1. Brian Yee, W6BY Microwave Update 2010 Cerritos, CA
Spectrum Analyzers
Brian Yee, W6BY
Microwave Update 2010
Cerritos, CA
This talk is an introduction to the basic design of spectrum analyzers and a survey of representative units available to the average or above average ham.

2. Presentation Outline Basic design and evolution Applications
Overview of available models
Issues for prospective buyers
And…

3. Basic Concepts – Key Components
MIXER
RF IN
Display
Detector IF
A simple swept receiver with a visual display
SWEEP OSCILLATOR
The most rudimentary form of spectrum analyzer is the same as a superhetrodyne receiver, except the local oscillator is replaced by a sweeping oscillator. Early spectrum analyzers used mechanical methods to sweep the LO, and they were limited to only a few kilohertz of sweep width.
A brief history of some spectrum analyzer developments can be found at:

4. Some Refinements… MIXER RF IN Display Problems with this approach:
FILTER
FILTER
Detector
Problems with this approach:
Limited frequency range
Can’t look at low frequencies
Spurious signals show up
Narrow sweeps difficult
SWEEP OSCILLATOR
Simple spectrum analyzers are limited by the frequency range of the sweep oscillator, unless they use harmonic mixing. Harmonic mixing can introduce spurs or false signals since the front end is not selective. Also, a free running sweep oscillator is not frequency accurate, especially during narrow sweeps. Analog displays also suffer from flicker and distortion.

5. More Refinements RF IN MIXER Display Better spur rejection
Resolution
bandwidth
Video
bandwidth
RF IN
MIXER
FILTER
Display
FILTER
FILTER
Detector
SWEEP OSCILLATOR/
SYNTHESIZER
Better spur rejection
Better frequency accuracy
By adding a tracking front end filter, spurious signals are nearly eliminated. Replacing the free running sweep oscillator with a synthesized sweep oscillator greatly improves frequency accuracy. The technique commonly used in modern spectrum analyzers is called “lock and roll”, meaning the sweep is locked using synthesis techniques at the beginning of the sweep and then the oscillator is allowed to sweep. During narrow sweeps, the local oscillator is completely synthesized for greatest frequency accuracy.
Variable filters before and after the detector improve the resolution of the displayed signals.

6. High-end approach 2 - 6 GHz Resolution Bandwidth 3.6 GHz 9KHz – 2 GHz
3.3 GHz OSC
Resolution Bandwidth
3.6 GHz
9KHz – 2 GHz
MIXER
321.4 MHz
21.4 MHz
MIXER
LP FILTER
FILTER
MIXER
FILTER
RF IN
FILTER
300 MHz OSC
2 – 22 GHz
MIXER
321.4 MHz
FILTER
FILTER
FILTER
Display
The trend in later model spectrum analyzers was to use a much more sophisticated front end. These tended to use a high frequency first IF which was downconverted to a mid-frequency second IF, and finally a low-frequency IF. This triple-conversion approach improves selectivity. The purely analog display is replaced by an A/D converter and digital storage. This eliminates flicker and enables accurate measurements using cursors and markers. Also a limited amount of signal processing is possible, such as averaging, peak search, and so on. Since all oscillators are locked to a precision frequency reference, the spectrum analyzer can accurately measure the frequency of displayed signals.
Since these analyzers use microprocessors, these analyzers can be controlled remotely.
Detector
SWEEP OSCILLATOR/SYNTH
Video Bandwidth
2 - 6 GHz

7. Applications Checking the power of amplifiers (with the proper pad)
Checking for harmonics
Phase noise measurement
Modulation measurement
Channel power measurement

8. Tektronix 7Lxx, 1Lx Plug-ins
In the beginning…
The old military units with mechanical sweep oscillators will not be discussed here since other than for historical purposes they are not suitable for modern use.
Some early spectrum analyzers were plug-ins for oscilloscopes. This was a natural fit since oscilloscopes had most of the basic elements needed for a spectrum analyzer.
The 1Lxx series was a popular plug-in for the Tektronix 545 series oscilloscopes. In the later 7000 series Tektronix scopes, the 7Lxx series was employed. The 7000 series
Scopes were some of the first to have digital readouts on the display.

9. Tek 7Lxx Series

10. Tektronix 49x Series – 22 GHz
492 popular, lots available, mm-wave mixers available, digital/analog display
Tektronix introduced a line of portable spectrum analyzers, starting with the 491, and later with the 492 and 494 series. The 492 and 492 series had much better accuracy, digital displays and used microprocessor control.
These units could also measure millimeter wave signals up to 300 GHz.
494AP top of the line
Some limited-frequency range units (496, 492P’s)

11. Tektronix 27xx Series – 1.8 GHz
Primarily made for field use, service shops and CATV servicing

12. Tektronix 275x Series – 22+ GHz
Rack mount variation of 494AP, some models mm-wave capable

13. Tektronix 279x – 26 GHz
Nice…
Tektronix continued to improve on its line of portable spectrum analyzers.

14. Tektronix 2782/4 – 40 GHz Even nicer… CRT has color LCD shutter
2782 has 33 GHz range, 2784 has 40 GHz range
MM-wave operation possible with internal duplexer
Expensive
This was Tektronix’ top of the line portable spectrum analyzer. These used proprietary liquid crystal shutters to produce color displays. The 2782 could measure to 33 GHz, and with slightly different firmware could be extended to 40 GHz (2874).
Both analyzers could measure up to 300 GHz with external mixers. These are still expensive, hard to find and very desirable.

15. HP 851A Display unit shown, RF unit goes on the bottom.
Very basic functionality, but more than 1L20
Back at HP, they were developing a unit called the 851A. These used a 2 to 4 GHz backward wave oscillator that was revolutionary because of its wide sweep bandwidth. Previous generations of spectrum analyzers used a tunable first LO followed by a
Narrow band sweep oscillator. The 851A could measure up to 12 GHz using harmonic mixing, and to 40 GHz (very high at the time) using external harmonic mixers. The display section is shown here, and there was a separate RF tuning unit.

16. HP 141T – 18 GHz Nice unit, lots available
3 types of plug-ins up to 18 GHz
Uses storage tube
A very popular successor to the 851A, and a significant improvement, was the 141T with plug-ins that enabled measurements from the kilohertz range up to 18 GHz, and to 40 GHz with external mixers. This also used harmonic mixing for the microwave bands, and a much improved display and IF system. This series had accessories such as tracking preselectors and different band plug-ins.

17. HP8557 Several plug-ins available
HP also made some low-cost “portable” analyzers that were oscilloscope plug-ins, following the Tektronix trend.

18. HP 8565A – 22+ GHz Many available
Storage display with digital readouts
Can use external mixers
The 8565A started to use some digital technology, but they were still behind Tektronix in their ability to incorporate a digital display on the CRT.

19. HP 8569A/B – 22+ GHz Very popular, lots available
Can use external mixers
Digital storage display, GPIB
HP finally started to catch up to Tektronix when it produced the 8569A/B. This is basically the 8565A (same analog circuits) with a digital display GPIB capability.

20. HP 8568A/B, 8566A/B, 8567A – 22 GHZ+ An industry standard
One of the best displays
Lots available
Serviceable
Can use external mixers, auto signal ID
HP took a great leap forward when it introduced the 8568A. Now they finally had a high-performance spectrum analyzer with a very nice digital display, superb frequency and amplitude accuracy, and advanced signal measurement capability.
These analyzers are still the standard in many EMC and design labs. About a year after the 8568A was released, HP announced the 8566A, which extended frequency coverage up to 22 GHz with the same accuracy and features as the 8568A.
The 8566A could measure millimeter wave signals with external mixers, and had the unique (at the time) ability to identify and display only the real signal out of the spurs produced by harmonic mixing.

21. HP 859x Series Low-end portable Subject to drift
Intended for field service market
Lots available
In the late 80’s, HP came out with a line of low-cost portable analyzers. These analyzers suffered from frequency drift and poor frequency accuracy.
Still, HP expanded this line with several models up to 22 GHz. Many of these are available, and still command a relatively high price.

22. HP 856x Series
Many of the features of the 8566 series and more crammed into a portable unit
HP later introduced the 856x series of portable spectrum analyzers, and many of these had the same or better performance than the 8566 series of bench analyzers. The high-end models could cover up to 50 GHz, fully preselected,
And had millimeter wave capability with external mixers. Many available at high prices.
Synthesized
Available in several frequency ranges
Some can use external mixers
Expensive

23. IFR 1 GHz unit Lots available
IFR originally made communications service monitors, but branched out into spectrum analyzers. Some of the service monitors include spectrum analysis capability.
IFR later acquired Marconi Instruments, and still produce several of their products.

24. Advantest 326x, R346x series
Nice units, lots of features, hard to get service info
Several Japanese manufacturers came out with some nice units, but parts and manuals are hard to come by.

25. Advantest U36xx Series
Microwave units to 26+ GHz

26. Anritsu MS710A/B/C/D
D model can accept either Tek or HP mixers

27. Anritsu MSxx Series

28. Others What to look for: Service manual available Physical condition
“unobtainium” parts
Serviceability
Marconi
Instek
AIL/Eaton
Systron Donner
Rohde and Schwarz – expensive
Wandel & Goltermann
Various Chinese makes

29. Recent advances Smaller units with more capability
DSP, more complex measurements
Demodulation of various communications formats
Better displays
Faster speed
Interface with other instruments such as power sensors
The current improvements in microcontrollers, A/D converters, DSP and displays enable smaller, lighter and more powerful instruments. DSP can replace much of the bulky analog circuits and can process a wider spectrum of signals simultaneously. DSP also enables signals not only to be displayed, but also demodulated and can measure the quality of complex signals.

30. And…..?

31. The MUD 2010 Grand Prize!!! 100 KHz to 3 GHz freq. Range
6.5” color TFT
4-hour battery life
One-button power measurement
Spectrogram
USB power sensor support
Field strength measurement
Much more…