Cooking the books on VSWR – Bird43 indication

A reader of Cooking the books on VSWR asked

…so you are telling me that I could measure this Prev>Pfwd with a directional wattmeter like my Bird43… I have never seen it and I don’t believe it.

For clarification, I did not discuss Prev or Pfwd in respect of the three scenarios (other than to say Pref cannot exceed Pfwd).

I did discuss line voltage measurements you can make with a simple RF volt meter which was in the article’s reference quote. But, let’s discuss what you might measure by inserting a 50Ω Bird43 directional wattmeter in the Load case 2 scenario.

Above is a calculated plot of the expected Pfwd and -Prev readings, Prev is shown negated so you can add it by eye with Pfwd to obtain the net power Power (blue line). Continue reading Cooking the books on VSWR – Bird43 indication

Last update: 17th October, 2019, 8:40 PM

Cooking the books on VSWR

Having recently seen the suggestion that …

Most tools and most derivations of SWR will produce negative SWR reports because they are more interested in mathematics than in measurements you can make with a simple RF volt meter.

…, this article explores the expected voltage on a practical transmission line under two mismatch scenarios, voltage that ought be measurable with a simple RF voltmeter.

VSWR concepts… by the book

Textbooks on transmission lines often introduce the concept of standing waves by presenting a plot of voltage along a mismatched lossless transmission line.

Above is a plot of calculated line voltage vs displacement from the load, -ve is towards the source. Continue reading Cooking the books on VSWR

Last update: 17th October, 2019, 4:57 PM

CNC6040 router project – spindle speed linearisation – using new grbl_ESP32 feature

A known issue with the common CNC6040 router and similar devices is very poor calibration / linearity of the spindle motor response to gcode Sx commands.

Above is the system block diagram. The grbl_ESP32 gcode interpeter processes a gcode S (speed) command, converting it to a variable duty cycle PWM waveform on parallel port pin 1. Continue reading CNC6040 router project – spindle speed linearisation – using new grbl_ESP32 feature

Last update: 28th December, 2020, 7:53 AM

CNC6040 router project – spindle speed linearisation

A known issue with the common CNC6040 router and similar devices is very poor calibration / linearity of the spindle motor response to gcode Sx commands.

Above is the system block diagram. The grbl_ESP32 gcode interpeter processes a gcode S (speed) command, converting it to a variable duty cycle PWM waveform on parallel port pin 1. Continue reading CNC6040 router project – spindle speed linearisation

Last update: 12th October, 2019, 5:14 AM

Noise Figure Y factor method calculator updated

Recent updates to Noise Figure Y factor method calculator expose the temperature of each of the attenuators in each scenario and so allow more flexibility in application to real world problems.

The screenshot above demonstrates its use where the DUT and Att12 are cryogenically cooled.

For most applications, the default value of 290K is appropriate, so though the form has a few more fields, there isn’t more data entry for most usage.

The calcs have not changed, just replacement of a global Tatt with T for each instance. The input form and output form have been reformatted to suit.

 

Last update: 6th October, 2019, 9:56 AM

Noise Figure – Equivalent Noise Bandwidth

Harald Friis (Friis 1944) gave guidance on measuring the noise figure of receivers, and explains the concept of Effective Bandwidth.

Effective Bandwidth

The contribution to the available output noise by the Johnson-noise sources in the signal generator is readily calculated for and ideal or square-top band-pass characteristic and it is GKTB where B is the bandwidth in cycles per second. In practice, however, the band is not flat; ie, the gain over the band is not constant but varies with frequency. In this case the total contribution is ∫GfKTdf where Gf is the gain at frequency f. The effective bandwidth B of the network is defined as the bandwidth of an ideal band-pass network with gain G that gives this contribution to the noise output.

Continue reading Noise Figure – Equivalent Noise Bandwidth

Last update: 4th October, 2019, 7:49 AM

nanovna-saver – a first look

The NanoVNA is a new low cost community developed VNA with assembled units coming out of China for <$50.

I have long held the view that these things are most useful when accompanied by a capable PC client that performs flexible text book presentations of data.

Considering buying one, my first step was to perform a desk evaluation of a popular PC client, which seems to be nanovna-saver.

Before downloading it, I examined the first screenshot on the github page.

It gives evidence that the author does not follow industry standard convention for transmission line terms and theory.

In the results shown above (s11) impedance is 39.105+j39.292Ω and some transformations of that value. Continue reading nanovna-saver – a first look

Last update: 2nd February, 2020, 7:11 AM

Is it 290K or 293K?

A reader of my articles commented on them and some of my calculators regarding the use of 290K as the reference temperature (T0) for Noise Figures.

(Friis 1944) suggested that temperature as reference temperature and it has been widely used since. One may also see 293K (eg in certain ITU-R recommendations), but in my experience, 290K is most commonly used and is for instance the basis for calibration of Keysight noise sources in Excess Noise Ratio (ENR).

The assumption in measurement of Noise Figure or of sensitivity is that the ‘cold’ source has a known source resistance with Johnson noise equivalent to 290K (16.85° C). That noise producing resistance is commonly achieved using a large attenuator at the generator output.

References / links

  • Friis, HT. Noise figures of radio receivers. Proceedings of the IRE, Jul 1944 p420.
  • Keysight. Jul 2018. Keysight 346A/B/C noise source operating and service manual.
Last update: 3rd October, 2019, 1:42 AM

Update for NFM software (v1.19.0)

NFM has been updated to v1.19.0.

The update corrects an error in conversion between ENR and temperature where Tcold<>290K.

References

  • Duffy, O. 2007. Noise Figure Meter software (NFM). https://owenduffy.net/software/nfm/index.htm (accessed 01/04/2014).
Last update: 28th September, 2019, 7:19 PM

Antenna analyser – what if the device under test does not have a coax plug on it?

I have written a few articles on fixtures for adapting the device under test (DUT) to an antenna analyser’s coax jack.

Antenna analysers come with a range of connectors, the UHF connector is very popular, perhaps less so are N-type, SMA and BNC.

I use a range of fixtures made to suit specific applications, but the most flexible are the two shown in the following pic.

Above are two adapters: Continue reading Antenna analyser – what if the device under test does not have a coax plug on it?

Last update: 27th September, 2019, 3:57 AM