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Using an external mixer with a spectrum analyzer.

Using an External Mixer with a Spectrum Analyzer

Introduction

Microwave spectrum analyzers usually include circuits for connecting external waveguide mixers for mm-wave operation. Some instruments permit the connection of external mixers at frequencies where an internal mixer is included. Most users do not avail themselves of this capability, thinking, "Why use an external mixer when the instrument comes equipped with a mixer?" However, external mixers can provide improved performance for specialized applications and may permit measurements that could not be performed otherwise.

Greater sensitivity and flatness, higher accuracy and greater dynamic range are some of the benefits to be derived through the use of an outside mixer. On the other hand, there are also drawbacks when using an external mixer, including spurious signal responses due to the lack of preselection. This paper provides advantages and disadvantages for using external mixers, and discusses the derived benefits from using this procedure.

The Front-End Block

Virtually all microwave spectrum analyzers include a waveguide mixer connection to extend the operating frequency into the mm-wave range, as shown in Figure 1. These instruments also provide a buffered local oscillator output on the front panel. Therefore, three-port mixers, as well as the usual two-port waveguide mixers, can be connected and used at mm-wave frequencies. Some spectrum analyzer permit IF signal routing from an external mixer even at lower frequencies, where an internal mixer is included.

The internal spectrum analyzer mixer is optimized to cover a wide frequency range. However, an external mixer can be adjusted to cover only a narrow frequency range. This may result in lower conversion loss and a higher compression drive level to yield higher sensitivity and greater dynamic range. The external mixer will bypass the internal preselector filter, thus eliminating the insertion loss of that device. Also, the external mixer can have any desired physical configuration or connector type. This permits operation with a coaxial connection when the instrument is set up for waveguide. The external mixer connection to the spectrum analyzer has to transfer the local oscillator signal to the mixer and the IF signal out of the mixer. These are known and controlled frequency signals. The impact on sensitivity and display flatness of longer cables usually is less than that of a longer connection to a higher frequency input signal. Therefore, better performance frequently is achieved by remote mixer location near the signal source, compared to using a longer signal cable from the source to the mixer.

Of course, there are limitations to using external mixers. Bypassing the preselector means that there are more spurious signal responses. Therefore, bypassing the preselector may not be possible for very wideband signals where spurious signal responses may mask desired signals. Change in mixer conversion efficiency and other front-end losses will uncalibrate the spectrum analyzer amplitude indication. In modern instruments, this may be corrected by entering into the spectrum analyzer's memory an appropriate correction offset value. Since the spectrum analyzer's internal attenuator is disconnected, the maximum input signal level is limited to the external mixer's power handling capacity unless an external attenuator is added.

Calibration

Figure 2 shows the display of a 3.5 GHz signal using the internal spectrum analyzer mixer (lower trace) and an external mixer (upper trace). The internal kTBF noise level of the spectrum analyzer is the same for both displays. However, the external mixer trace shows a 17.2 dB higher display level (upper left readout) than that of the internal mixer. This results from a 5 dB lower conversion loss of the narrowband external mixer compared to the wideband internal mixer, and the elimination of preselector and broadband input matching circuit losses. The result is a sensitivity improvement of 17 dB. Once the display level difference is established, a 17.2 dB calibration offset level is entered into the spectrum analyzer's memory, which provides absolute amplitude calibration for future measurements.

The signal amplitude used for Figure 2 was - 62.8 dBm. Clearly the lower trace (internal mixer) is more than 30 dB below the full screen -30 dBm reference level. But the upper trace (external mixer) would show the wrong amplitude. Once the proper offset is entered into memory, the external mixer will show correct amplitude values.

Improving the Measurement

Range

A higher CRT signal display level and no change in noise level will provide greater dynamic range, as long as the instrument gain is reduced. Lower gain means that larger signals can be displayed on screen, and this can generate undesired signal compression and spurious signal responses. A narrowband external mixer, optimized for large drive level and compression operation, can use the gain reduction mode to good advantage. Figure 3 shows a full screen of 80 dB with very low noise for the lower (reduced gain) trace as compared to the normal trace showing 1.5 divisions of noise. The asterisk in front of the reference level (upper left) indicates that the number shown includes an offset value.

The normal signal detection sensitivity for the Tektronix 2756P spectrum analyzer is specified at -125 dBm at 3.5 GHz. Using a high efficiency narrowband external mixer provides sufficient improvement to detect a -140.5 dBm signal, as shown in Figure 4. The vertical display is at 2 dB/division and the noise level shows at -145 dBm using a 10 Hz filter bandwidth.

Another potential area of performance improvement using an external mixer involves large signal drive levels, such as in intermodulation measurements. Most spectrum analyzers using broadband mixers provide third order intermodulation intercept points (TOI) of less than +10 dBm. A good narrowband mixer will exhibit a TOI of +15 dBm or better. This will provide a 10 dB increase in measurement dynamic range, as the third order distortion level is inversely proportional to twice the TOI. Thus, a 6 dB change in signal input level shows an 8.4 dB change in third order intermodulation level for the internal mixer input, as shown in Figure 5. The excess 2.4 dB comes from the spectrum analyzer and not the signal. Performing the same test with a + 15 dBm TOI narrowband external mixer results in a 6 dB IM component change, as it should be when all the intermodulation is produced by the signal. The external mixer shos an IM level of 61.2 dB (Figure 6), whereas the internal mixer can only be reduced to 58.8 dB. This does not mean that the measurement could not have been performed using the internal mixer. A 60 dBc requirement is not difficult to achieve.

Spurious Signal Responses

Bypassing the preselector and associated circuits will reduce insertion losses and improve signal sensitivity. However, this will permit the display of spurious signal responses. The spacing of the spurious signal responses depends on the frequency of the IF amplifier in the superheterodyne conversion chain. The higher the IF, the greater the frequency spacing of the spurious signals. A good rule of thumb is that the spurious signal free range is equal to the IF. Thus, for the Tektronix 2756P, the IF is either 829 MHz or 2072 MHz depending on input frequency, which is also the spurious signal free display span when using an external mixer. Wider spans also can be used, but the spurious signal responses need to be identified.

Figure 7 shows a 16 GHz signal at 2000 MHz across the screen, displayed using an external mixer optimized for 2 GHz operation and via the internal preselected input. The display level difference between the preselected input and external mixer output was 11.6 dB. This is the reference level offset value to be entered into memory in order to maintain correct amplitude calibration. The image spurious signal response at 1658 MHz to the left of the true response is shown for the external mixer input. The true response in the identify mode moves up and down, but does not shift horizontally. A spurious signal response would not stay in the same horizontal frequency position.

Dos, Don'ts and Hints

Use an external mixer with spectrum analyzers that provide a LO output, IF input and internal switching to route the signals between internal and external mixer operation. The mixer must be designed to operate with the IF and local oscillator frequencies associated with your spectrum analyzer. For the 3.5 GHz signal examples presented here, the requirements were 829 MHz IF, 2 to 6 GHz range LO and an input signal of 3.5 GHz. The reason for using an external mixer is to improve measurement performance. Hence, the external mixer needs to provide capabilities not available with internal mixer operation. Since internal spectrum analyzer mixers usually are of high quality, this means trading some performance aspects for other performance aspects. The usual trade-off is between a restricted frequency range in return for greater sensitivity and/or greater dynamic range. Other reasons for using external mixers are coaxial operation in waveguide bands and for remote mixer connection. Amplitude measurements will be in error when using external mixers because the instrument is calibrated for internal mixer operation. However, many spectrum analyzers permit the entry into memory of reference level offset values. This permits re-calibration of the amplitude for the external mixer. Some spectrum analyzers permit additional gain reduction. This can be used to increase the display dynamic range with external mixers that have exceptionally good intermodulation or gain compression levels. Bypassing the preselector will generate spurious signal responses. The approximate spurious signal free full screen span is equal to the IF.

References

[1] M. Engelson, "Learn to Gauge Spectrum Analyzer Dynamic Range," Microwaves & RF, January 1990.

[2] M. Engelson, "Skills Improve Spectrum Analysis With Preselectors," Microwaves & RF, December 1988.

[3] M. Engelson, "Measuring IMD By Properly Using the Spectrum Analyzer," Microwaves & RF, February 1988.

Morris Engelson received his Bachelor's and Master's degrees from the College of the City, New York. He is microwaves chief engineer at Tektronix Inc., and Director of JMS, a Portland consulting firm. Engelson also is an adjunct associate professor at Oregon State University and is a fellow of the IEEE.
COPYRIGHT 1991 Horizon House Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:microwave spectrum analyzers
Author:Engelson, Morris
Publication:Microwave Journal
Date:Apr 1, 1991
Words:1673
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