How-to

How to: FM modulation with RF Explorer Signal Generator

RF Explorer Signal Generator RFE6GEN does not directly create FM or AM modulation signals, but can be easily used to produce low rate modulation including SINAD signals using Frequency Sweep.

For a FM modulation of 1KHz tone typically used for SINAD, proceed as follows:

  • Select the Start Frequency the value you need (e.g. 2.000000GHz)
  • Select the Stop Frequency the FM deviation you need. For instance for a 5KHz deviation, set exactly 5KHz above (e.g. 2.000005Ghz)
  • Select the Frequency Step same value as FM deviation (e.g. 5KHz)
  • Select the Step Delay. For a 1KHz tone, you need 1ms delay, therefore define Step Delay:00.001
  • Select RF power you need and start transmission using Frequency Sweep operational mode.

With this technique the highest rate tone generated is 1KHz (which correspond to 1ms step delay). With a 2ms step delay, frequency tone is 500Hz, etc.

How to: SNA Measuring RF Filter response

Using Network Analyzer functionality

RF Explorer Signal Generator enables advanced Network Analyzer functionality (SNA) when a RF Explorer Spectrum Analyzer is connected to the same computer. This is an advanced feature, used to identify Insertion Loss/Gain vs Frequency to fully identify S21/S12 scalar response of a 2-port RF device. These include filters, amplifiers, terminated couplers, etc.

In this self contained, 5 minutes tutorial video, you can learn how to measure frequency response of a 2-port RF device such as band pass filter. The same process can be used to measure any other RF device.

What you will learn:

  • How to measure response of a band pass filter, including all RF connections required and RF Explorer for Windows
  • The same process can be used to measure any 2-port RF device.

What you need:

  1. RF Explorer Spectrum Analyzer model for the frequency required for test
  2. RF Explorer Signal Generator RFE6GEN
  3. Quality RF cables
  4. SMA wrench (8mm or 5/16")
  5. Device Under Test (DUT)- in this case we will use a 1675MHz BPF

Video Best viewed in HD 1920x1080 full screen

Definitions

  • Normalization: This is standard SNA requirement to rule out any cable, connector or environmental imperfection. The application will properly measure the response of your setup and will consider that 0dB response in order to refer any measurement to it.
  • DUT: Device Under Test. It is the standard term to refer to the RF device being tested. The DUT can be an amplifier, a filter, etc. Any DUT will require the same steps and can be considered a 2-port black box.
  • SMA Gender: SMA connectors (as most RF connectors) can be male or female. Male is the one with a center pin connector, and female is the one with center hole connector. The gender of the RF Explorer units is standard SMA female, whereas DUT can be any gender. In order to connect RF Explorer Signal Generator on the input of the DUT, and RF Explorer Spectrum Analyzer to the output of the DUT, you need quality cables and connectors. The higher the frequency, the more critical is selecting quality cables, adapters and connectors. We suggest quality 6GHZ semi-rigid cables we offer from SeeedStudio and other distributors (click on images below for more details). You may also need a Female-Female adapter as pictured below when normalizing a DUT response that is Male-Male gender. Warning: Using low quality, long cables may significantly reduce repeatability and resolution of your measurement.

Procedure summarized

  1. Select the START / STOP frequencies to sweep, and the number of steps. The more steps you use, the longer the sweep will take. Recommended values are from 50 - 200 steps for a good compromise of speed and resolution.

  1. Select power: By default you should select -30dBm in the CW power value, that is good for most testing. However, if you are using an attenuated line for normalization and test (e.g. attenuator is in series for some reason, or using a Directional Coupler for reflection / SWR) then increase to -10dBm or more. On the other hand, if the normalization connection is amplified (e.g. with mixer or amplifier of some sort) then decrease to -40dBm. As a general rule, you should select a power level that feeds the Analyzer input port at -25 to -35dBm ideally, as that is the best compromise between high power to reduce environmental noise and still being in the linear range of the analyzer.
  2. First step is normalizing the setup response. To proceed with normalization, you need to use the exact same cables, connectors, adapters and everything you will use with the DUT, except the DUT itself. Sometimes you may need an adapter to plugin both cable ends if the DUT is Male-Male, use a suggested adapter above in that case. Click on [Normalize SNA] and wait a few seconds for the tool to alert on the normalization step is done. The RF Explorer Spectrum Analyzer unit will show TRACKING in clear text and no other functionality is available while SNA is working.
  3. Once normalization step is done, you can now connect the DUT and click on [Start SNA] for continuous tracking sweep. Screen will be updated with SNA results every few seconds.
  4. Important: Do not change START/STOP/STEPS configuration or otherwise you would need to Normalize again. Normalization is valid for a specific setup and needs to be done again if modified. For instance selecting a different number of steps or frequency range or power, will immediate clear normalization data and Start SNA button will be disabled to force a new normalization.
  5. Click on [Stop SNA] to stop tracking anytime.

How to: Measure Directivity of Directional Couplers

Directional couplers are very useful devices, in particular we can use them for two major goals in measurement lab:

  • Characterize return loss / impedance matching / VSWR
  • Measure output power and reflection in real application circuit

For an example of return loss measurement process, check How-to: SNA Measuring Reflection - Return Loss.

Good directional couplers are expensive devices, but in many cases lower cost devices can produce good enough results on limited, reduced frequency ranges. The main topic of this article is how to get the most convenient directional coupler for a particular application, at the lowest possible cost. By understanding what are relevant parameters and how couplers work, we can save time and budget.

We are comparing these three couplers.

DISCLAIMER: every coupler is an unique device that may not perform exactly the same as other of the same model, due to tolerances. Manufacturers typically describe minimum and typical values for most critical parameters, and in most cases you need to really test an unit to know how it performs. We cannot guarantee a device of the same manufacturer and model will perform exactly the same as the ones tested here.

The single most important parameter in a directional coupler used for reflection measurement is the Directivity. This parameter should be in these ranges to be useful:

  • 10dB minimum for any minimally indicative measurement, no accuracy expected
  • 20dB minimum for reliable measurement
  • 30dB or larger ideal for measurement of well-matched devices

What makes Directivity interesting is the fact that changes with frequency. You cannot expect a directional coupler to have a constant directivity at all frequencies. Even the most expensive ones used for instrumentation have directivity variations among frequency range. That is the reason why a directional coupler will be usable to measure reflection over a limited frequency range. Normally the wider the range and higher the directivity, the higher the price too.

In this article we will compare three devices of very different price, as a practical example, and explain how directivity plays a role so the best price/performance compromise can be made.

This table compare performance among them

  DC1 DC2 DC3
Price $295 on Minicircuits shop $25 on eBay, $50 on Minicircuits shop $25 on eBay
Model ZHDC-10-63-S+ ZADC-13-2000-1 / ZNDC-13-2G+ BG7TBL
Freq range for Directivity >= 30dB 200-6000MHz 1200-1500MHz 25-1200MHz
Freq range for Directivity >= 20dB 50-6000MHz 800-2000MHz 25-1600MHz

Other parameters of couplers are not so important for reflection measurement and therefore we will ignore them for the sake of simplicity.

Measure Coupler DC1 Directivity

We will use RF Explorer SNA to measure directivity of these couplers. This same process can be used to measure the directivity of any directional coupler with unknown directivity, or to check the directivity described in the datasheet is correct.

The first step is to normalize the SNA using RF Explorer Spectrum Analyzer and Signal Generator connected to the directional coupler with the INPUT open.

Connect RF Explorer Signal Generator and Spectrum Analyzer as depicted below and then to the PC with USB cables. On the PC load RF Explorer for Windows and define frequency range you want to test the coupler for. Then click on [Normalize SNA].

With this step completed, RF Explorer for Windows memorizes coupler normalized response for maximum reflection and is ready for next measurement, which will indicate directivity as a direct measurement.

Now connect the termination load to the INPUT coupler pin, and [Start SNA] tracking. The response you will get is the best possible impedance match this coupler is able to measure, also known as Directivity.

For the DC1 you will get this response for return loss

This is similar to that of the datasheet, actually a bit better. At high frequencies close to 2500MHz the directivity is exceeding 45dB in this unit.

Note the reading is inversed sign, because the SNA measures return loss in dB (a negative value) whereas interpreting it as Directivity we must change the sign to positive. Therefore a return loss of -35dB translates to a Directivity of +35dB. 

Measure Coupler DC2 Directivity

 Repeating the process for this lower cost directional coupler, normalize with INPUT port open...

... then complete tracking with INPUT port correctly terminated

 Results for directivity are also close but actually better than that of the datasheet

Note 20dB directivity starts at 600Mhz in this unit, which is better than 800MHz predicted by datasheet

 Measure Coupler DC3 Directivity

The interesting difference about this coupler is that was produced as a SWR Bridge, and therefore the port connections come labelled as required for reflection measurement. That is not the case with standard directional couplers which comes labeled for normal coupler operation.

SWR Bridge is a special case of Directional Coupler. Both perform the same function, but they have the same physical ports marked differently. Make sure to observe correct connection for sensible results. This table below may be useful for reference connection when measuring reflection of a device.

Connection Directional Coupler  SWR Bridge 
Signal Generator Output port  Input port 
Spectrum Analyzer Coupled port  Output port 
Device under test (DUT) Input port  DUT or Coupled 

Normalization step as usual...

 ... then complete tracking

Results cannot be compared to a datasheet as we didn’t find any reliable info available, but the vendor advertise this as “1-500MHz with 30dB directivity or better” and we found to be better than that. This unit had a good 35dB directivity or better up to 1000MHz, and better than 20dB up to 1600MHz

 Conclusions

Being directivity the most important parameter of a directional coupler used for reflection / VSWR measurement, it is very easy to characterize your coupler to know how good is in a particular frequency range.

Out of these 3 couplers presented here, it is no surprise the higher cost DC1 clearly outperforms the other two in range. However, DC2 can be used in a fraction of the range and, if combined with DC3, can provide a low cost combo for ranges 1-2000MHz. For higher frequencies, none of these two couplers DC2 or DC3 are recommended.

An upcoming follow up article will show how this directivity plays an important role when characterizing VSWR of a device. Stay tuned.

How to: SNA Measuring Reflection - Return Loss

RF Explorer product family offers advanced features when combining Spectrum Analyzer and Signal Generator working together as a Scalar Network Analyzer for 1-port and 2-port RF devices.

In this tutorial we will describe how to use RF Explorer to measure the <return loss> S11 / S22 of a device in dB. The return loss can be described as the fraction of signal reflected back to the source, and therefore we will use the term <reflection> in this document to make it easier to read by non-expert users. The reflection in dB is a parameter used to describe how good the impedance matching when compared to specifications is. In most cases we want to know the reflection of a 50ohm impedance, the higher the reflection, the worse the impedance matching.

The reflection is a measurement of how much energy incident to a device is being reflected back and, therefore, not entering the system. When measured in dB, the following values can be used as indicative:

  • Reflection 0dB: all RF energy is being reflected. This is an ideal open or short circuit, equivalent to VSWR=infinite.
  • Reflection -3dB: half of the RF energy is being reflected, and thus half of the energy is being received by the device. This is equivalent to VSWR 5.8.
  • Reflection -10dB: 1/10th of the RF energy is being reflected. Usually this is the threshold when most devices are considered to be tuned and have a reasonably good impedance matching. This is equivalent to VSWR 1.9.
  • Reflection -20dB: 1/100th of the RF energy is being reflected. This is a very good matching, expected for good designed and matched filters. This is equivalent to VSWR 1.2.
  • Reflection -30dB or less: 1/1000th or less of the RF energy is being reflected. This is considered exceptionally good matching, and usually found in lab grade devices such as precision attenuators and filters. This is equivalent to VSWR 1.07 or less.

As an example of real world reflection, a narrow band antenna is expected to reflect/reject the RF energy for frequencies that are far from the antenna center tuning frequency, intended by design so the RF input circuit is not overloaded by undesired signals. On the other hand, the narrow band antenna should not reflect but actually be a good receiver for the specific frequency it has been designed for.

Similarly, a good Low Pass Filter (LPF) should offer very small reflection (-10dB or less) for frequencies lower than cutoff frequency, and large reflection (very close to 0dB) for frequencies higher than cutoff frequencies.

With these examples in mind, we will learn how to use RF Explorer SNA to measure reflection in dB.

Items needed

  • RF Explorer Signal Generator RFE6GEN
  • RF Explorer Spectrum Analyzer model for the required frequency
  • 2 or 3 high quality RF cables. We recommend these ones.
  • A good 50ohm termination. We recommend this one.
  • The device to test (antenna, filter, etc)
  • Windows computer with latest RF Explorer for Windows installed.
  • And last but not least, a good directional coupler. We recommend Mini-Circuits ZHDC-10-63-S+ as a good price/performance device for frequencies range 50MHz - 6GHz with good directivity. If you do not need full wideband support but a more restricted frequency range, you can consider a lower cost limited bandwidth coupler. Select one with 30dB directivity or more for best results, and never use couplers with directivity lower than 20dB. Look at this article for other options.

(click on images to get a larger view)

Normalizing

This step is required so the components are calibrated for use with the SNA and all signal references are internally recorded by RF Explorer for Windows. All measurements done with a SNA start with a normalization step, and reflection measurement is no exception.

The most confusing detail for novice users is how to correctly connect a directional coupler for reflection measurement. See example below on how to connect for normalization. Note the open (unconnected) INPUT port in the directional coupler.

 

 

  • Connect RF Explorer Signal Generator to the OUTPUT port of the directional coupler
  • Connect RF Explorer Spectrum Analyzer to the COUPLED port of the directional coupler
  • Keep the INPUT port of the directional coupler OPEN for normalization step. If you need a cable to later connect the antenna with the right orientation, then add the same cable to the INPUT port but keep the cable end OPEN.
  • Connect RF Explorer Spectrum Analyzer to the computer USB port
  • Load RF Explorer for Windows. The RF Explorer Spectrum Analyzer should be automatically connected.
  • Connect RF Explorer Signal Generator to the same computer, different USB port
  • Using the Signal Generator tab in the RF Explorer for Windows tool, find the right COM port and click on Connect. At this point both Analyzer and Generator should be connected and therefore the SNA functionality is available.
  • Select Power=-30dBm for directional couplers with coupled port of 10dB or less, as the one recommended. If you are using a coupler with a coupled port of 20dB or more, you may need to increase power up to -10dBm.
  • Select Start/Stop frequency you are trying to characterize response for. Select steps in the range of 25-50 to start with, you may want to increase to 100-200 for ultra-detailed graph, at the cost of slower trace updates.
  • Click on [Normalize SNA…] button, wait till the process completes.

To confirm the normalization works as expected, click on [Start SNA…] button using the same connections, you should see an almost flat line, with small noise like the graph below, usually in the range of +-0.5dB or less.

 

 

Measuring reflection for an antenna

An antenna is a 1-port device and therefore easy to connect. And that is the only easy thing we can say about measuring reflection in an antenna.

As a matter of fact, antennas are very tricky devices because while it is being measured, is influenced by all sort of items in the environment:

  • External unintended RF sources will interfere with measurements, as the antenna is receiving at all times
  • Walls and metallic objects will produce external reflection which are bounced back to the antenna while the SNA is performing measurement, distorting performance
  • Most antennas, such as monopole and dipoles, are influenced by ground. An antenna designed for a handheld 2-way radio, for instance, is adjusted for best performance when a human is holding it on his/her hand and, therefore, connected to a bench setup will not be the exact same environment where it is expected to work best.

Due to all this, high precision antenna measurements must be done in special RF chambers, where all external and internal radiations are dramatically reduced.

For most users, an anechoic RF chamber is not available and so only imperfect measurements can be done. That said, the closer we get to an environment where the antenna is expected to work in real life, the better. Keep it far from metallic objects, set it vertical with the help of a good semi rigid RF cable, and minimize external RF sources as much as possible (don’t measure antennas any close to your cell phone, WiFi AP, etc).

Connect the antenna to the coupler INPUT port, and click on [Start SNA…]

 

 

An example of the connection and graph for a 216MHz antenna is shown below.

 

 

Measuring reflection of a LPF

Filters are easier to measure than an antenna but, being 2-port devices, is important to remember it must be correctly terminated. In other words, one of the filter ports must be connected to a SMA 50ohm load, and the other port used for reflection measurement.

In RF terminology, one filter port is 1 (input) and the other is 2 (output). Most filters are bidirectional and therefore port 1 and 2 are expected to work the same. When you terminate port 2 with a load and measure port 1, the resulting measurement is S11 return loss. By doing otherwise, you measure S22. Most filters will exhibit S11 and S22 identical responses, so there is no real need to measure response on both ports except if trying to find problems in a faulty filter.

Below is a sketch of all items you need to first normalize, then measure your filter

 

Connect the filter (terminated with 50ohm load) to the coupler INPUT port, and click on [Start SNA…]

 

 

An example of the connection and graph for a 200MHz LPF is shown below.

 

5 minutes video tutorial

This video tutorial is only 5 minutes long and drives you through the steps required to measure VSWR of a 800MHz antenna.

Video Best viewed in HD 1920x1080 full screen

 

Additional notes and special cases