A new harmonic heterodyne frequency converter plugin adds automatic GHz frequency measurements to the universal capabilities of HPs top counter
by Richard F. Schneider, Ronald E. Felsenstein, and Robert W. Offermann
O BE USEFUL in the widest possible range of applications. a microwave counter should be capable of measuring the carrier frequencies of pulsed or CW signals, and for pulsed signals, should also provide for liine interval or frequency measurement of the pulse modulation. The design should be optimized with wide IF band-widths for narrow pulses and wide KM deviations, and should have high sensitivity. Cost-effective frequency range selection and automatic operation are essential.
A new system that operates as a plug-in to the HP 5345A 500-MHz Universal Counter1 is designed to meet these requirements over a carrier frequency range of 0.4 to 40 GHz. The system, which consists of the 5355A Automatic Frequency Converter and the 5356A.'B/C Frequency Converter Heads, is shown in Fig. 1. It provides an effective alternative to the complex specially assembled systems that formerly were the only way to measure up to 40 GHz, The 5356A/B/C Frequency Converter Heads eliminate the need for microwave transmission lines to connect the measured source to the counter. Coaxial cables, while convenient, cannot always be used, since two-metre coaxial lines typically have about 10 dB loss at 18 GHz and get worse at higher frequencies. To circumvent this, hybrid microwave circuits in the heads down-convert incoming frequencies to intermediate frequencies (IF) that are easily transmitted over a 1.7-metre miniature coaxial cable to the 5355A Automatic Frequency Converter Plug-in. This eliminates the transmission line loss and effectively improves system sensitivity by that amount. Heads with various coaxial or waveguide connectors can be selected to meet the measurement requirement |see article, page 14).
Microprocessor control in the plug-in makes operation automatic in either pulse or CW mode. The new system uses the single-sampler harmonic heterodyne technique.* The microprocessor computes the input frequency according to the desired resolution set on the 5345A Counter front panel
Fig. 1. Model 5355A Automatic Frequency Convener plug-m for Model 5345A Counter measures the frequencies of CW or pulsed signals up to 40 GHz. Down-conversion of the input frequency takes place in the interchangeable 5356A-BiC Frequency Converter Heads, eliminating the need for high-frequency transmission lines between the source and the counter External gating makes it possible to measure the frequency profile within a pulse.
Input
5355A.B.'C Head
Sampler and Driver
5355A Automatic Frequency Converter
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■ 1 | |
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2-Modulus Frequency Synthesizer | |
' AAD = Aulomalit Amplitude Discrimination
- External Gate
Synthesizer Control
' AAD = Aulomalit Amplitude Discrimination and displays the frequency on the counter's eleven-digit display.
The 5355A plug-in has a simplified keyboard that allows the user to select autumatic or manual CW or pulse operation, to specify frequency offsets or multiplication of the measured frequency by a constant, to display frequency deviation, and to select the prescaler built into the plug-in. The prescaler divides the input frequency by eight. It is used to measure frequencies from 0.4 to 1.6 GHz; no frequency converter head is needed in this range. The prescaler has its own fused front-panel connector.
Pulse repetition frequency measurements and time interval measurements such as pulse width, pulse repetition interval, pulse repetition period, and pulse-to-pulse spacing are made by the reciprocal-taking 5345A Counter, using the detected IF from the 5355A Converter plug-in. The counter mainframe also measures frequencies from 50 /¿Hz to 500 MHz. The counter has a maximum time interval resolution of two nanoseconds for single-shot intervals and iwo picoseconds for time interval average measurements.
The complete microwave counter syslem consisting of the 5345A Counter with the 5355A Frequency Converter and the 5356A/B/C Heads measures any frequency from 50 /¿Hz to 40 GHz. Its sensitivity with the 5356A/B Heads is -20 dBm from 1.5 to 12.4 GHz and -15 dBm from 12,4 lo 20.5 GHz. With die 5356C Head, sensitivity is 5 dB better up to 26.5 GHz and decreases to -10 dBm at 40 GHz. Prescaler sensitivity is -15 dBm from 4011 MHz to 1,6 GHz.
In the automatic mode, the system measures the frequencies of RF pulses from 100 ns to 20 ms wide at pulse repetition frequencies of 50 Hz to 2 MHz, in manual mode, pulses as narrow as 60 ns can be measured, and external gates as narrow as 20 ns may be applied to the counter lor applications such as measuring the f requency profile within a pulse.
For pulsed RF-signals, the FM lolerance is 50 MHz peak-to-peak for a 100-ns pulse in the automatic mode, and SI) MHz p-p for a 60-ns pulse in the manual mode. Automatic calibration of the 5345A mainframe assures accuracy to 3 kHz in pulsed carrier frequency measurements. Resolution is selectable to as tineas 100 Hz by frequency averaging. For example, a 26.5-GHz pulse radar with a 1-jU.s wide pulse could be measured with a 10-ms gate time to a resolution of 10,3 kHz and an accuracy of 43 kHz or about 2 parts in IIP [assuming no time-base error).
For CW [continuous) signals, the maximum resolution is 0.1 Hz up to 10 GHz and 1 Hz from 10 to 40 GHz. The FM lolerance is 15 MHz p-p in the normal mode and 60 MHz p-p in the special FM mode.
Fig. 2. Simplified block diagram of the harmonic heterodyne frequency conversion technique for CW and pulsed signals
Harmonic Heterodyne System
The harmonic heterodyne technique has been described in previous articles.- Basically, a microwave sampler is driven at programmed synthesized frequencies, as shown in Fig. 2, until a signal occurs in the passband of the IF amplifier, indicating lhat some harmonic of the sampling frequency is mixing with the incoming microwave signal to produce a countable IF. The microprocessor then executes an algorithm to identify the harmonic number N and the sign of' the IF (sum or difference!, and solves for the input frequency, according to the equation:
where fx is the input frequency, N is the harmonic number, and fs is the programmed synthesized frequency.
The harmonic number is determined by changing the synthesized frequency slightly and measuring the change in the IF frequency.
The sign of the IF in equation 1 is determined by whether fipj is larger or smaller than fIP1.
Pulse Mode
The basic design parameters of the system were derived from the pulse requirements and the mainframe counter's capabilities. Linear programming3 was used to optimize the system. Seven equations in five variables were solved sub-jecl to various boundary conditions, including the minimum input frequency, ihe IF bandwidth, the IF guard band, the maximum harmonic number, and the minimum synthesizer frequency. The linear programming equations were entered into the computer and families of solutions were obtained for the five variables. Tradeoffs were then made to minimize the tuning range of the synthesizer oscillator and optimize the IF bandwidth. Finally, a separate computer program was derived to determine the minimum number of frequencies required to obtain complete frequency coverage. The result is a set of frequency tables, one for each frequency converter head. For example, with the 1fl-GHz Model 5356A head, only 13 synthesizer frequencies are required.
In the search routine the synthesizer is stepped instead of swept. The synthesizer frequency tables are stored in a ROM, and the synthesizer is stepped to the next frequency in the table after waiting the longest specified pulse repetition interval of 20 ms. This is repeated until a signal appears in the IF passband of 157 to 330 MHz. Next the synthesizer frequency is digitally incremented 4 MHz and the IF passband is tested. If the incremented-synthesizer IF falls outside Ihe passband, the search routine proceeds to the next frequency in the table. If the IFs for both synthesizer settings are within the passband, the calculation of N and the sign of the IF can proceed.
After the initial acquisition in the IF passband of 157 to 330 MHz, the IF can shift into the IF guard band without affecting the measurement. The guard band extends down to 78 MHz and up lo 375 MHz. as shown in Fig. 3. If the IF moves out of the guard band, the 5355A reacquires the input and discards the results of any measurement in progress.
Automatic gain control of the IF amplifier in the pulse mode minimizes the required input signal on'off ratio and maintains the signal-to-noise ratio. An IF detector and a
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