Is a receiver test with different resistors connected it its input terminals meaningful?

The question arises from time to time, is a receiver test with different resistors connected it its input terminals meaningful?

Scope

This discussion applies to linear receivers. A receiver using a diode AM detector, with or without BFO injection is NOT a linear receiver for this purpose, nor is an FM receiver. A good traditional superheterodyne SSB Communications Receiver is a linear receiver for the purpose of this discussion, but for example any techniques designed to reduce / cancel noise will render it non-linear

Noise in resistors

Thermal agitation within a resistor gives rise to broadband noise (Johnson-Nyquist noise, thermal noise), the noise power that can be captured from a resistor in a given bandwidth is given by \(P=k_0 T B\) where:

  • k0 is Boltzman’s constant;
  • T is the absolute temperature; and
  • B is the bandwidth.

Receiver sensitivity

If:

  • a receiver is designed for a 50+j0Ω source; and
  • has noise figure specifications or specifications that imply a noise figure; and
  • and is tested to specification

the measurement is done with a 50+j0Ω source that contributes thermal noise (50Ω @ 290K) and the equivalent internal noise contribution of the receiver can be calculated. Continue reading Is a receiver test with different resistors connected it its input terminals meaningful?

A little transformer challenge

A little challenge was posted online, a request to explain this nominal 50-25Ω transformer.

Don’t get tricked by the 2:1 impedance ratio, it is probably nominal.

Since this is an RF transformer, let’s assume that the coax line sections work in TEM mode. It is likely to be very low loss, let’s assume it is lossless for ease of analysis. Continue reading A little transformer challenge

Replacement of skylight in the little shed

The ‘little shed’ is about 30 years old and the fibreglass sheets in the roof providing a skylight have reached end of life, so they were replaced before hail destroyed them. A fairly local company rolls the exact same profile and colour of Colorbond steel sheet, so two sheets were ordered in the required colour and length and collected a couple of days later at Minto.

The sheets were replaced, but a solution was needed for the missing skylights. A solar / LED solution was chosen.

Above, a nominal 30W PVA, not optimally tilted, delivers about 15W max. Cost $50. Continue reading Replacement of skylight in the little shed

How is SND different to NF?

Signal to Noise Degradation, SND is a measure the extent of how the off-air or ultimate S/N ratio is degraded by a receive system.

We can calculate \(SND=10 \log (1+\frac{T_{sys}}{T_{amb}})\) where:

  • Tsys is the equivalent system noise temperature at the space interface; and
  • Tamb is the equivalent ambient noise temperature at the space interface.

For more explanation of the metric Signal to Noise Degradation (SND), see Signal to noise degradation (SND) concept.

The Noise Figure (NF) of a system or system component is often defined as the extent of how the system (or component) degrades S/N. The unstated assumption is that the source has an equivalent noise temperature of 290K.

We can calculate system \(NF=10 \log (1+\frac{T_{sys}}{T_{0}})\) where:

  • Tsys is the equivalent system noise temperature at the system interface; and
  • T0 is the assumed reference temperature, 290K.

System NF can be thought of as a measure of system degradation of S/N for a specific source noise temperature of 290K.

The two expressions above might be similar in form, but are different and give very different results.

NF does not bring to book the external or ambient noise as it applies to a specific scenario, so it does not provide a complete picture of S/N degradation. Continue reading How is SND different to NF?

Receive only antenna for 160m – maximum receiver NF for SND

This article explores the design / analysis of a passive receive system for the 160m band, determining the maximum receiver noise figure (NF) to achieve a specified maximum S/N degradation (SND) by receiver internal noise.

For explanation of the metric Signal to Noise Degradation (SND), see Signal to noise degradation (SND) concept.

The antenna data table given uses a tabulated average gain for a set of interesting 160m antennas published by Tom Rauch (W8JI) at https://www.w8ji.com/receiving.htm . Read the whole article, it is interesting and relevant.

External noise is estimated using ITU-R P.372-16, and results are tabulated for the five noise environment categories used in ITU-R P.372-16.

The example and calculations assume linear systems, if there is significant nonlinearity that gives rise to significant IMD, IMD noise depends on the specific scenario (including receive spectrum) and is not captured by this analysis.

Maximium receiver NF for given SND

Let us assume that noise arrives equally from all directions. In that case, the average gain of the antenna is used to determine the noise power captured, \({Gain}_{avg}={Gain}_{max}-\text{Directivity}\).

The table above uses Rauch’s tabulated average gain for a range of 160m antennas, and calculates the maximum receiver NF to achieve the specified 1dB maximum SND. Continue reading Receive only antenna for 160m – maximum receiver NF for SND

Receive only antenna for 160m – K9AY matching and performance discussion

This article explores the design / analysis of a passive receive only antenna, a K9AY loop, for the 160m band (1.8MHz).

The example and calculations assume linear systems, if there is significant nonlinearity that gives rise to significant IMD, IMD noise is not captured by the analysis.

Results are for the scenarios calculated and may not be extensible to different scenarios.

Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.

Above is the K9AY loop from a model by W7EL.

Design objective

The objective here is to design a receive only antenna system that can be relatively remote from local noise sources (like house wiring), and captures enough external signal and noise that the receiver internal noise does not degrade S/N too much. Continue reading Receive only antenna for 160m – K9AY matching and performance discussion

Receive only antenna for 160m – matching and performance discussion – improved SimNEC model

This article is a follow on from Receive only antenna for 160m – matching and performance discussion. It develops a SimNEC model that imports the loop impedance, and transforms it with an ideal transformer to quickly find an optimal transformer ratio and predict system losses.

Results are for the scenarios calculated and may not be extensible to different scenarios.

Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.

Above is the SimNEC model, it assumes reciprocity of the antenna system. Continue reading Receive only antenna for 160m – matching and performance discussion – improved SimNEC model

Receive only antenna for 160m – matching and performance discussion – 8000pF?

A reader of Receive only antenna for 160m – matching and performance discussion referred me to an online discussion with a simpler solution.

A couple of quotes from posters…

It almost doesn’t matter what the actual receiver Zin is. What matters is the 100ft of 50 or 75 Ohm coax that is used to get the Rx-only antenna signal from where the antenna is placed to the receiver.

and…

That’s what I thought, also.

A few hundred feet of coax can be ~8000 pF of parallel capacitance which should short out signals by having only about 11 Ohms of reactance.

So the gist of this is that:

  • receiver Zin does not matter;
  • the 100′ of coax or 300′ of coax matters; and
  • 300′ of coax can be adequately represented by a shunt capacitance of 8000pF.

Receiver input impedance is not necessarily a tightly controlled parameter, but it is measurable, so lets take a real example.

IC7300 example

Let’s measure Zin of an example IC7300 around the 160m band.

Above is Zin plotted from a saved .s1p file. At 1.85MHz, Zin=56.2-j5.4Ω, a little off nominal, but pretty close (VSWR50=1.35). The dotted line is X and the dashed line is R, ignore the solid line… it is for people who don’t understand impedance. Continue reading Receive only antenna for 160m – matching and performance discussion – 8000pF?

VNA measurement – small is beautiful

I have written online and in many many emails that a very common failure of VNA measurements of components is the test fixture, and the standout problem is most often the length of connecting wires.

This article works a couple of theoretical designs based on a validated model and experience of building and measuring many baluns of similar or identical design. We will then look at extracts from a Youtube video by ferrite manufacturer Fair-rite and appraise the results.

Validated theoretical choke designs

FT240-43

It is possible to calculate a pretty good estimate of the impedance of a common mode choke wound on a #43 material ferrite core over 1-30MHz. Measurement of a real choke suggests an equivalent shunt capacitance to calibrate the model to measurement. Whilst I have given the generic name to this core, it is based on Fair-rite’s 5943003801 and Fair-rite’s published 2020 #43 mix characteristics. There are imposters, and they may be significantly different.

Let us take a practical example design and calculate the expected choke impedance and from that, the expected |s21|dB in a VNA series through measurement setup.

Above is a SimNEC model of a FT240-43 with 11t winding and 2.5pF equivalent shunt capacitance to calibrate the self resonant frequency. The model calculates and plots choke impedance, and |s21|dB in the series through measurement configuration shown. Continue reading VNA measurement – small is beautiful

VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

VNA fixture for measuring Zcm of a common mode choke – coax wound discussed issues with common ham practice for measuring coax wound common mode chokes.

The article left readers with some homework:

  • Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?
  • Are the resistors beneficial?
  • Do they degrade fixture behavior?
  • Then, why are the used so often?

This article addresses those questions.

Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?

Let’s treat the common mode choke as a black box with two input terminals at left and two output terminals at right with voltages as annotated above. Continue reading VNA fixture for measuring Zcm of a common mode choke – twisted pair wound