Preselector

The broadband harmonic input mixer and wide-sweeping first local oscillator in the HP spectrum analyzer allow simultaneous observation of all input signals between 10 MHz and 12.4 GHz. The analyzer acts like a wide-open superheterodyne receiver. It accepts, in coax, any input signals between 10 MHz and 12.4 GHz, and mixes them with the fundamental and the harmonics of a local oscillator which sweeps from 2 GHz to 4 GHz. (The harmonics are generated in the mixer.) The result ing IF signals, centered at 2 GHz, are converted down to 20 MHz, amplified and detected, and displayed on the CRT.

This wide-open, untuned input system is an advantage when widely spaced signals must be observed simultaneously. Tuning a parametric amplifier is one such occasion; signal, pump, and idler frequencies can be seen on a single display. Another task which calls for a wide spectrum display is observing the output of a varactor multiplier chain.

However, there are situations when the untuned input, combined with the action of the harmonic input mixer, can make the display difficult to interpret. For example, 1 GHz on the display also corresponds to input frequencies of 4, 5, 7, 8, and 11 GHz; thus signals separated widely in frequency can appear close together. Also, a single input signal can produce responses at more than one point on the display. Hence the display can become ambiguous and complicated, especially if there are several input signals.

Display ambiguities can be eliminated by using frequency-selective circuitry (preselection) ahead of the input mixer to confine the displayed signals to a definite frequency range. The HP Model 8441A Preselector, designed primarily for use with the HP spectrum analyzer, employs an Yttrium-Iron-Garnet (YIG) bandpass filter whose center frequency is electrically tunable from 1.8 to 12.4 GHz. The preselector obtains from the spectrum analyzer a tuning voltage proportional to the analyzer's local-oscillator frequency. Its center frequency then automatically tracks a selected harmonic of the spectrum analyzer's local oscillator, with a frequency offset equal to the intermediate frequency. Thus only signals in a selected frequency range can reach the mixer and be displayed on the CRT. Nominal bandwidth of the preselector is 40 MHz, wide enough to avoid interference with desired signals (maximum IF bandwidth of the analyzer is 1 MHz), but narrow enough to provide good rejection of most unwanted signals.

Arrows indicate regions of possible double responses. (Tuning curves for two harmonics are closer than approx. 40 MHz)

V

/

/

£

V

g

X

___

2.25 2.5 2.75 3 3.25 3.5 3.75 LOCAL OSCILLATOR (LO) FREQUENCY (GHz)

Fig. 1. Coaxial input mixer of HP spectrum analyzer converts input signals to 2-GHz intermediate frequency. An IF response is produced whenever f, — nfL0 ± 2 GHz, where n — 1,2,3,4,...

2.25 2.5 2.75 3 3.25 3.5 3.75 LOCAL OSCILLATOR (LO) FREQUENCY (GHz)

Fig. 1. Coaxial input mixer of HP spectrum analyzer converts input signals to 2-GHz intermediate frequency. An IF response is produced whenever f, — nfL0 ± 2 GHz, where n — 1,2,3,4,...

Responses of the Harmonic Mixer

To understand multiple responses and how the preselector rejects them, consider the tuning curve for the spectrum analyzer, Fig. 1. This is a graphical presentation of the harmonic mixing responses of the spectrum analyzer. A signal of frequency fs produces a response whenever fs = n fr.o ± fiF where fL0 = first-local-oscillator frequency, 2 to 4 GHz, fIF = first intermediate frequency, 2 GHz

(A 200 MHz IF can also be selected, for observing signals in the vicinity of 2 GHz), n= 1, 2, 3, . . . Responses produced by a particular harmonic with a plus or minus sign in the above equation are spoken of as having been generated by the n+ or n~ mixing modes, respectively.

Harmonic Responses

Without preselection, the analyzer will respond to several different input frequencies for a particular local-oscillator setting. For example, if the local-oscillator frequency is 3 GHz, the analyzer will respond to signals at 1, 4, 5, 7, 8, and 11 GHz. Corresponding mixing modes are 1", 2-, 1+, 3", 2+, and 3+.

Harmonic responses like these can be prevented by the preselector. The preselector is connected to the analyzer as shown in Fig. 2, and a front-panel switch on the preselector is turned to the desired mixing mode. The preselector's center frequency then automatically tracks the proper harmonic of the analyzer's local oscillator —

offset above or below it by an amount equal to the first intermediate frequency — so that only responses produced by the desired mixing mode are displayed. Only where the tuning curves for two mixing modes come within approximately 40 MHz of each other is there a possibility that an unwanted harmonic response can occur. Such areas are indicated in Fig. 1; they can be avoided by selecting another mixing mode.

1

LI

1

US

SB^SSSSSSSSS^

HP 852A SPECTRUM ANALYZER DISPLAY SECTION

Preselector Drive iHP 8441A PRESELECTOR

HP 855IB SPECTRUM ANALYZER RF SECTION

Fig. 2. New HP Model 8441A Preselector, connected to spectrum analyzer as shown, confines displayed signals to a selected frequency range. The preselector is a narrowband Y1G filter which automatically tracks a selected harmonic mixing mode of the analyzer's LO and mixer.

How a YIG Filter Works

The electrically tunable filter used in the Preselector described in the accompanying article is of the recently developed Yttrium-Iron-Garnet (YIG) type. Highly polished spheres of single-crystal YIG, a ferrite material, when placed in an RF structure under the influence of a dc magnetic field, exhibit a high-Q resonance at a frequency proportional to the dc magnetic field.

To understand the phenomenon of ferrimagnetic resonance, consider diagrams (a) through (e). In the ferrite with no dc magnetic field applied, there is a high density of randomly oriented magnetic dipoles, each consisting of a minute current loop formed by a spinning electron. Viewed macroscopically, there is no net effect because of the random orientations. When a dc magnetic field, H„ of sufficient magnitude is applied, the dipoles align parallel to the applied field, producing a strong net magnetization, M„, in the direction of H0. If an RF magnetic field is applied at right angles to H0, the net magnetization vector will precess, at the frequency of the RF field, about an axis coincident with H„. The precessing magnetization vector may be represented as the sum of M„ and two circularly polarized RF magnetization components m, and my. The angle of precession 0, and therefore the magnitudes of m, and m„ will be small except at the natural precession frequency. This frequency, known as the ferrimagnetic resonant frequency, is a linear function of the dc field H0.

Diagram (f) shows the basic elements of a YIG bandpass filter.1 The filter consists of a YIG sphere at the center of two loops, whose axes are perpendicular to each other and to the dc field H0. One loop carries the RF input current, and the other loop is connected to the load. When Ho is zero there is large input-to-output isolation, since the two loops are perpendicular. With H0 applied, there is a net magnetization vector in the direction of H0. The magnetic field h, produced by the RF driving current in the input loop causes the net magnetization vector to precess about the z-axis. The resulting RF magnetization component, m;, induces a voltage into the output loop. At frequencies away from the ferrimagnetic resonant frequency, m, and the voltage it induces are small, so input-to-output isolation is high. When the input current is at the ferrimagnetic resonant frequency, <p and my are maximum. There is a large transfer of power from input to output, and insertion loss is low. Thus the filter center frequency is the ferrimagnetic resonant frequency and can be tuned by varying H„. Commonly, the YIG sphere and RF structure are located between the poles of an electromagnet, and tuning is accomplished by furnishing a control current to the magnet coils.

To achieve improved selectivity and offband isolation, the YIG filter used in the preselector employs two YIG filter stages in series and in the same magnetic field. This filter was developed and is manufactured by the Watkins-Johnson Company.

1 For a detailed treatment see, for instance, P. S. Carter, Jr., 'Magnetically Tunable Microwave Filters Using Single-crystal Yttrium-Iron-Garnet Resonators,' IRE Transactions on Microwave Theory and Techniques, Vol. MTT-9, No. 3, May 1961.

(a) Randomly oriented magnetic dipoles in the unmagnetized ferrite.
(b) Magnetic dipoles aligned under the influence of a magnetic field.
(c) An equivalent representation of (b) showing the combined effect of the aligned dipoles.

ct hRF-►

9

, M

y

(d) Precession of the net magnetization vector due to RF magnetic excitation.

z

M0 k

—►-y

X ^

my

(e) Equivalent representation of precessing magnetization vector.

mm

(f) Tuned bandpass filter consists of YIG sphere at center of two mutually orthogonal loops.

mgmmmm

fflL

, 1

(b)

iU

||§j|i§il

! !

», ,..;.

»TV"

Fig. 3. (a) Spectrum-analyzer display for 5-GHz input signal, without preselection. Input signal produces responses by mixing with one or another LO harmonic when LO is tuned to 2.33, 3.00, and 3.50 GHz. (b) Model 8441A Preselector eliminates multiple responses by removing input when LO is tuned to 2.33 and 3.50 GHz.

Fig. 3. (a) Spectrum-analyzer display for 5-GHz input signal, without preselection. Input signal produces responses by mixing with one or another LO harmonic when LO is tuned to 2.33, 3.00, and 3.50 GHz. (b) Model 8441A Preselector eliminates multiple responses by removing input when LO is tuned to 2.33 and 3.50 GHz.

Multiple Responses

A single input signal above 4 GHz will produce responses for two or more different local-oscillator frequencies. A 5-GHz input, for example, will produce responses by mixing with one or another LO harmonic when the local oscillator is tuned to 2.33, 3.00, and 3.50 GHz. Like harmonic responses, these multiple responses can be eliminated by the preselector.

Fig. 3(a) shows the spectrum analyzer display resulting from a 5-GHz input signal when the local oscillator is sweeping over its entire range from 2 to 4 GHz, without preselection. The three multiple responses from left to right are the 3_, 1\ and 2~ mixing responses.

With preselection, any selected one of these responses is tracked by the YIG filter and the others rejected. Fig. 3(b) shows the resulting display when the 1+ response is preselected. The YIG filter sweeps from 4 to 6 GHz in synchronism with the local oscillator. When the local oscillator is at the frequencies required to produce the and 2+ responses from the 5-GHz signal, the filter is tuned away from 5 GHz, so these responses are rejected. Again, the amount of rejection is large except at the crossings of two response curves (indicated in Fig. 1).

Possible harmonic and multiple responses which can occur, even with preselection, when the tuning curves for two mixing modes intersect, can be avoided by switching to another mixing mode. By proper selection of analysis bands, multiple responses can be reduced by more than 38 dB throughout the range of 1.8 to 12.4 GHz.

Spurious Responses

Spurious responses are harmonic and intermodulation distortion products generated by the nonlinear behavior of the analyzer's input mixer when it is driven by signals greater than —30 dBm. Intermodulation distortion products are caused by the interaction of two or more strong input signals in the mixer.

The new preselector is effective in reducing spurious responses. Fig. 4 shows the analyzer displays both with and without preselection when two high-power signals are applied to the input mixer. Without preselection, harmonic and intermodulation distortion products make it difficult to find the desired signals. Preselection greatly simplifies the display.

With preselection, harmonics of either input signal are actually still generated in the mixer, but only while the preselector is tuned to the signal fundamental. At this time the local oscillator is tuned so that only the signal fundamental produces a 2-GHz IF signal; thus no response to the harmonics is produced. When the local oscillator is at such frequencies as to produce responses from signal harmonics, the preselector is tuned off the fundamental, so no harmonics are produced in the mixer. The only responses that appear on the display represent the true harmonic content of the signal.

Intermodulation distortion is also reduced by the preselector, if the strong signals causing it are separated by more than the YIG filter bandwidth (40 MHz). The preselector passes only one signal at a time to the mixer, and attenuates the other signal by an amount given by the selectivity characteristic, Fig. 5. For example, if the preselector is tuned to an 8-GHz signal, another signal 200 MHz lower in frequency will be attenuated 31 dB.

Using the preselector to reduce spurious responses increases the spectrum analyzer's spurious-free dynamic range — and, therefore, the analyzer's distortion-measur ing capability — by at least 30 dB. Normally, spurious responses are more than 50 dB down for —30 dBm input signals, but increase drastically for higher input levels. The preselector allows input signals up to one milliwatt to be applied without the multitude of unwanted responses that occur without preselection (Fig. 4). The resulting wide dynamic range may be used to advantage, for example, in observing the harmonic output of microwave signal sources. For a fundamental input level up to one milliwatt, harmonics may be observed at as low a level as spectrum-analyzer sensitivity will permit. Harmonics as much as 100 dB below the fundamental may be observed with assurance that they are not being generated in the mixer.

Observing Impulses

In analyzing the spectra of high voltage, narrow width pulses, such as those from impulse generators used for receiver calibration, preselection is a definite requirement. These pulses often have amplitudes of 100 V or more. If such a pulse were connected directly to the analyzer, its amplitude would be limited and its shape would be distorted by mixer saturation. This would change the spectrum being measured. The preselector, on the other hand, passes only the spectral lines within its bandpass, so the total power applied to the mixer at any time is extremely small, and no mixer saturation occurs.

Using the Preselector as a Tunable Filter

Although it is designed primarily as a preselector for the HP spectrum analyzer, the preselector can also be used independently as a tunable bandpass filter. The center frequency can be set anywhere between 1.8 GHz and 12.4 GHz. This can be done manually, using the calibrated front-panel control, or remotely, by means of a control voltage. There is also an internal sweep generator which can sweep the filter over its entire frequency range or any portion of it. In this sweeping mode of operation the preselector, in conjunction with a broadband detector and a sensitive oscilloscope, becomes a low-sensitivity but remarkably wideband spectrum analyzer (see Fig. 6).

0 0

Post a comment