GHz Synthesizer Tests Frequency Converter Heads
by Mohamed M. Sayed
APG-3.5connector, which is mode-free to 34 Gf lz,' is used up to 4U GHz in the 5356C. The effect of this connector's moiling between 34 and 4(j CHz is taken into account by reducing the specified sensitivity by 5 dB in this region, A special collar was designed for the APC-3.fi connector to strengthen it and to protect it from damage. Waveguide connector WR-28 is also offered, ior customers using only R-band (26.5 to 40 GHz).
To test the accuracy of the 5356/5355 system in CW and pulse modes, a synthesized source Is needed, especially lo check for -1 -count accuracy with a one-second gate time. Both synthesizers and pulse modulators are commencatiy available up to 18 GHz. and these are used in [he 5356/5355 test system To cover the 18-tO-26 5-GHz band, an amplifier and a K-band doublet are used. The input frequencies to the doubler are 9 to 13.25 GHz and the output frequencies are 18 to 26 5 GHz as shown in Fig. 1
Synthesizer 2-18 GHz
Amplifier Doubler
9-13,25 GHz
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Fig, 1. Generation of an 18-to-26.5-GHz signal to test the 53568iC Heads.
In the 26.5-tO-4Q-GHz range an R-band doubler can be used, with the primary synthesizer operating from 13.25 to 20 GHz. This means thai the synthesizer, the pulse modulator and the amplifier must also operate up to 20 GHz. Since most of the instruments operate only up to 18 GHz. major modifications would have been needed
The simpler method that is actually used to generate a synthesized signal to 40 GHz is to use an amplifier and two doublers in cascade. The first stage is an amplifier-doubler. and the second stage is another doubler. The primary synthesizer, the pulse modulator, and the amplifier operate in the frequency range 6.625 to 10 GHz. Fig 2 shows a block diagram of the 26.5-to-40-GHz synthesizer
To 5356C 26.5-40 GHz
Frequency Converter Head Design
Fig. 2 shows the block diagram of the 5356A/B/C, The input voltage-controlled oscillator (VCO) frequency to the 5356A L3 (J varies from 885,^ MHz to 1056 MHz and the output IF to the 5355A varies from 1 MHz to 528 MHz. The IK output is proportional to the RF input within the 5356A B/'C's dynamic range. All of the components in the head are built in thin-film microcircuit configurations.
The coaxial assembly shown in Fig. 2 is replaced by a high-pass filter for the 5358A Option 001. Since the 5356/ 5355 is a pulse counter, the input pulse may contain a video signal. This signal may be so large that it overloads the counter, especially since the IF gain is about 80 (IB, To attenuate such video signals. Model 535GA option 001 has a high-pass filter between the input connector and the sampler. The filter's maximum insertion loss from 1.5 to 13 CHz
Synthesizer 6.625-10 GHz
Amplifier
Amplifier Doubter
6.625-10 GHz 13,25-20 GHz
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Fig. 2. Generation of a 26 >to-40-GHz signal to test the 5356C.
Acknowledgments
Special thanks are due Roger Stancliff of the HP Santa Rosa Division for his help in designing the amplifier doubler
Input BBS.2 to 1056 MHz
Step Recovery Diode
Output 1 to 40 GHz
Fig. 3. Pnoto and schematic of the 5356A 'BlCsarvpler driver L, is drive inductance and C. is tuning capacitance. L2, C.. L and C3 form a matching network to mafefj the impedance seen at C, fo 5011.
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10 12 14 16 18 20 22 24 26 20 30 32 34 36 3fi 40 Frequency (GHz)
Fig. 4. Spectrum ot the sampler driver output shows comb lines to 40 GHz The input frequency is i GHz is 1 dB, and its minimum insertion loss below 10U MHz is more than 35 dB.
The power amplifier is housed in a TO-8 package using a thin-film alumina substrate. It consists of two stages, a gain stage and a power stage. The amplifier is driven to saturation so that its output is insensitive to input variations. The IF amplifier is housed in a TO-12 package using a thin-film sapphire substrate. It also consists of two stages.
The head casting consists of an upper half and a lower half. The sampler and the two printed circuit boards are mounted on the lower half. Heat sink materials are attached to the upper half to dissipate the heat from the power amplifier and the IF amplifier. There is also a heat sink on the lower half for the power amplifier. As a result, (he temperature rise is less than tl C. The heat sink materials also serve as shock absorbers for mechanical vibrations. The casting is designed to accept the different input connectors: N, SMA, AP03.5. waveguide WR-42. and waveguide WR-28. The casting is also designed for improved EMI [electromagnetic interference) performance. All of the parts inside the head can be disassembled easily, using a screwdriver and an SMA wrench. As a result, the 5350A/B'C is easy to troubleshoot.
Sampler Driver
The heart of the frequency converter head is the sampler driver. Fig. 3 shows a schematic diagram. The driver is built in a coaxial package. The step-recovery diode has a very fast rise time that generates a comb of harmonics d! the VCO frequency. One of the comb frequencies is heterodyned with the input microwave frequency to produce an IF output in the proper range.
When time domain measurements were used to test the sampler driver, the test parameters were very sensitive to operator error and test equipment limitations. Therefore, frequency domain measurements are used. Since a 1-to-40-GHz spectrum analyzer (without external mixer) wasn't commercially available, an in-house 40-GHz spectrum analyzer was designed. Fig. 4 shows typical comb lines up to 40 GHz for a 1 -GHz VCO. A 40-GHz test fixture was also designed to test the driver and to adjusl the tuning element before sealing it. The input VCO frequency is varied from «85 MHz to 1058 MHz and the 38 coml: lines are adjusted to meet the required counter sensitivity. The input to Ihe
HF Input
fAW^AVv-w
Microstrip r, R3
Balun
Microstrip r, R3
Balun
Hybrid Circuit
Fig. 5. 40-GHz sampler used in the 5356C Head is a thin-titm hybrid circuit. R. and R, are closer) to optimize bandwidth and dynamic range
Pulse
Input from Sampler Driver
Hybrid Circuit
Fig. 5. 40-GHz sampler used in the 5356C Head is a thin-titm hybrid circuit. R. and R, are closer) to optimize bandwidth and dynamic range sampler driver is almost constant since the power amplifier is working in the saturation region. Therefore, the comb lines are insensitive to power amplifier variations and temperature.
Sampler
Fig. 5 shows a schematic diagram of the sampler, it consists of a thin-film hybrid mounted in an aluminum package. Two versions of the sampler are used. The 5356A;B sampler is the same as the one in the 5343A Counter without the thin-film buffer amplifier.2"''4 The 5356C 40-GHz sampler is the same basic design with slightly different component values. For the 5356C, R: and R2 are chosen to maximize the sampler's dynamic range up to 40 GHz,
In both versions of the sampler, two beam-lead Schottkv-barrier diodes are placed on the hybrid across the slotted line. This type of diode provides a low, easily controlled inductance and is easy to mount on the thin-film substrate by the thermocompression bonding technique.
To work up to 40 GHz. the diodes are chosen for minimum series resistance, junction capacitance, and stray capacitance. The diode capacitance is incorporated in a low-pass filter that has a cutoff frequency of 41 GHz. The circuit is optimized for low SWR up to 40 GHz using an in-house OFSNAP computer program. Fig. 6 shows the relative sampler conversion efficiency up to 40 GHz. and Fig. 7 shows the return loss up to 40 GHz. Four experimental setups were used for these measurements: 1.5-3.4 GHz. 2-18 GHz, 18-28.5 GHz and 26.5-40 GHz.
The sampler's IF output is designed to be insensitive to sampler driver variations. The minimum uutput from the sampler driver is enough to drive the sampler into saturation. Therefore the IF output is also insensitive to temperature variations,
5355A Compatibility
The design goal was to make any 5356A,|'B,,IC work with any 5355A, The interface between these two instruments is analog, since the VCO signal comes from the 5355A to drive the power amplifier in the 535IjA/U.'C. and the IF comes from the 5356A/B/C to drive the IF amplifier in the 5355A. To guarantee complete compatibility the following conditions were established.
The IF output from the 5356A/B/C is insensitive to the level of the VCO input from the 5355A. The lowest VCO input power level at any frequency is sufficient to drive
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Fig. 7. Return loss o! the 40 GHz sample!
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Frequency (GHzJ
Fig. 7. Return loss o! the 40 GHz sample!
the power amplifier to saturalion. Thus the output ol the sampler driver is insensitive to the 5355A VCO level. The IF output from the 5356A/B/C to the 5355A is sufficient to guarantee the minimum sensitivity of the combined system. The counter sensitivity is defined as the 535fiA.'B/G conversion efficiency [RF to IF) plus the 53 55A IF sensitivity. The 5355A IF sensitivity is adjusted to meet the required specifications and the 5356A/B/C conversion efficiency is tested from "1.5 to 40 GHz to assure that it meets the necessary levels. The input of the 53 55 A is unconditionally siabl&so that it will not oscillate with any 5356A'B-CAlso, the output of the 5356A/B/C is unconditionally stable with any 5355A. This is especially important since the IF gain of the combined system can exceed HO dH.
Sensitivity, Flatness, and Distortion
Since the 5356C is so broadband, there were tradeoffs to be made among sensitivity, frequency response flatness. and distortion caused by sampler overload. The bias resistor between the sampler IF output terminals (Fig. 5] was chosen to maximize the dynamic range of the combined 535KC and 5355A up to 40 GHz.
Dynamic range is a function of frequency. For the 5356C it ranges from -25 dBm to +5 dBm below 12.4 GHz and from - 20 dBm to +15 dBm above 12,4 CHzfor full accuracy (— 1 count). However, the harmonic number is correctly
10 20 30 40
Frequency (GHz)
10 20 30 40
Frequency (GHz)
Fig. 6. Relative conversion efficiency ol the 40 GHz sampler The IF is 300 MHz and the VCO frequency from the 5355A is 7 GHz o a
Frequency (GHz)
Fig. 8. Relative conversion efficiency ot the 5356A & C up to 40 GHz. The IF is 100 MHz and the VCO frequency is 1055 MHz. Note the Signer sensitivity of the 5356C
Frequency (GHz)
Fig. 8. Relative conversion efficiency ot the 5356A & C up to 40 GHz. The IF is 100 MHz and the VCO frequency is 1055 MHz. Note the Signer sensitivity of the 5356C
HI V
Temperature ("C)
Fig. 9. Relative change ot 5356.4S.C .-.inversion efficiency with temperature The IF is 100 MHz and theVCO frequency is 1055 MHz determined for a wider range of input signal levels: 3D to +8 dHm below 12.4 GHz and -25 to +18 dBm above 12.4 GHz.
The dynamic range of the 5356A can be shifted by using one of the Hf 849313-Series Attenuators to replace the coaxial assembly (see Fig, 2). For example, the 8493B Option 010 will make the dynamic range —10 to +15 dBm instead of -2(J to +5 dBm. The damage level will change from +25 dBm to +33 dBm CW and *35 dBm pulse.
Fig. 8 shows the relative conversion efficiency of the 5350A up to 18 GHz. the 5356B up to 2fi.5 GHz, and the 5356C up to 40 GHz. These curves are for25°C. Fig. 9 shows
Frequency (GHz)
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Measured Measured Frequency (GHz) Fig, 10, (a) Sensitivity ot the 5356C 5355A system ft}J Sensitivity ot the 5356.4,5355A and 5356B5355A systems In ail cases the 5355A is a worst-case unit the variation of the conversion efficiency with temperature. A 5355A that has the lowest IF sensitivity within the system specifications is used to lest each 535GA B C. Fig. 10 shows the combined sensitivity for CW and puises. Acknowtedgments The author would like to thank all members of the hybrid department of the HP Santa Clara Division, especially Kathy Luiz, who assembled the original thin-film circuits. The 5356 product design was accomplished very effectively by Keith Leslie. Special thanks are due Jeff Wolfington and Al Barber for their constructive criticism during the course of this project. Many individuals from the HP Santa Rosa. Stanford Park, and Microwave Semiconductor Divisions deserve credit for their help and constructive discussion, especially Young Dae Kim of Stanford Park for his help in designing the high-pass filter for the 5356A Option 001. The product introduction of the 5356AB was handled by Martin Neil, and of the 5356C by Larry Johnson and Doug Nichols, Service engineers were Randy Goodner for the 5356A/B and Joe Dore for the 5356C. Production engineers were Bob Offermann for the 5356A/B and Art Bloedom for the 535GC. Special thanks are due to Luiz Peregrino for his continuing encouragement. The author would like to express his appreciation to Roger Smith, microwave section manager and Tan Band, engineering lab manager, for their support and interest in this project. References 1. C.R Kirkpatrick. R.E. Pratt, and D.K. Chambers, "Coaxial Components and Accessories for Broadband Operation to 26.5 GHi," Hewlett-Packard Journal, June 197?. 2. J. Merkelo, "A dc-to-20-GHz Thin-Film Signal Sampler for Microwave Instrumentation." Hewlett-Packard Journal. April 1973. 3. A Hologln and V.A. Barber, "Microprocessor-Controlled Harmonic Heterodyne Microwave Counter also Measures Amplitudes." Hewlett-Packard Journal. May 1978. Mohamed M. Sayed Mohamed Sayed joined HP's Microwave Technology Center in 1973 After working on microwave silicon and GaAs PET transistors for two years, he joined HP's Santa Ciara Division, and since that time has worked on microwave counters Born in Egypt, he res'?^ ceived his BSEE and MSEE degrees from Cairo University and his PhD from Johns Hopkins University in Baltimore. Maryland He has taught al Cairo. Johns Hopkins, and Howard Universities, and is currently teaching al San Jose State University Before joining HP he spent a year doing research on solar cells at the University ot Delaware He has published papers in the field o) microwave measurements, microwave transistors, and solar energy He's a member of IEEE and is active in the National Alumni Schools Committee of Johns Hopkins University Mohamed ¡s married, has a daughter and lives in Cupertino, California. He Is currently attending Santa Clara University to obtain his Master's degree in engineering management In his spare time he likes to read and [ravel 
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