How to Demonstrate SSB Upconverter Performance

Upconverters are ubiquitous in modern RF systems, translating everything from baseband quadrature DDS signals to many of today’s mmWave signals. An X-band upconverter with sideband suppression utilizing an IQ mixer is prototyped in this article to enable the reader to better understand the up-conversion process and the mathematics behind sideband suppression itself.

While integrated forms of upconverters are widely available for many applications, it is interesting to construct a modular upconverter with a significant number of Mini-Circuits’ parts to see how the components interact and how each contributes to overall system performance. Filtering, frequency multiplication, amplification, signals in quadrature, and sideband suppression are a few of the concepts covered when reviewing the system architecture. Finally, we discuss use cases where up-conversion combined with sideband suppression is essential for achieving RF system performance goals.

The Traditional Upconverter for Single Sideband (SSB) Transmission

The traditional or fundamental upconverter shown in Figure 1 is comprised of a mixer and filter and has typical outputs of f_LO ± f_IF. It is understood that, in reality, the output will include spurs and harmonics. The benefits of the fundamental mixer are that it is very common, very broadband, and relatively inexpensive to implement. The main drawback is that filtering hardware is often very large, very difficult and costly to design due to the filter shape factor required to achieve sufficient sideband suppression.

In the following paragraphs it is shown that sideband suppression may be better achieved through the utilization of an IQ mixer and mathematical cancellation of the upper sideband than by the traditional filtering shown in Figure 1. The performance of the demonstration hardware described herein provides the designer with a greater appreciation for the operation of a SSB upconverter than perhaps a purely theoretical article would.

The Single Sideband Upconverter Using an IQ Mixer

The single sideband (SSB) “modulator” using phasing, or signals in quadrature, was first invented by Ralph V. L. Hartley 100 years ago. From that pioneering work by Hartley, we fast-forward those 100 years to 2025 and to a block diagram of the modular single sideband (SSB) upconverter system demonstrated by Mini-Circuits, shown in Figure 2. The four primary sections of the RF system are the test equipment (essential for any product demonstration), the LO chain, the upconverter section itself, and the amplifier chain. The frequency plan for the upconverter is quite simple, mix an IF frequency of 1.5 GHz with an LO of 11.5 GHz to achieve 10 GHz and 13 GHz RF outputs (f_RF = f_LO ± f_IF).

We will take a “bottoms-up” approach in describing the block diagram, beginning with the test equipment and proceeding through the LO chain to the upconverter section, and out the amplifier chain, returning to the test equipment level.

Signal Generation

The IF and LO signals originate in a pair of Mini-Circuits’ SSG wideband signal generators, the SSG-15G-RC and the SSG-44GHP-RC, respectively. The SSG-15G-RC is a 10 MHz to 15 GHz RF signal generator capable of outputting signals over a dynamic range of -50 to +16 dBm, which is perfect for a 1.5 GHz IF signal. The SSG-44GHP-RC can output +23 dBm of power and operates from 100 MHz to 44 GHz, making it an optimum LO generator. Our system requirements do not push the frequency limits of either signal generator, as we only use 1.5 GHz IF and only require half the final LO frequency (5.75 GHz) of the 44 GHz generator, but their spectral purity and especially versatility in terms of remote control come in handy.

Creating the Local Oscillator

As mentioned previously, the LO originates as a 5.75 GHz signal sourced by the SSG-44GHP-RC signal generator which drives +18 dBm into a wideband, 4 to 14 GHz frequency doubler, the CY2-143+. The emphasis is on simulating a scenario in which only a C-band signal (5.75 GHz) is available for formulating the LO. Demonstrating the diverse nature of the Mini-Circuits frequency conversion portfolio, a total of five components in the LO chain convert the 5.75 GHz C-band signal into the X-band LO (11.5 GHz). Once doubled to 11.5 GHz by the CY2-143+, the LO is stripped of the fundamental, any doubler spurious products, subharmonics and higher order harmonics by the BFHKI-1252+ LTCC bandpass filter, which has a passband of 10.9 to 13.9 GHz. This filter attenuates the 5.75 GHz fundamental by greater than 68 dB and the 17.25 GHz third harmonic by approximately 55 dB.

The next component in the LO chain is the BLK-18W-S+ DC block. If omitted, the bias voltage of the low phase noise LO driver amplifier, the LVA-273PN+ will be pulled down to the point at which it is disabled. The LVA-273PN+ is an excellent choice of LO driver due to its compatible compression level as well as the fact that it’s a low phase noise amplifier. The spot noise at 10 kHz offset is impressive at -172 dBc/Hz and its integrated phase noise measures just -112.76 dBc (integrated from 30 Hz to 1 MHz offset). The extremely low additive phase noise exhibited by the LVA-273PN+ is essential in most all LO chains in order to preserve the often-pristine nature of the LO generators themselves. The LVA-273PN+ drives an LO signal at its P_1dB level of +18 dBm into the IQ mixer. Note also that the LO driver amp is biased using Mini-Circuits’ MBT-283+ MMIC wideband, 1.5 to 28 GHz bias tee, which tends to be an easier and often higher-performance method of applying V_CC than that of using discrete components.

The Upconverter and Sideband Suppression

The heart and soul of the entire RF upconverter system can be found in the upper lefthand section entitled “Upconverter,” and consists of a quadrature hybrid (QCN-19+), the SMIQ-5143H+, and the ABF-9R3G+. The 1.5 GHz IF input is first split into a pair of quadrature signals (I and Q) by utilizing the QCN-19+ LTCC, 1100 to 1925 MHz quadrature hybrid. The I and Q signals are driven into the IF input ports of each of the internal mixers of the SMIQ-5143H+, as shown in Figure 2. Two LO signals in quadrature are formed by the 90⁰ hybrid internal to the SMIQ-5143H+ mixer, and those LO signals drive the LO ports of each internal mixer. The upconverted RF output is constructed using the SMIQ mixer’s internal, in-phase combiner.

The desired (lower, 10 GHz) RF sideband output resides almost exactly on the upper -3 dB frequency of the ABF-9R3G+ thin-film bandpass filter. That frequency location is so optimum, that the ABF-9R3G+ attenuates the undesired (upper, 13 GHz) by nearly 43 dB. Other harmonic and spurious content of the RF signal is rejected by this bandpass filter as well. The cleaned-up 10 GHz RF sideband is then passed on to the amplifier section. It is essential to take a closer look at the up-conversion process both mathematically and spectrally, so that the performance of the demonstration hardware is well-understood.

SSB Upconverter Demonstration Hardware – A Deeper Dive

The upconverter section has been isolated and repeated in Figure 3, and also simplified slightly due to the removal of the clean-up bandpass filter. We stated previously that when we beat the 11.5 GHz LO against an IF frequency of 1.5 GHz, through the mixing (multiplication) process we get 10 GHz and 13 GHz RF outputs (f_RF = f_LO ± f_IF). We now put the emphasis on determining how the upper sideband is suppressed, not utilizing a filter component, but by orienting quadrature signals such that mathematical cancellation occurs.

The IF signal is split into two signals in quadrature, or 90 degrees apart. Note that we make the in-phase signal a cosine function, and the quadrature signal a sine, since the two are 90 degrees apart. Likewise, the LO signal is split into an in-phase (cosine) and quadrature (sine) pair, and applied to the two internal mixers at their respective ports. What happens next is remarkable. When the two cosine functions are mixed (multiplied) with each other, we have:

cos(f_LO)*cos(f_IF) = cos(A)*cos(B) = ½[(cos(A-B) + cos(A+B)] = ½[cos(f_LO – f_IF) + cos(f_LO + f_IF)]

and for the sine functions:

sin(f_LO)*sin(f_IF) = sin(A)*sin(B) = ½[(cos(A-B) – cos(A+B)] = ½[cos(f_LO – f_IF) – cos(f_LO + f_IF)]

Finally, internal to the SMIQ-5143H+ mixer is an in-phase combiner on the RF output port, which simply sums the mixer outputs mathematically, so we find that:

RF output = ½[cos(f_LO – f_IF) + cos(f_LO + f_IF)] + ½[cos(f_LO – f_IF) – cos(f_LO + f_IF)] = cos(f_LO – f_IF) = cos[2π(10 GHz)]

Mathematically, the upper sideband is eliminated. The IQ mixer in the upconverter application performs the sideband suppression as long as the two IF inputs that are fed into it are in quadrature, affording additional “filtering” in the design.

Note that we could have utilized polar notation in the calculations above, with a power magnitude and angle, but if we’re being honest, it really doesn’t look as cool as trigonometric functions, and the upper sideband cancellation does not look as intuitive either.

The Amplifier Chain

The amplitude of the 1.5 GHz IF signal from the SSG-15G-RC is not given, but let’s suppose it is -10 dBm. The insertion loss of the QCN-19+ quad is only approximately 0.7 dB and the conversion loss of the SMIQ-5143H+ is just short of 8 dB, so let’s round up to 10 dB for the pair. We’ve already stated that we’re at the -3 dB point of the ABF-9R3G+, making the RF output attenuation value from the signal generator to the amplifier chain -13 dB. A close look at the amplifier lineup reveals that the PMA3-5123+ preamp exhibits a typical gain of 21.6 dB and the PMA5-123-3W+ final stage a typical gain of 28.3 dB. The two-stage cascade achieves very nearly 50 dB of gain. 13 dB of insertion/conversion loss followed by 50 dB of gain yields 37 dB net gain through the upconverter/amplifier. Consequently, -10 dBm + 37 dB = +27 dBm, which is approximately 7 dB backed off from P_1dB for the final stage which will help mitigate spectral regrowth that occurs due to compression. Finally, since +27 dBm is near the maximum input power limit of many spectrum analyzers, the BW-S10W20+ 10 dB, 20W attenuator is inserted in-line with the final stage-to-spectrum analyzer plumbing.

What would we expect to find on the spectrum analyzer? If the phase relationship between the IF I and Q inputs was random, Figure 4a would apply, whereas for the I and Q inputs in quadrature, implemented in Mini-Circuits’ SSB upconverter demo system, Figure 4b applies. The level of sideband suppression has two major contributors, (1) The sideband suppression of the SMIQ-5143H+ mixer itself which is approximately 32 dB at an LO frequency of 11.5 GHz, and (2) the attenuation of the RF cleanup filter, the ABF-9R3G+ between 10 GHz and 13 GHz, or 43 dB. In addition to excellent upper sideband attenuation, the ABF-9R3G+ also knocks the 11.5 GHz LO signal down by 36 dB.

Use Cases and Applications for Upconverters Similar to the Demonstration Model

SSB has been around for many decades, in fact for more than 100 years. SSB is a staple in many legacy systems such as ham (amateur) radio, maritime radio, aircraft or avionics communications, shortwave broadcast communications, and many defense applications for secure voice and data.

In recent years, the level of data throughput transmitted using digital waveforms has grown exponentially, while the amount of available frequency spectrum has not. Consequently, there is even more pressure on upconverters and transmission systems to achieve increasing levels of spectral efficiency. This is especially true of 5G and 6G wireless infrastructure, where IQ upconverters beat digital OFDMA and beamforming waveforms up into the sub-6 GHz and mmWave bands. Their presence and numbers are even greater in MIMO and massive MIMO radios, and many sizes/types (i.e. macro, micro, pico and femtocells) of 5G and 6G base stations.

SSB IQ upconverters are prolific in radar and EW systems. Modern defense systems utilize direct digital synthesis (DDS) and ultra-high-speed DACs to generate advanced jamming and tracking techniques. In order to transmit over the total required bandwidth, digital baseband or even IF band digital signals most often require translation to a higher frequency. The baseband DDS, the DAC, and the IQ upconverter are a powerful triad in the world of EW, making it possible to effect jamming and detection methods that were deemed only theoretical just 10 years ago.

As with any technique, system or application, it must always be tested. Consequently, since the most sophisticated IQ modulators must be put through an even more sophisticated characterization, the test equipment required to test them (vector signal generators, spectrum/signal analyzers) also contains many high spectral purity, low phase noise upconverters.

There are so many applications and uses cases for upconverters, particularly those capable of transmitting digital IQ data that an entire paper devoted to just some of them could be written. Internet of Things (IoT) includes all manner of Wi-Fi networks and frequencies, medical imaging is now utilizing ultrawideband (UWB) radar for monitoring vitals and to detect tumors, which requires very narrow, very precise pulses created using an upconverter, Satcom has long used upconverters (since at least the 1970s) and software-defined radios (SDRs).

The main advantages of both traditional and IQ SSB upconverters are that there is a reduced bandwidth requirement when compared to double-sideband (DSB) or any number of other modulation techniques. Frequency translation is achieved while spectral efficiency is improved, once the LO is removed and the undesired sideband is removed. In some instances, and for certain in legacy AM systems, lower levels of power consumption can be achieved.

Mini-Circuits Has the IQ to Get the Job Done

A live demonstration utilizing physical Mini-Circuits’ components was designed and operated, and a pair of Mini-Circuits’ signal generators provided the IF and LO inputs. System characterization showed that with the I and Q IF inputs in quadrature, one sideband could be suppressed greatly. Mathematically, this was also shown to be the case. Throughout the article we described how each RF part or piece of test equipment from Mini-Circuits was applied toward the end goal of suppressing the upper sideband and delivering the SSB-modulated and upconverted signal to the spectrum analyzer. Filtering, frequency multiplication, amplification, signals in quadrature, biasing, splitting and combining were some of the topics of discussion. We also provided links to each and every component utilized. The article closes with a brief section on use cases for SSB-modulated data streams and examples of applications in which such techniques are essential. Once we had completed the article, we were left with an appreciation of Mini-Circuits’ IQ and the ability of a properly-configured IQ mixer to suppress either the upper or lower sideband.

Courtesy of Mini-Circuits