Akademisk Radioklubb

LA1K / LA1ARK / LA1UKA

Category: Guides

Measuring LNA characteristics

As mentioned in the post on our future ADS-B setup we have conducted a series of measurements on amateur radio low noise amplifiers (LNAs). I had the opportunity to spend some time at NTNUs microwave laboratory while working on my masters thesis. Some of these results and figures are also presented in the thesis (“Ground station considerations for the AMOS satellite programme”, Øyvind Karlsen, NTNU, 2017). This post shows how we measured the Noise Figure (NF), Gain, Third-order intercept (TOI/IP3), spurious free dynamic range (DR_f) and insertion loss of the LNAs in the figure below.

LNAs under test. From top left to bottom: SSB-Electronic SP-7000, SP-2000, SP-200, SP-70, VHFDesign.com LNA 04-2m-v.03, Kuhne MKU LNA 131 AH and LNA4ALL

To start I would like to elaborate which characteristics are important for a LNA and how to measure them. If you are more interested in the results, feel free to skip ahead by clicking this link. The LNAs under test are:

SSB-Electronic:
– SP-7000 (432 MHz)
– SP-2000 (144 MHz)
– SP-70 (432 MHz)
– SP-200 (144 MHz)
VHFDesign.com:
– LNA 04-0m-v.03 (144 MHz)
Kuhne:
– MKU LNA 131 AH (1296 MHz)
LNA4ALL:
– LNA4ALL @ 5 V (30 MHz to 2000 MHz)

A simple way to check if LNAs are working, is to turn on the power supply with a multimeter inline. The multimeter should then be able to read that the LNA is drawing the amount of current specified in the datasheet.
By toggling on and off the power to the LNA there should also be an observable increase of the noise floor at the receiver compared to when the LNA is off. To get a better idea of how they contribute to receive performance more sophisticated measurements must be made.

For all measurements an Aim-TTi EL302Tv precision power supply is used to power the LNAs. All devices are operated at 12 V, except the LNA4ALL which is operated at 5 V.

Noise figure and gain analysis

The noise figure of an active component describes how much noise it adds to an incoming signal. The noise and gain performance of the first stage in a receiver is particularly important as it contributes the most to the system noise figure. The background for this lies in Friis formula for noise, for a more thorough explanation see this link. Using the FSV-K30 option on a R&S FSQ signal analyser together with a HP 346B noise source, the gain and noise figure of the LNAs can be measured. The measurement setup is seen the figure below.

Gain and NF measurement setup

The measurement works by having a wideband noise source, which is a device with a very precise noise contribution over a large frequency range, connected and calibrated to a spectrum analyzer (in this case called a signal analyzer). The noise contribution of this noise source is listed in a table at the back of the device, as seen below.

Signal analyzer calibration procedure. Noise source table shown on the back of the noise source to the right.

Once the spectrum analyzer is calibrated to the noise source, the noise source is connected to the input of the device under test (DUT). The output of the DUT is then connected to the spectrum analyzer. The resulting signal measured at the spectrum analyzer will now be the amplified noise from the noise source. Since the power level and noise contribution of the source is precisely known it is possible to calculate both noise and gain of the DUT.

This is where option FSV-K30 makes the measurement very simple. The signal analyzer sweeps  and calculates automatically, the output is a plot of both noise figure and gain vs frequency.

These measurements are very temperature sensitive. They are done at room temperature, and for thermal stability the equipment is powered on for an hour before starting measurements. Furthermore, the signal analyser is regularly re-calibrated to the noise source.

The SSB-Electronic LNAs feature adjustable gain by tuning a potentiometer. Each LNA is tested for three cases: maximum position, middle position and minimum position.

Linearity analysis

The linearity of a LNA will limit what signals it can reliably amplify. Any active device will generate spurious emissions, for a LNA the close-by intermodulation products that occur between two or more tones in close proximity are the most detrimental. We will show two linearity measures that originate from the third order intermodulation product (IM3) of a two tone test, third order intercept point (TOI or IP3) and Spurious Free Dynamic Range (SFDR or DR_f).

The n-th order intercept point (IPn) is where the n-th order intermodulation-product (IMn) crosses the linear growth curve. This is commonly used as a measure of linearity for non-linear devices, and a higher IPn value is better. Since our devices are highly linear, the measurement is noise floor limited beyond third order intercept (IP3). The relation between the two tone test and IP3 is shown in the figure below.

Relationship between two tone test and IP3 measurement

The two tone test is conducted by setting up two equally strong carriers with a given spacing. By setting up a two-tone test, intermodulation products will be generated in any non-linear device. We are interested in the third order intermodulation products, these can be found at F_high + F_spacing and F_low – F_spacing.
The measured level of these intermodulation products may be used to calculacte output referred IP3 (OIP3) and input referred IP3 (IIP3), as illustrated in the figure above.
The third order intercept point is located at the intersection between the linear line and the third order line. In the logarithmic domain this may be mathematically expressed as:

Solving for X gives output referred location of the third order intercept point

Where P_fundamental is the power of one of the fundamental tones and P_IM3 is the power of the resulting intermodulation. Since the power difference between the output and the input of a LNA is gain, the input referred third order intercept point is found by

There is no guarantee that the low and high intermodulation power is equal, so both low and high side powers must be recorded.

Two-tone measurement is performed with 100 kHz spacing generated from a R&S SMU 200A signal generator that is connected to the device under test. The resulting distortion is measured by a R&S FSQ 40 signal analyzer. Inherent test setup distortion is measured for all frequency ranges, but intermodulation products were buried in the noise floor for generator output power of -37 dBm per carrier.

For the SSB-Electronic devices the IP3 measurement for mid-level of potentiometer is done right after NF and gain measurement to ensure that these occur at the same level.

Saturation

In presence of a strong transmitter the LNA might saturate. If this happens it may be necessary to add filtering to attenuate the saturation source.

The measure of how much input power an amplifier can take before spurious emissions are generated is the Spurious Free Dynamic Range (DR_f). DR_f is given as the magnitude relation between the fundamental power and the third order intermodulation product.

If the goal is to measure signals that are at the noise floor, the signal that will induce detrimental spurious behaviour is located at DR_f dB over the noise floor.

It is important to note that DR_f is measured at a specific frequency spacing, and that DR_f is typically lower for close spacing. For HF amateur radio it is more common to use smaller spacing for two tone tests, this way interference from nearby stations during contests may be assessed. A common spacing is 2 kHz, where an adjacent station may be located during contests. The intention with these LNAs is to work weak signals. Local interference is more likely than interference from the intended mode of propagation so wide band blocking is

Finally the insertion loss of the LNAs with internal bypass relays are measured. This is measured by a R&S ZNB 8 Vector Network Analyser (VNA) . Calibration is done with a HP 85052B calibration kit.

Results

In the following figures the gain of the devices is shown with a black trace with values in dB on the right hand axis. Similarly the noise figure is shown with a blue trace with values in dB on the left hand axis.

Click each text section to expand the results for the different LNAs.

 

SSB-Electronics SP-7000

SSB-Electronics SP-7000

SP7000 High gain setting

SP7000 High gain setting:

Gain: 22.6 dB

Noise figure: 1.252 dB

Low fundamental tone power (434.95 MHz): -21.22 dBm

High fundamental tone power (435.05 MHz): -22.6 dBm

Low IM3 power (434.85 MHz): – 96.8 dBm

High IM3 power (435.15 MHz): -98.9 dBm

Low side OIP3: 37.73 dBm

High side OIP3: 38.84 dBm

Low side IIP3: 16.24 dBm

High side IIP3: 15.13 dBm

Low side DR_f: 77.68 dB

High side DR_f: 75.46 dB

Insertion loss: 0.28 dB

SP7000 Medium gain setting

SP7000 Medium gain setting:Gain: 16.96 dB

Noise figure: 1.514 dB

Low fundamental tone power (434.95 MHz): -26.93 dBm

High fundamental tone power (435.05 MHz): -26.79 dBm

Low IM3 power (434.85 MHz): -102.2 dBm

High IM3 power (435.15 MHz): -104.3 dBm

Low side OIP3: 37.635 dBm

High side OIP3: 38.755 dBm

Low side IIP3: 20.675 dBm

High side IIP3: 21.795 dBm

Low side DR_f: 75.27 dB

High side DR_f: 77.51 dB

Insertion loss: 0.28 dB

SP7000 low gain setting

SP7000 Low gain setting:Gain: 12.23 dB

Noise figure: 1.841 dB

Low fundamental tone power (434.95 MHz) : -31.35 dBm

High fundamental tone power (435.05 MHz): -31.3 dBm

Low IM3 power (434.85 MHz): -106.8 dBm

High IM3 power (435.15 MHz): -109.1 dBm

Low side OIP3: 37.73 dBm

High side OIP3: 38.9 dBm

Low side IIP3: 25.5 dBm

High side IIP3: 26.67 dBm

Low side DR_f: 75.45 dB

High side DR_f: 77.8 dB

Insertion loss: 0.28 dB

SSB-Electronics SP-70

SSB-Electronics SP-7000

SP70 high gain setting

SP70 High gain setting:

Gain: 22.13 dB

Noise figure: 0.68 dB

Low fundamental tone power (434.95 MHz): -21.64 dBm

High fundamental tone power (435.05 MHz): -21.6 dBm

Low IM3 power (434.85 MHz): – 95.6 dBm

High IM3 power (435.15 MHz): -99.4 dBm

Low side OIP3: 36.98 dB

High side OIP3: 38.9 dB

Low side IIP3: 14.85 dB

High side IIP3: 16.77 dB

Low side DR_f: 73.96 dB

High side DR_f: 77.8 dB

Insertion loss: 0.17 dB

 

SP70 medium gain setting

SP70 Medium gain setting:

Gain:  19.26 dB

Noise figure: 0.78 dB

Low fundamental tone power (434.95 MHz): -24.1 dBm

High fundamental tone power (435.05 MHz): -24.04 dBm

Low IM3 power (434.85 MHz): -98.2 dBm

High IM3 power (435.15 MHz): -102.1 dBm

Low side OIP3: 37.05 dBm

High side OIP3: 39.03 dBm

Low side IIP3: 17.46 dBm

High side IIP3: 19.44 dBm

Low side DR_f: 74.1 dB

High side DR_f: 78.06 dB

Insertion loss: 0.17 dB

SP70 low gain setting

SP70 Low gain setting:

Gain: 12.72 dB

Noise figure: 1.22 dB

Low fundamental tone power (434.95 MHz) : -31 dBm

High fundamental tone power (435.05 MHz): -30.95 dBm

Low IM3 power (434.85 MHz): -105.3 dBm

High IM3 power (435.15 MHz): -108.5 dBm

Low side OIP3: 37.15 dBm

High side OIP3: 38.78 dBm

Low side IIP3: 24.43 dBm

High side IIP3: 26.06 dBm

Low side DR_f: 74.3 dB

High side DR_f: 77.6 dB

Insertion loss: 0.17 dB

SSB-Electronics SP-2000

SSB-Electronics SP-2000

SP2000 high gain setting

SP2000 High gain setting:

Gain: 23.08 dB

Noise figure: 1.9 dB

Low fundamental tone power (144.95 MHz) : -20.95 dBm

High fundamental tone power (145.05 MHz): -20.79 dBm

Low IM3 power (144.85 MHz): -84.5 dBm

High IM3 power (145.15 MHz): -87.8 dBm

Low side OIP3: 31.78 dB

High side OIP3: 33.51 dB

Low side IIP3: 17.86 dB

High side IIP3: 19.60 dB

Low side DR_f: 63.55 dB

High side DR_f:  67.01 dB

Insertion loss: 0.09 dB

SP2000 medium gain setting

SP2000 Medium gain setting:

Gain: 18.58 dB

Noise figure: 1.92 dB

Low fundamental tone power (144.95 MHz) : -25.23 dBm

High fundamental tone power (145.05 MHz): -25.07 dBm

Low IM3 power (144.85 MHz): -89.1 dBm

High IM3 power (145.15 MHz): -92.5 dBm

Low side OIP3: 31.94 dB

High side OIP3: 33.72 dB

Low side IIP3: 8.86 dB

High side IIP3: 10.64 dB

Low side DR_f: 63.87 dB

High side DR_f:  67.43 dB

Insertion loss: 0.09 dB

SP2000 low gain setting

SP2000 Low gain setting:

Gain: 13.91 dB

Noise figure: 2.03 dB

Low fundamental tone power (144.95 MHz) : -29.95 dBm

High fundamental tone power (145.05 MHz): -29.78 dBm

Low IM3 power (144.85 MHz): -93.8 dBm

High IM3 power (145.15 MHz): -96.6 dBm

Low side OIP3: 31.93 dB

High side OIP3: 33.41 dB

Low side IIP3: 13.34 dB

High side IIP3: 14.83 dB

Low side DR_f: 63.85 dB

High side DR_f:  66.82 dB

Insertion loss: 0.09 dB

SSB-Electronics SP-200

SSB-Electronics SP-200

SP200 high gain setting

SP 200 High gain setting:

Gain: 20.5 dB

Noise figure: 0.4 dB

Low fundamental tone power (144.95 MHz) : -22.79 dBm

High fundamental tone power (145.05 MHz): -22.64 dBm

Low IM3 power (144.85 MHz): -95.5 dBm

High IM3 power (145.15 MHz): -95.6 dBm

Low side OIP3: 36.36 dB

High side OIP3: 36.48 dB

Low side IIP3: 23.63 dB

High side IIP3: 23.75 dB

Low side DR_f: 72.71 dB

High side DR_f:  72.96 dB

Insertion loss: 0.06 dB

SP200 medium gain setting

SP 200 Medium gain setting:

Gain: 16.88 dB

Noise figure: 0.36 dB

Low fundamental tone power (144.95 MHz) : -26.67 dBm

High fundamental tone power (145.05 MHz): -26.53 dBm

Low IM3 power (144.85 MHz): -98.6 dBm

High IM3 power (145.15 MHz): -98.4 dBm

Low side OIP3: 35.97 dB

High side OIP3: 35.94 dB

Low side IIP3: 15.47 dB

High side IIP3: 15.44 dB

Low side DR_f: 71.83 dB

High side DR_f:  71.87 dB

Insertion loss: 0.06 dB

SP200 low gain setting

SP200 Low gain setting:

Gain: 12.73 dB

Noise figure: 0.57 dB

Low fundamental tone power (144.95 MHz) : -30.91 dBm

High fundamental tone power (145.05 MHz): -30.75 dBm

Low IM3 power (144.85 MHz): -102.6 dBm

High IM3 power (145.15 MHz): -102.2 dBm

Low side OIP3: 35.85 dB

High side OIP3: 35.73 dB

Low side IIP3: 18.97 dB

High side IIP3: 18.85 dB

Low side DR_f: 71.69 dB

High side DR_f:  71.45 dB

Insertion loss: 0.06 dB

VHFDesign.com LNA 04-0m-v.03

VHFDesign.com LNA 04-0m-v.03

VHFDesign.com 2m LNA

Gain: 23.58 dB

Noise figure: 0.52 dB

Low fundamental tone power (144.95 MHz) : -20.3 dBm

High fundamental tone power (145.05 MHz): -20.1 dBm

Low IM3 power (144.85 MHz): -97.3 dBm

High IM3 power (145.15 MHz): -96.6 dBm

Low side OIP3: 38.5 dB

High side OIP3: 38.25 dB

Low side IIP3: 14.92 dB

High side IIP3: 14.67 dB

Low side DR_f: 77 dB

High side DR_f:  76.5 dB

Kuhne MKU LNA 131 AH

Kuhne MKU LNA 131 AH

Kuhne 23cm LNA

Gain: 21.42 dB

Noise figure: 0.43 dB

Low fundamental tone power (1294.95 MHz) : -23.18 dBm

High fundamental tone power (1295.05 MHz): -23.21 dBm

Low IM3 power (1294.85 MHz): -94.5 dBm

High IM3 power (1295.15 MHz): -102.2 dBm

Low side OIP3: 35.66 dB

High side OIP3: 39.5 dB

Low side IIP3: 14.24 dB

High side IIP3: 39.5 dB

Low side DR_f: 71.32 dB

High side DR_f:  78.99 dB

LNA4ALL

LNA4ALL

The IP3 is found to be on average 34.75 dB by measurements done by DG1TRF, these can be found on the LNA4ALL product page. Below is the noise and gain measurements from 30 MHz to 2 GHz measured with 5 V supply.

LNA4ALL gain and noise figure

 

 

 

 

 

 

 

 

Measuring coax length with burst generator and oscilloscope

I have a quite long Aircell 7 cable that I would like to know the length of, but didn’t want to uncoil. This is a good opportunity to showcase a technique for measuring the length and attenuation of a coaxial cable, using a function generator and an oscilloscope.

Fig 1: Time delay for RG58 patch cable

Measurement background

Using a function generator in burst mode we can measure the reflection from the open end of a coaxial cable. An oscilloscope is connected through a t-junction between the function generator and the test cable. Since the internal resistance of the oscilloscope is high, and current prefers the path of least resistance, and the burst signal will travel to the coaxial cable. A small amount of the signal will coupled to the oscilloscope. We denote this as the incident voltage, U.
Upon reaching the open end of the cable, the wave will reflect and travel back towards the function generator. As the wave passes the oscilloscope a small amount will be coupled. We denote this as the reflected voltage, Ur. The reflected wave finally dissipates when it reaches the function generator.

This is very similar to tying a rope to a pole, swinging it and having the rope reflect back.

Fig 2: Measurement setup

The time difference between Uand Ur is the time it takes for the wave to propagate to the open end of the coax and back again. Using this we can calculate the length of the coaxial cable using the following formula:

Vf is the velocity factor of the coaxial cable and c is the speed of light. Since the time between incident and reflected is the round-trip time we divide the result by two.

By seeing how much the voltage has dropped on the reflected wave relative to the incident wave we can calculate how much loss the coax has at generator frequency. Since the reflected wave passes through the cable twice we should divide by two to find the one-way attenuation.

Some measurements

As mentioned, I have quite long Aircell 7 cable that I would like to know the length of, but didn’t want to uncoil. To keep everything neat I used a short RG58 cable to patch it together. This is the setup shown in figure 2.

The two cables are made using different dielectrics, and will have different velocity factors.  Aircell 7 has a Vf of 0.83, RG58 has a Vf of 0.66. To account for this we should first measure the delay and attenuation caused by the RG58, and then subtract that contribution from the Aircell 7 measurement.

To be able to measure on the short RG58 cable I am using a 1 cycle 100 MHz sine wave burst. The burst is set up to repeat every second, this means that any remaining oscillations should have fully died out. A generator frequency of 10 MHz is sufficient to get accurate results, but only if the cable you are measuring is longer than 20 m.

Fig 3: Unknown length Aircell 7

Fig 4: Voltage drop of RG58 patch

 

 

 

 

 

 

 

 

 

 

 

Figures 1 and 4 show the measurement results from the RG58 patch, we put the results into the formula and get the following results:

I also measured the RG58 coax with a measuring tape, and found the physical length to be 1.55 m.

Fig 5: Voltage drop Aircell 7

Fig 6: Time delay Aircell 7

 

 

 

 

 

 

 

 

 

Finally using the results from figures 5 and 6 we find the length and attenuation of the Aircell 7 cable

In conclusion this method is a quick and efficient way to measure the loss and length of a coaxial cable. If you have a broken cable the breakage point will also reflect, so this can be a very useful tool to pinpoint where you need to mend the cable.
It should also be said that the accuracy of this method depends largely on the accuracy of the velocity factor given in manufacturer specifications, meter order deviations can easily arise from a wrong spec. The influence of the oscilloscope could also matter, some people connect 10X or 100X probes to the t-junction for these measurements, I found it to be fine using just the internal impedance.

Introduction to the libpredict API

libpredict is an ANSI C library for predicting satellite orbits based on TLEs, developed by ARK. This was primarily developed for use in flyby, but can also be useful on its own. If you just want to track a satellite, flyby is usually a better choice, but if you want to go down to a deeper level and be able to apply satellite prediction to more advanced and complex usecases in a more flexible way, libpredict might be suitable.

The goal of libpredict was mainly to separate the satellite calculations from predict for use in its fork, flyby, and enable reuse of the API in other satellite applications. C implementation became a requirement due to the well-defined binary compatibility for C libraries and the use of C in both predict and flyby. While the core routines are in C, we will also at some point be providing high-level bindings for other languages like python. See also: Development of flyby and libpredict.

This post outlines in detail how libpredict can be used to track satellites in a programming language, and is long and technical and probably mostly for those with special interest in the topic. If life gets too frustrating and boring, you can scroll down to the plots and rest your eyes on colorful satellite tracks:-). An earlier post, Satellite tracking using flyby, gives a top-down motivation for why we are doing this at all and a more user-friendly approach to satellite tracking.

Continue reading

Satellite tracking using flyby

A lot of satellites typically have beacons, interesting data transmitters or transponders that can be received or used by radio amateurs. Knowing when and where the satellites are passing overhead is essential. Flyby, through its companion library libpredict, is a software package for estimating such satellite positions. See also: Development of flyby and libpredict.

Exact calculation of the position of the satellites in complicated gravitational fields is very difficult, so simplified perturbation models are generally used. The exact position and other parameters of the satellite is measured (typically by NORAD) at a given time (epoch time). These properties can then be estimated at other times by using a simple gravity model for earth, and including the more complicated gravitational effects from earth and deeper space using perturbation theory. Given updated parameters and “small” time differences from the epoch time (less than a couple of weeks is probably good enough), we can generally predict the position of the satellite to a high enough accuracy that we can point our antennas towards the predicted position and expect the satellite to be there. The set of measured parameters (after being crunched through a related model) is called a TLE, while the prediction model is called SGP4/SDP4. The details on this are not really that important, you can generally assume that a TLE corresponds to a satellite, and that it can be used to calculate a time-dependent position using the appropriate prediction software.

The satellite is speeding overhead while we are approximately standing at rest. The electromagnetic waves transmitted by the satellite will therefore be doppler shifted when we receive them. The satellite transmits at a specific frequency known a priori, but the received frequency is shifted significantly and will change continuously as the satellite passes overhead. The satellite will experience the same thing with respect to our transmitted frequency if we are transmitting anything towards it. Without proper correction of our transmitted signal, the satellite might not even receive anything within its fixed frequency range. In addition to pointing the antenna in the right direction, it is therefore important to also adjust the frequencies correctly. Flyby does both, using the satellite prediction library libpredict, and can automatically communicate the frequency and position changes to a rig and antenna controller using hamlib.

This post will outline the steps involved from compiling Flyby to tracking satellites. Continue reading