RF Workbench Page 2

This is Part 2 of the RF Workbench series - a simple examination of basic RF measurement and bench practices. This installment builds on the information from the RF Workbench Page 1 web page.

I share introductory and practical content on attenuation, the return loss bridge, insertion loss or gain and spectrum analyzers. Consult EMRFD and use your favorite web search engine for additional information.

The Attenuator Network

An attenuator is a passive network which places an intentional power loss between 2 components independent of the frequency. I summarize some basic attenuator principles in the following bullets:

Most attenuator networks have fixed input and output impedances.
The input and output impedances may be similar or different.
Attenuation networks are balanced or unbalanced.
The attenuation value may be fixed or variable. Most often they are simple, fixed resistive pads which function as voltage dividers.
For signal measurement, attenuators are utilized to lower AC voltage, dissipate power, or help to match the impedance between 2 stages. Example uses include placing an attenuator after a signal source to decrease the power delivered to a 50 ohm load, or to calibrate an S-Meter.
Attenuation is expressed in dB.
Attenuators increase return loss and lower VSWR
Attenuators can function as buffers to isolate stages during measurement
The attenuators on this page should be fed with a 50 ohms signal source and terminated in 50 ohms.

The desired response of an attenuator network. Practically speaking, device construction techniques including shielding will limit how high in frequency your attenuator will properly operate.

A pair of commercial 50 ohms input/output impedance step attenuators from the past. Such devices are often for sale at Ham festivals or estate sales. They are reasonable tools for HF and perhaps even VHF work depending on their design and condition. Visually examine and test the attenuator before use in actual circuit testing.

Designing and Building Attenuator Networks

Numerous programs for designing attenuator networks exist, but a table is easy to use. Click here for such a table.

Fixed attenuator networks are easy to build. The degree of precision is up to you. Nearest value 5% tolerance resistors work well for popcorn circuits, however combinations of 1% or 1% plus 5% tolerance resistors may allow closer matching to calculated resistor values.

Carbon film or other low inductance type resistors are best used for making attenuator pads. Keep lead lengths short. Consider the resistor power dissipation values. For example, an 8 dB attenuator pad will dissipate 84% of the RF passed through it. I have seen attenuator pads which were exposed to high power and some or all of the resistors were burnt and turned to charcoal. Clearly the operator did not regard the power rating of the attenuator resistors. Refer to EMRFD Section 7.4 for practical information concerning attenuator design and power dissipation.

50 ohm attenuator pads from my bench drawer

Three of the attenuator pads from my bench attenuator drawer. When using Ugly Construction or its variants, you can solder in, change or remove attenuator pads at a whim. A small stock of these pads speeds up experimentation.

A 10 dB attenuator pad from my collection. This box uses two 100 ohm (5%) resistors and a 68 ohm (1%) resistor for the 96.2 and 71.2 ohm resistances called for. The 1% tolerance part was used because my collection of 68 ohm resistors are metal film 1% tolerance 1/4 watt types. Perhaps, a closer match to the calculated resistor values will be pursued one day, but this box works fine.

Two commercial BNC in-line attenuators. I use them every day.

Step Attenuators

A step attenuator belongs on every serious RF workbench. They allow in-situ attenuation adjustment with a degree of precision as low as 1 dB. Step attenuators are nothing more than switched calibrated resistances and the switches can be SPDT, relays, rotary or digitally-controlled types. The quality and price of commercial attenuators varies widely. Experimenter concerns include the minimum attenuator insertion loss, power rating, return loss, noise from switch contacts and noise from the resistors themselves.

A homebrew step attenuator makes a great weekend project and almost every radio handbook contains 1. Web linked projects plus commercial kits may be found online — use your favorite search engine to locate them.

Many homebuilders use 1% metal film resistors to keep resistor noise down. A serious step attenuator should be encased in a metal, RF-proof box and have quality interfaces such as BNC, N or SMA connectors. Your needs, budget and parts collection determine the outcome when you home build one.

Serebriakova Attenuator - Серебрякова аттенюатор (50 Ом)

The Serebriakova; a simple, variable attenuator well suited for QRP homebuilding. Filled with gratitude to its Russian designer's family, I share this contribution with my readers. This attenuator network makes signals smaller or larger in a 50 ohm environment via a potentiometer. My analysis indicates acceptable performance considering its simplicity. The input match is close to 50 Ω across the range of the potentiometer. The output match across the potentiometer range is mediocre. Click here for a DC match analysis from Wes, W7ZOI. As shown, you wouldn't place this device on your main bench signal generator output as the output impedance diverges widely during amplitude adjustment.

Add fixed attenuator pads on the input and/or output to improve matching into 50 ohms. This circuit could serve in multiple applications including an RF gain control on a receiver front end, for bench measurement (when adapted) and for a low-level transmitter gain control. The Serebriakova attenuator may function up to 500 MHz in a carefully constructed, shielded box. The input and output capacitors may be omitted below 30 MHz. The attenuation varies a minimum of 20 dB when turning the potentiometer from CCW to CW.

A variant of the Serebriakova attenuator is shown above. Input and output matching are enhanced by fixed attenuator pads. The input match into 50 ohms is fine. After testing, I learned that the fixed 4 dB output attenuator pad is likely too low to ensure a wide range output match into 50 ohms. A 6 or 10 dB output pad is preferable, however, if this is your only variable attenuator, the device would then only be usable for very low-level work. You can decide what value of input or output pads to use.

A new, clean and small size 500 ohm pot works best. Store your potentiometer collection in sealed plastic bags to keep out workshop and house dust.

Shown above are return loss (RL) and VSWR measurements performed on the adapted Serebriakova attenuator shown above. Clearly the input match is better than the output match. The output match did not significantly change when the attenuation switch was moved from 4 to 10 dB attenuation or back.

Based upon these values, it would likely be a better compromise to put a 3 to 4 dB pad on the input and a 10 dB attenuator pad on the output to ensure an output RL of at least 20 dB. Some might argue that the output RL should be higher. Perhaps, but the match is pretty good for such a simple circuit. Let's put it in perspective; a commercial signal generator that sells on the Internet for $450.00 U.S dollars was measured by a builder I know in the UK and had a best case RL of 10 dB! He later sold it and built a homebrew signal generator that was vastly better in terms of matching. A lot of stuff we use does not have well defined input/output impedances and we get along pretty well despite this.

Fixed attenuator pads are a good remedy for mismatching issues and I discuss them in the next section.

The shielded, adapted Serebriakova attenuator. When home building your personal version, strive to make the AC connections as short as possible. The above device has nearly 23 dB of variable attenuation at 14 MHz. If you can't build, find, nor afford a precision step attenuator for your QRP workbench, this device may work reasonably well for you.

Impedance Matching, Return Loss and VSWR

A lot of gear we build, buy and use has a stated nominal 50 ohms input or output impedance. In truth, a pure 50 Ω impedance is rare and components in an RF system are frequently mismatched. Almost every Ham Radio operator will try to match the impedance of their antenna to the feed line and radio for maximum transmitter output power. Radio and antenna system matching is often the only case where impedance matching is considered.

On your bench, measuring the impedance match between 50 ohm RF system components may also be useful. RF components commonly measured include signal generators, filters, splitters or attenuator networks. You can easily measure the impedance match of your RF components using basic circuits based upon a Wheatstone bridge. On the RF workbench, terms like return loss, reflection coefficient and VSWR are used to quantify impedance matching. Only return loss and VSWR are considered.

When 2 system components are impedance matched, maximal power transfers from one device to the other. If the impedances are different, RF power is reflected back to the signal source. This reduces the amount of power delivered to the load. Transmitted and reflected waves moving along a transmission line superimpose and cause standing or stationary waves. The greater the impedance mismatch between the 2 components, the larger the amplitude of the standing waves. Mathematical formulas are used to compute how much power is lost due to mismatch. Wenzel Associates provides a great tutorial on SWR, Return Loss, and Reflection Coefficient linked here.

Return Loss

Return Loss = the difference between the outgoing incident power and the reflected power as a result of the mismatch between the the signal source and its load. Return loss is expressed in dB as a positive number on this web page. The higher the return loss, the better the impedance match. An ideal prefect match would have a RL of infinity; that is no power is reflected back to the signal source and all of the incident power is delivered to the load. If a circuit has no load (open circuit), the RL is 0 dB, as all of the power is reflected back to the signal source.

Another term for return loss = S11, however S11 is the negative of return loss: RL = 20 dB or S11 = -20 dB.

VSWR

Voltage standing wave ratio is another measure of how well the components of an RF network are impedance matched. Increasing the return loss lowers the VSWR and vice-versa. Most amateur radio enthusiasts are familiar with VSWR and often refer to it as "match" or "SWR". RL and VSWR can be derived mathematically from one other. VSWR = [10^(RL/20) + 1] / [10^(RL/20) - 1]. Note X ^ Y means X raised to the power of Y therefore 2^3 = 2x2X2 = 8.

Thus a RL of 10 dB = 1: 1.92 VSWR and 20 dB = 1:1.2 VSWR and 30 dB = 1:1.07 VSWR

In EMRFD, a return loss bridge is presented as Figure 7.41. This circuit is shown below and is very easy to build and use.

The 50 ohm impedance detector may include a spectrum analyzer, power meter, receiver with an attenuator, or a 50 ohm terminated oscilloscope. On my bench, a 50 ohm terminated scope is used. A practical example is to measure the return loss of a signal generator. The procedure is as follows:

Connect the 50 ohm output impedance signal generator to the bridge RF input port with 50 ohm coax. Connect a 50 ohm terminated oscilloscope to the detector port via 50 ohm coaxial cable. Turn on the signal generator with no load (open circuit) on the Unknown Impedance port. Record the peak-to-peak voltage. Next connect a 50 ohm resistive load to the Unknown Impedance port and record the peak-peak-voltage.

The power in dBm of each of the 2 peak-to-peak voltages are calculated. Return loss of the signal generator equals the difference in dB between these 2 values. You can use math or software to calculate power in dBm. Please refer to the RF Workbench Page 1 for information how to calculate power using formulae or software. A JavaScript Applet which take the 2 peak-to-peak voltages and calculates RL and VSWR is labeled as K on this web page. Numerous related applications are available on the World Wide Web.

Shown above are measurements taken using a homebuilt, low distortion, 14 MHz, crystal controlled signal generator. The return loss = 39.2 dB which is quite good. The return loss of some commercial gear in my shack and yard was measured. Two examples are shared: A high grade, commercial transceiver I was borrowing had an input port return loss of 15 dB. This is a 1.4:1 VSWR. The return loss of 15 dB indicates that the reflected wave power is 15dB lower in power than the incident wave. My antenna tuner matched antenna had a return loss of ~60 dB, although it was difficult to achieve this as the tuner capacitor knobs do not have reduction gears and this match was difficult to obtain with my clumsy hands.

The RL bridge from my QRP workbench. 51 ohm 5% tolerance resistors were used. These could be replaced with 49.9 ohm 1% metal film resistors, however this would not improve the bridge to any great extent. Click for a photo of some return loss measurement tools.

Return Loss and the Attenuator Network (How Do Attenuator Pads Improve Component Matching?)

Attenuator pads are routinely used to increase return loss in an RF environment. For example, lets say you are testing a signal generator and measure a return loss of 6 dB. If you place a 10 dB attenuator pad after the signal generator, the return loss increases to 26 dB. If we used a 6 dB pad instead, the return loss would be 18 dB. In both cases the return loss is increased by 2x the attenuator pad value. The doubling of return loss occurs because both the incident wave and reflected signals pass through the attenuator pad. This is how attenuator pads improve matching.

The biggest problem with attenuator pads is signal loss, however, you can often mitigate this when designing or choosing the output power of the signal source and/or amplifiers in your circuit or measuring equipment.

What is the minimally acceptable return loss for a device such as a signal generator? There is no single correct answer. The minimum return loss depends on the context; for example, performing precision measurement versus just tuning an antenna.

Precision measurement:

For amateur experimenter bench measurement, aim for a return loss of at least 20 dB. This often means adding attenuator pads to the output of your signal generator, amplifier, or other device to get a minimum 20 dB return loss. For an electronic engineer, the minimal return loss is probably higher; maybe 30 dB or so. I have read conflicting opinions about this and for some people, design overkill is normal.

Antenna tuning:

When tuning an antenna for full transmitter output power, the minimal return loss is around 14 dB (a VSWR of 1:1.5). If your antenna system return loss is 14 dB or better, the match is fine. Many Hams will protest a 1:1.5 VSWR and ardently chase a 1:1 VSWR on every frequency with their antenna tuner.

A Method to Measure Insertion Loss or Gain

Often experimenters wish to measure the gain of an amplifier or the insertion loss of a filter or attenuator pad. A method is presented that uses a 50 ohm terminated scope and measurement environment. The signal generator should provide a low distortion RF output to ensure accurate power measurement.

The circuit starts with a signal generator set to the frequency of interest. This signal generator is connected to a 6 to 10 dB attenuator pad module to increase the return loss and match. The reason this pad is included as shown is that I have observed that many signal generators built and used by experimenters lack a definitive output impedance. The signal generators on my workbench typically have a built in 6 - 10 dB output termination pad and have a return loss of at least 20 dB.

A Device Under Test having a 50 ohms input and output impedance is connected to the attenuator pad via 50 ohm coax. Finally the device output is connected to a 50 ohms terminated oscilloscope. The signal generator is turned on and signal peaking is performed. In the case of a low-pass filter, the signal generator frequency control is tweaked to give the maximum output voltage in the scope. If you are evaluating a band-pass filter, the filter trimmer capacitors are adjusted to give maximum signal at the desired center frequency.

Signal voltage peaking ensures that losses caused by the filters are not caused by the filter being mistuned or in the case of the low-pass filter, to allow for cutoff frequency deviation caused by component value variations. It may be necessary to increase the signal generator amplitude to allow a good quality signal to be observed in the scope.

Record the peak-to-peak voltage (or RMS voltage if available and preferred). The Device Under Test is then removed and replaced with a BNC barrel connector. The peak-to-peak voltage is again recorded. The power of each of the 2 recorded voltages are calculated in dBm and the difference is the insertion loss or gain as appropriate.

This measurement circuit has a controlled input and output impedance and uses the same coaxial cables with and without the Device Under Test. This represents an excellent measurement technique. Some builders may choose to terminate the Device Under Test with a 50 ohms resistor and measure with a 10X scope. The capacitance of the probe can alter measurement in some cases. As always, choose your measurement technique based upon whatever gear you own and how exacting your standards are.

Spectrum Analyzers - Comments from the Workbench

Electronics professionals ruminate that spectrum analyzers are uncommon because experimenters perceive them as esoteric and difficult. My own opinion differs. Spectrum analyzers are relatively uncommon because of one reason - cost. I have watched prices on sites like eBay with amazement. The ads go something like this: 1.5 GHz spectrum analyzer for sale. Built in 1982. Ships in 2 pieces weighing over 22 kilograms. Minimum bid $1850.00. And...sorry, I live in Florida, U.S.A. and in all likelihood, shipping these 2 heavy pieces is going to cost you a fortune. In the attached ad photos you can see lots of wear and tear, plus some screen burn-in on the display.... Guaranteed to turn on however!

Perhaps I exaggerate or even lampoon the perceived value of old boat anchor spectrum analyzers, but I have bought and sold cars for less money. Be prepared - spectrum analyzers are not cheap. They are however, very cool and open the door into a truly fascinating world. Frequency domain circuit measurement (spectrum analysis) addicts and intrigues. Homebuilding a spectrum analyzer is a serious option, but requires advanced building skills.

Prior to using a spectrum analyzer, the concepts of shielding and putting critical pieces in RF-proof boxes received casual interest. Quickly I learned that RF in our home and community can and does get into your projects.

The center frequency of the display is ~150 MHz. These are local police and ambulance FM radio conversations. The spikes appear and disappear after 4-9 seconds or so. This interference was noticed when I took the lid off a band-pass filter chassis in a circuit under test. This RFI is very low level, however, serves to inform. Numerous sources of RF were found in our home using a spectrum analyzer. The worse RFI generator in out home is our clothes washing machine during its spin cycle. RF-tight shielding eliminated many of the RFI problems during my experiments.

Spectrum Analyzer Calibrator

Shown above is a spectrum analyzer (SA) calibrator designed by Wes, W7ZOI and used by permission. This output is rich in harmonics to assess calibration. The 10K potentiometer is adjusted to provide the output voltage which will calibrate your spectrum analyzer. Mine was set to either -27 dBm or - 17 dBm. A 10 dB attenuator pad is often used in conjunction with this circuit. You must be very careful when connecting a signal generator to a spectrum analyzer as a higher than rated SA input voltage can destroy the mixer/front-end of your spectrum analyzer and may be very expensive to repair.

The above filter was used to set the correct output voltage for the SA calibrator. The procedure is explained on page 1 of the RF Workbench series. The spectrum analyzer calibrator is low pass filtered and the peak-to-peak or RMS voltage is measured while adjusting the 10K trimmer pot. RMS was chosen for this measurement. For a -27 dBm output, the RMS voltage from my Rigol scope was 10 mV. When a 10 dB attenuator pad is used, the RMS voltage should be 31.6 mV. Measurement was performed with a 50 ohm terminated scope.