SPECTRUM ANALYSIS

energy distribution across the frequency I _. - . spectrum for a given electrical signal. Evaluation of the relative amplitudes and frequencies of the discrete components of rf signals yields information on band- widths, modulation characteristics, spuri- ous signal generation and other valuable data impossible or impractical to obtain by any other means.

Microwave spectrum analysis has as- sumed added importance since the intro- duction of the Hewlett-Packard Model 851A/8551A Spectrum Analyzer. With its fully calibrated controls and displays, plus wide spectrum coverage, this ana- lyzer brings welcome practicality to fre- quency-domain measurements and opens up new areas of application.

Broadband applications Radio Frequency interference (RFI)

testing, spectrum surveillance and gather- ing of spectrum signatures-these are im- portant fields being revolutionized by the hp spectrum analyzer. The far-ranging sidebands of radar transmitters, inter- modulation products of multiple trans- missions and spurious signals generated by electronic and electrical devices can be quickly detected and measured with the analyzer. Wide dynamic range and broad spectrum coverage in the hp spec- trum analyzer permit measurements of signals widely separated in frequency and amplitude. Transients and random inter- ference can be recorded by a time-exposed photo of the analyzer’s crt display taken with an oscilloscope camera. Displays of repetitive signals may be plotted on an x-y recorder, using the vertical and hori- zontal output signals from the analyzer. Figure 1 shows the radiation present throughout the entire vhf spectrum in a large metropolitan area as viewed on the hp spectrum analyzer. Note the cluster of FM broadcast stations on the left and the television aural and video carriers ap- pearing at center-right of the display. This display represents only 15% of the analyzer’s maximum spectrum width ca-

-.I--. I

0.5 nr/cm TIME FUNCTION

0

2cc I I Cc FREQUENCY FUNCTDN

Figure 2. Nanosecond pul

pability. Power density measurements are another important application of the spectrum analyzer, made possible by cali- brated IF bandwidths. By knowing the effective noise bandwidth of the IF am- plifier, a calibrated output in terms of noise power per megacycle is possible using an rf indicator such as the hp 411A RF Millivoltmeter (page 123) to measure the analyzer’s 20 mc IF output. Calibration is achieved by feeding a known signal level into the analyzer rf input from a signal generator and noting the output level on the rf millivoltmeter. This level then becomes a reference to which all power density measurements may be referred.

Solid-state applications Tuning varactor multiplier strings and

parametric amplifier circuits can be tedi- ous and time consuming by conventional techniques. There also is a good chance that spurious signals may be present in the output of such devices, even when everything seems “peaked up” correctly. The hp spectrum analyzer allows people working with microwave solid-state cir- cuits to observe all output frequencies of such devices simultaneously and make adjustments for optimum output free of spurious signals.

Figure 1. VHF spectrum as viewed on hp spectrum analyzer.

Display shows frequencies from 10 m c to 300 m c a t a spectrum

width setting of 30 mc/cm.

se and spectrum resulting

Fast rising, short duration pulse wave- forms in the nanosecond region can be generated by semiconductor diodes driv- ing a shorted transmission line. Often, it is desirable to obtain a uniform output across large segments of the spectrum with such devices (Figure 2). With the broad frequency display and flat ampli- tude response of the 851&855lA, it is a simple task to measure narrow, fast rising pulse spectra and make adjust- ments for discontinuities in the generat- ing system.

Narrowband measurements In addition to the broad spectrum ca-

pability of the analyzer, calibrated spec- trum widths down to 10 kc/cm allow detailed analysis of very narrow seg- ments of the band. A unique phase-lock stabilization system reduces local oscil- lator residual FM in the analyzer to less than 1 kc peak-to-peak deviation when viewing narrower spectrum widths. This system permits stable displays of narrow spectra, plus the convenience of remain- ing stabilized while tuning across the band. Narrow spectrum widths are use- ful for applications such as FM deviation measurements and residual FM checks on signal sources. The 60 db dynamic range and display makes FM measure- ments by the “carrier-zero” method ex- tremely accurate since the modulation frequency may be adjusted to the precise point where all the signal energy is con- tained in the sidebands. The modulation frequency is measured on an electronic counter and noted. Then, using a table of Bessel functions, carrier deviation is a simple calculation:

f , = mf,, where f, = carrier deviation

m = modulation index (from Bessel table)

fa = modulation frequency

214

Application Note 63 Well illustrated applications and spe-

cific information on spectrum analysis are yours for the asking in hp Applica- tion Note 63. An introduction to spec- trum analysis and interpretation of spec- tral displays explain the basic principles of this important branch of microwave measurements. More rigorous treatment of the subject is included in an appendix showing the application of Fourier Anal- ysis to spectrum analyzer displays.

One section of the note contains spec- trum analyzer applications in detail, sug- gesting time-saving methods and solu- tions of difficult measurement problems. Your copy of Application Note 63 is available on request through hp Sales Offices in your area.

2Gc I F AMPLIFIER

Spectrum analyzer requirements The basic functions of a spectrum ana-

lyzer are to translate electrical functions into their various frequency components and present their amplitudes on a visual display. To be versatile and do an ef- fective job, the spectrum analyzer should have: 1) the ability to locate and identify signals over a wide frequency spectrum, 2 ) the ability to magnify portions of the spectrum for detailed analysis with stable calibrated sweeps and resolution, 3) min- imum display clutter from spurious re- sponses in the analyzer, and 4 ) wide dynamic range and flat frequency re- sponse.

A simplified block diagram of the hp 851A/8551A Spectrum Analyzer is shown in Figure 3 . The rf section contains the

200MC I F AMPLIFIER

local oscillators, mixers and two of the three IF amplifiers, comprising a triple conversion superheterodyne receiver. The first local oscillator is a backward wave oscillator which is capable of being swept or tuned from 2 to 4 gc. Input signals of 10 mc to 10 gc pass through the 0-60 db rf attenuator to a crystal harmonic mixer and are converted to the 2 gc IF. After amplification, the 2 gc IF is converted to 200 mc, amplified, and converted again to 20 mc. The use of a 2 gc first IF keeps images 4 gc apart, preventing a confusing double response for a single input fre- quency. The first mixer is carefully de- signed for minimum spurious generation and flat frequency response.

The display section contains the 20 mc IF attenuator, bandpass filters, amplifiers, and video detector, plus the crt, sweep generator and display shapers.

Except for the crt, this section is designed with solid-state components throughout. The input consists of an accurate 0-80 db attenuator calibrated in 1 db steps. Bandpass filters, controlling the analyzer’s resolution, follow the at- tenuator. These have accurately con- trolled bandwidths of 1,3,10,100 kc and 1 mc. The switching logic of the Sweep Time and Spectrum Width selectors auto- matically select the optimum filter for best resolution without sacrifice in gain. Manual selection of the filters also is provided. A current controlled attenua- tor and feedback network comprise a dis- play shaper which allows calibrated readout on the crt in terms of input power (square law), db (logarithmic) or voltage (linear). A full discussion of spectrum analyzer design considerations is included in Application Note 63.

SWEEP GENERATOR

AMPLIFIER HORIZONTAL

I F ATTEN

FILTERS

2 0 M C - IF AMPLIFIER - EL BAND PASS V I DE0 AMPLIFIER -

DETECTOR

LOCAL OSCILLATOR

180MC OSC I L L ATOR GENERATOR

---

HELIX POWER SUPPLY

8551A R F SECTION

851A DISPLAY SECTION ---------------------------

Figure 3. HP Model 851A/8551A Spectrum Analyzer simplified block diagram.

21 5

8 5 l A , 8 5 5 l A SPECTRUM ANALYZER Totally new concept in microwave spectrum analysis

The new Hewlett-Packard 85 lA/t355 IA Spectrum Analyzer is truly an advance in the state of the art. It provides a 60 db display dynamic range, flat response over ,spectrum widths from 100 kc to 2 gc and image separation of 4 gc; all con- trols are calibrated, including the logarithmic, linear and squared vertical displays. High sensitivity and broad fre- quency range, plus a unique signal identifier, are additional features which make this instrument the most versatile and useful spectrum analyzer available today.

RF characteristics

The 851A/8551A Spectrum Analyzer covers the frequency range from 10 mc to 40 gc, 10 mc to 10 gc in coax, 8.2 to 40 gc in waveguide with external mixers and adapters (Figure 1 ) . The coaxial input is inherently broadband; however, the range can be limited with pre-selection filters such as the hp 360 and 8430 Series (page 213) to eliminate interference from signals outside the frequency range of interest.

Ten calibrated spectrum widths from 100 kc to 2 gc are available. This wide range of spectrum widths permits obser- vation of widely separated signals and broad spectra, as well as detailed examination of individual signals, distortion prod- ucts, etc. The 4 go image separation (a 2 gc first IF) assures a display uncluttered by overlapping images. For investigation of signals close to 2 gc, a 200 mc first IF can be selected.

Amplitude control

Signal amplitude is controlled by a 0 to 60 db rf attenuator (10 db steps) in the coaxial input system, plus a 0 to 80 db IF attenuator (1 db steps plus vernier). When an external waveguide mixer is used, rf attenuation can be accomplished with the appropriate hp 382 Waveguide Attenuator (page 225).

Calibrated IF bandwidths, important in power density meas- urements, can be selected either manually or automatically. In the automatic mode, IF bandwidth is selected for best resolu- tion of a cw signal with each combination of spectrum width and sweep time. In addition, sweep time is calibrated, and the crt has an internal graticule for parallax-free viewing. A base- line clipper is provided to dim the base line on the crt for clearer and more comfortable viewing and improved photography of low-repetition-rate signals.

Specifications, 8551A RF Section

Frequency range: coaxial input: 10 mc to 10 gc; waveguide input: 8.2 to 40 gc (accessory mixers and adapters required).

Spectrum width: 10 calibrated spectrum widths, 100 kc to 2 gc in a 1, 3, 10 sequence to 1 gc; vernier allows continuous ad- justment between calibrated ranges and can reduce width to zero.

Swept-frequency linearity*: * 5% when local oscillator (LO) is stabilized and swept 10 mc or less; f10% (typically 5 % ) when LO is swept more than 10 mc.

Image separation: 4 gc. Phaselock internal phase-lock provided for stabilizing LO;

LO sweep tracks sweep of voltage-tuned 10 mc reference oscillator.

Phaselock range: unit can be phase locked for spectrum widths up to N x 10 mc, where N is the harmonic of the LO.

Phase lock tuning: reference oscillator automatically tracks with Tune control over 2 gc LO range.

Tuning: selectable continuous coarse, fine and stabilized (phase locked) tuning determines center frequency about which LO is swept; tuning is accomplished with a single front-panel Tune control.

*Correlation between LO frequency and sweep position on Model 851.4 crt as a percentage of the selected spectrum width.

. 4 85 14/8551A

11521A

11520A

11518A

Figure 1. hp 11517A-11521A external mixers and adapters.

216

Fine tuning: frequency change of L O fundamental is 10 mc 2 2 nic per revolution of the Tune control; masimum accumu- lative error across the band, 2 2 0 mc; settability of a signal on the crt with the Tune control, i-50 kc (fundamental mising).

Stabilized tuning: frequency change of LO fundamental is 10 mc * I mc per revolution of the Tune control; maximum accumulative error across the hand, i 2 nic; settability of a signal on the crt with the Tune control, f 5 kc i fundamental mixing).

Tuning accuracy: * 1% of LO fundamental or harmonic Sensitivity (10 kc IF bandwidth): 10 mc to 2 gc, -95 dbm: 1.8

to -1.2 gc, -100 dbm (400 mc image separation); 2 to 4 gc, -80 dhni; 4 to 6 gc, -95 dbm; 6 to 10 gc, -80 dbni; 8.2 to 18 gc. -80 dhm; 18 to 26 gc, -75 dbni; 26 to 40 gc, -65 dbni; with a source stability better than 1 kc, greater sensi- tivity can be achieved by using a narrower IF bandwidth.

Coaxial input attenuator: range: 0 to 60 db in 10 db steps; in- sertion loss: 0 at 10 mc, less than 2 db at 10 gc. Maximum input power (for 1 db compression)

Input atten. setting 0 db

Max. input Typical max. 2 QC IF input 200 mc IF

0 dbm -5 dbm 10 db 20 db 30 db

40 thru 60 db I +30 dbm I +30 dbm Waveguide input

+10 dbm +5 dbm +20 dbm +15 dbm t 3 0 dbm +25 dbm

I 11521A 18.2 to 12.4 ec) I tvDical lv -10 dbm I I 11517A 112.4 to 40 ec) I tvDical lv -15 dbm I Frequency response: coaxial input: *1.5 db over any 200 mc

range using fundamental mixing, 2 3 db over any 200 nic range using 2nd harmonic mixing, *5 db (typically 2 3 d b ) over any 2 gc range except when signal or LO is within 60 mc of 2 gc, including mixer and rf attenuator response with attenuator setting >_ 10 db.

Signal identifier: switch shifts display in inverse proportion to harmonic of LO used in mixing; direction of shift depends upon whether signal frequency is higher or lower than LO harmonic.

IF output frequency: 20 mc. Residual LO fm: 1 kc peak to peak or less when phase locked;

appSosimately 30 kc peak to peak when not phase locked. Residual responses (no input signal): less than -90 dbm re-

ferred to Signal Input on fundamental mixing (-85 dhm when LO is within 60 mc of 2 or 4 gc ) .

LO noise sidebands: greater than 60 db below cw signal level 90 kc or more away from signal.

LO type: 2 to 4 gc backward wave oscillator. LO output: approximately 1 mw available for use with wave-

meters or frequency counters; output connector, female Type N on rear panel.

RFI: conducted and radiated leakage limits are below those specified in MIL-I-6181D and MIL-I-16910C.

Power: 11 5 or 230 v * l O % , 50 to 60 cps, approx. 250 w. Dimensions: 16%” wide, 121/2” high, 18%” deep (425 s 318 x

467 m m ) ; hardware furnished for rack mount 19” wide, 12-7/32” high, 163/” deep behind panel (483 s 310 x 416 m m ) .

Weight: net 95 Ibs (43 k g ) ; shipping 136 Ibs (61,2 kg ) . Accessories supplied: 4 cable assemblies to interconnect RF

and Display Sections; 908A Termination for LO output. Accessories available: 11517A Mixer for P-, K- and R-bands

(12.4 to 40 gc) , $160; 11518A Adapter, adapts 11517A Mixer to P-band waveguide (12.4 to 18 gc) , $65; 11519A

Adapter, adapts 11517A Miser to K-hand waveguide (18 to 26 g c ) , $65; ll52OA Adapter, adapts 1151’A Mixer to R - band waveguide ( 2 6 to -10 kc) , $65: l l 5 2 l A Miser for X-band waveguide, $75.

Price: hp 8551A, $7100.

Specifications, 851A Display Section Vertical display: linear, square i power) or logarithmic.

Dynamic range: linear, 70 : l : squarc, :():I ; log. 60 dh. Accu- racy: linear. * 3 % of full scale; square, of fu l l scale-x-; log, i- 2 db+.

Manual: bandwidths of 1, 3 , 10, 100 kc and 1 mc can he selected.

Auto-Select: one of the above bandwidths automatically selected for best resolution of a cw signal with each com- bination of spectrum width and sweep rate.

Bandwidth accuracy: individual bandwidths are calihrated within i- 20%; bandwidth repeatahility and stability typically better than i37r.

IF input: 20 nic center frequency; 50 ohms input impedance Maximum cw input signal: - 1. t dbm IF gain set: two-section attenuator provides 0 to 80 db attenua-

tion in 1 db steps; one section provides 0 to 70 dh attenuation in 10 dh steps; the other, 0 to 10 d b in 1 dh steps; vernier provides continuous adjustment between 1 dh steps.

IF gain set accuracy: 70 db section, i0.5 dh; 10 db section, i 0 . 1 db.

Sweep rate: six calibrated rates, 3 nisec/’cm to 1 sec/cm in a 1, 3 , 10 sequence; vernier provides continuous adjustment hetween calibrated rates and estends slowest rate to at least 3 sec/cm.

IF bandwidth:

Sweep rate accuracy: % 3% Sweep synchronization: internal: sweep free runs; line: sweep

synchronized with power line frequency: esternal: sweep synchronized with esternally applied signal + 3 to + 15 volts peak; single sweep: sweep actuated by panel pushbutton.

Output signals: vertical and horizontal signals applied to crt are available for external monitoring; vertical: 0 to --.I volts, output impedance, 4700 ohms; horizontal: 10 volts p-p 1 0 . 3 volt, sweep approsimately symmetrical ahout zero, output impedance -1700 ohms.

Cathode-ray tube: 7 . 5 kv post-accelerator tube with P2 niediurn- persistence phosphor (others optional ) and internal grati- cule; light blue filter supplied.

Internal graticule: parallax-free 7 s 10 cm, marked in cm squares with 2 mm subdivisions on major vertical and hori- zontal axes.

RFI: conducted and radiated leakage limits are below those specified in MIL-I-6181D and MIL-I-16910C.

Power: 115 or 230 v ? 10yc, 50 to 1000 cps, appros. 25 w Dimensions: 16%” wide, 71/4” high, 18%” deep (425 s 185 x

,467 m m ) ; hardware furnished for rack mount 19” wide, 6-31/32” high, 163/8” deep behind panel (483 x 177 x 416 m m ) .

Weight: net 34 Ibs (15 k g ) ; shipping 35 Ibs (20,3 kg). Accessory supplied: joining bracket kit for semi-permanently

Price: hp 851A, $2400 Options

mounting 851A on 8551A.

07. P7 phosphor in lieu of P2 (amber filter supplied), no

31. P31 phosphor in lieu of P2 (green filter supplied), no additional charge.

additional charge. *Except pulse spectrums on 1 mc bandwidth.

21 7