather is not kind to aluminium antenna parts, often giving rise to corrosion that may result in high resistance joints that then reduce radiation efficiency.

It is good practice to document antenna behavior at installation, and through life, measurements can be compared to that benchmark to possibly reveal changes resulting from corrosion. See Diagnosing a possible antenna problem by comparison with a baseline for more discussion.

WARNING: wear eye protection when using any of the appliances mentioned in this article.

Corrosion preventative compounds

Above is the kind of structure that often develops corrosion in the overlapping / clamped tube sections. They can be recovered by cleaning the corrosion products out, and reassembling them with purpose specific conductive waterproof grease containing irregularly shaped shavings of zinc. The way in which it works is that the sharp particles of zinc penetrate the insulating oxide layer on clean aluminium providing a low R conducting path, and the grease prevents ingress of water and oxygen, so preventing corrosion.

Some forms use a silicon grease base for applications where mineral grease is incompatible with wire insulation, but they are more expensive. Continue reading Refurbishing aluminium antenna parts affected by weather / corrosion

nanoVNA-H – can firmware be updated if it has a broken USB socket?

The usual method used for firmware upgrade is DFU (Direct Flash Update) using the USB interface and one of many PC clients to load the firmware.

Before attempting a firmware upgrade, be certain of the hardware you have, and the appropriate / compatible firmware file and format. Look for a label on the back, or on the silkscreen of the PCB (though sometimes hidden under the battery… doh!)… know what hardware you have to ensure you load compatible firmware.

Before discussing how to upgrade firmware if the USB interface is not functional, be sure that this problem is not driver related, that there is a real hardware problem. Continue reading nanoVNA-H – can firmware be updated if it has a broken USB socket?

Motorola R2009D OCXO startup

This article documents the startup characteristic of the OCXO in a Motorola R2009D comms analyser.

Frequency error from cold start was logged using the FA-3-6G counter to make record of the time for reasonable stabilisation from cold. The R2009D had been calibrated to within 100mHz some days before this test.

This test was performed with power applied to the box, but the front panel switch in standby position.

Above is a plot of the startup.

At powerup, the oscillator is 514Hz high (error 5.14e-5), at 3min error is 2.8e-5, at 5min, error is 6.4e-6, at 20min error is -1.2e-7, improvement beyond this is very slow.

The front panel “Oven ready” LED is not a good indicator that the oscillator is very close to frequency, it comes on when OCXO heater current reduces, but the oscillator has not yet stabilised.

Calibration was carried out with the whole analyser powered and operating for 24h, the oscillator will not reach calibration frequency unless the whole box is operating. Experience is that setting the reference oscillator closer than 100mHz is wasted effort.

BG7TBL FA-3-6G frequency meter – initial impressions

A BG7TBL FA-3-6G  is a 6GHz frequency counter of the reciprocal counter type, well for Ch1, <=300MHz, at least.) This one was purchased on eBay for just under $300 delivered.

Above is the seller’s pic of the FA-3-6G frequency counter.

It was supplied with a non-compliant Chinese plug pack, it lacks rubber feet, and the nuts on all the BNC connectors were loose, not even hand tight. Continue reading BG7TBL FA-3-6G frequency meter – initial impressions

Balanced ATUs – the Holy Grail?

It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”

The word “true” in there bodes poorly!

It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.

Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.

A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.

Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?

(tr)uSDX BS170 woes

(tr)uSDX users seem beset by a number of common problems, and one of them is failure of the BS170 PA FETs.

Further, lots of users have bought replacements that appear as a dead short circuit when wired into the board. This seems to be that pins 1 and 3 are transposed on some product bought online.

These FETs have an integral body diode, and if Source and Drain are swapped, the body diode will conduct, appearing to be a near short circuit on the supply rail in the (tr)uSDX. Continue reading (tr)uSDX BS170 woes

tinyPFA – new Nanovna-H4 arrived

TinyPFA is firmware from Erik Kaashoek for NanoVNA-H4 hardware for accurate measurement of oscillator and clock source stability.

I was so impressed with the possibilities of the project, I purchased a new Nanovna-H4 v4.3 a couple of weeks ago to dedicate to this application.

Anticipation got the better of me, and I put tinyPFA onto my existing Nanovna-H4 v4.3 to evaluate it, and was not disappointed.

Anyway, the recently ordered device arrived.

Above, the new tinyPFA at work. Continue reading tinyPFA – new Nanovna-H4 arrived

tinyPFA – HP8656B vs Trimble Thunderbolt GPSDO

TinyPFA is firmware from Erik Kaashoek for NanoVNA-H4 hardware for accurate measurement of oscillator and clock source stability.

This article documents calibration and stability comparison of a HP8656B signal generator’s high stability reference oscillator.

Calibration

Above is a screenshot of the tinyPFA with only Trimble Thunderbolt GSPDO input, and after freq A is nulled (colours have been inverted for printing). Continue reading tinyPFA – HP8656B vs Trimble Thunderbolt GPSDO