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Fundamentals of Microwave
Frequency Counters
Application Note 200-1
Electronic Counters Series
1
Table of Contents
Down-Conversion Techniques for Automatic
Microwave Frequency Counters ................................................. 3
Prescaling .................................................................................................... 3
Heterodyne Converter ............................................................................... 4
Transfer Oscillator ..................................................................................... 5
Harmonic Heterodyne Converter ............................................................ 6
Comparing the Principal Microwave Down-Conversion
Techniques .................................................................................... 8
Measurement Speed .................................................................................. 8
Accuracy ...................................................................................................... 8
Sensitivity and Dynamic Range ............................................................... 10
Signal-to-Noise Ratio ................................................................................. 11
FM Tolerance .............................................................................................. 12
AM Tolerance ............................................................................................. 14
Amplitude Discrimination ........................................................................ 14
Summary of the Comparison ................................................................... 15
Additional Considerations in Choosing a
Microwave Counter ...................................................................... 16
Signal Inputs ............................................................................................... 16
Systems Interface ....................................................................................... 16
IF Offsets ..................................................................................................... 16
Microwave Power Measurements ........................................................... 16
Some Applications of the HP 5342A Microwave
Frequency Counter ...................................................................... 17
2
Down-Conversion Techniques for Automatic
Microwave Frequency Counters
A frequency counter, being a digital instrument, is limited in its fre-
quency range by the speed of its logic circuitry. Today the state of the
art in high-speed logic allows the construction of counters with a
frequency range of around 500 MHz. Continuing advances in IC tech-
nology should extend this range beyond 1 GHz in the not-too-distant
future.
The designer of an automatic microwave counter must look to some
form of down-conversion in order to extend frequency measurement
beyond 500 MHz. Four techniques are available today to provide this
down-conversion:
1. Prescaling, with a range of 1.5 GHz;
2. Heterodyne Converter, allowing measurements as high as 20 GHz;
3. Transfer Oscillator, used in counters with ranges to 23 GHz;
4. Harmonic Heterodyne Converter, a new technique which can
provide measurements to 40 GHz.
Prescaling
Prescaling involves simple division of the input frequency resulting in a
lower frequency signal which can be counted in digital circuitry. The
frequency measured by the counter section is related to the input
simply by the integer N. A display of the correct frequency is accom-
plished either by multiplying the counter's contents by N or by increas-
ing the counter's gate time by a factor of N. Typically, N ranges from 2
to 16.
Figure 1 shows the block diagram of a high frequency counter using
prescaling as its down-conversion technique. The input signal is
conditioned to interact correctly with the prescaling circuit, and then it
is divided by N before entering the main gate. Beyond this point the
block diagram looks like a conventional counter, with the main gate
being opened and closed (by the main gate flip-flop) in timing precisely
determined by the crystal time base of the instrument. The decade
counting assembly (DCA) now accumulates the under-500 MHz fre-
quency measurement, which is multiplied by N and transmitted to the
display.
Modern frequency counters using this technique are capable of mea-
suring above 1.0 GHz. Recent developments in solid-state technology
promise to extend this range into the low microwave range within a
few years.
3
Input Signal Conditioning
fx