Septmber 2005, Issue 182

Signal Generation Solution
Build an Inexpensive RF Signal Generator

IMPEDANCE MATCHING

Voltage standing wave ratio (VSWR) is an important term to understand when it comes to impedance matching at RF. In it’s simplest form, a VSWR = 1, which is sometimes written as 1:1, means that the source and load have a matched impedance such as a 50-W source driving a 50-W load. If a purely resistive load impedance were twice that of a purely resistive driving source impedance, as would be the case if a 50-W resistive source were driving a 100-W resistive load, then the VSWR would be 2:1. 

So why so much focus on impedance matching at RF? At RF, if there’s an impedance mismatch, some of the signal gets reflected back from the load to the driving source. A consequence is that some of the power that you wanted to deliver to the next stage in the circuit is reduced. For example, if you have a VSWR of 5:1, you lose 2.55 dB of your transmitted signal.

Another consequence is that the signal reflected back to the driving source can cause problems. For example, with a VSWR of 5:1, 44% of the power is reflected back to the source. In a high-power amplifier, this reflected signal could be so strong that it damages the amplifier’s output stage.

Another example is when you have a mismatch at the output of a mixer. The reflected signal gets sent back into the mixer and is remixed. This could produce spurious outputs. A table showing some typical VSWR values and what the power loss/reflection would be for various mismatches is available, along with some other useful items, on the Circuit Cellar FTP site.

As it turns out, most RF parts don’t have the typically used characteristic impedance of 50 W. In fact, some parts are dramatically different. Take a simple low-pass filter for example. In the passband for the filter, the VSWR is typically 1.1. Pretty good! Well, not so fast. In the stopband the VSWR of the filter typically can be 20:1 or worse. What that means is that almost all of the power (82%) outside the passband of the filter is reflected back to the driving source. If the filter were at the output of a mixer, the consequences of this mismatch could cause significant problems in the mixing process.

So, what can you do if most parts aren’t the nominal 50 W and you can’t live with the consequences? There’s plenty of literature available about using L/C networks to match impedances for narrowband designs. However, in wideband systems like the signal generator described in this article, a common technique is to use resistor pads between the mismatched impedances. Because they are purely resistive, they work their magic over a broad frequency spectrum.

What happens with a resistor pad is that the signals that eventually will be reflected back from the load will be attenuated on the path forward to the load and on the return trip. This effectively alters the VSWR as seen by the source because a lower reflected signal appears back at the source. Of course, the VSWR is improved at the expense of attenuation to the wanted signal as well. This is tolerable in many cases, as you’ll see in the RF generator design I’ll describe later.

The VSWR improvement using resistor pads can be dramatic. For example, a VSWR of 20:1 can be changed to a VSWR of 1.59:1 with the addition of a 6 dB pad. A table showing how various VSWR values can be changed with different pads is posted on the FTP site.