Frequency Counter Measurement Techniques

Inexpensive frequency counters that will measure frequency well into the microwave range are available to the hobbyist today. Units are available for as low as $100 USD that still have frequency capabilities of 500 to 1000 MHz or better. A frequency counter is an excellent means of accurately determining the frequency of unknown signals, or to see if an oscillator or a multiplier stage in a receiver or transmitter is working. However, one must watch out as what is really being measured and exactly what the counter is "seeing". This is not always what you think it is for several reasons. Often, a counter will give a false reading that is erroneous. This is not the fault of the counter. After all, it is a device that ideally reads events per unit time. For sine or square waves, or pulse trains signal encountered in RF and digital work, ideally this is reading the number of cycles per second, or level transitions in a particular direction. However, for complex waveforms,especially if random signals such as noise pickup, video or audio program components, or other extraneous frequencies are present at the counter input, the counter may produce a reading entirely different from what was expected or sought after. Further, the reading may jump wildly around. After all, it is counting transitions in level about a reference point (Generally zero crossings, ect) and a waveform containing several frequency components, especially if non harmonically related or not phase coherent, will have many possible zero crossings depending on the amplitudes of all the frequency components present at the counter input. Due to the counter frequency response, it may favor some of these components over others as the counter may have much higher sensitivity at the favored frequencies. A counter connected to a circuit point containing both RF and audio signal may not see the audio component if the counter preamp cuts off below say 1 MHz as one of our small hand held counters does. There fore, if we were trying to measure a 1 KHz audio frequency and some stray RF was present, the counter might just show the RF signal and ignore the audio. Another case is a frequency multiplier. For example, we have a stage that is multiplying from 107 to 428 MHz. If the multiplier produces 428 MHz out but some 107 MHz output is also present (This is normal for most multipliers), the 107 MHz may be several dB below the 428 MHz output. A counter coupled to the stage may show 107 MHz or 214 MHz as the output if its sensitivity is much higher at 107 or 214 MHz than 428 MHz, or may show a jumping reading or one that changes as counter coupling method or preamp sensitivity is varied. In this case it is hard to come to any definite conclusions as to whether the circuit is operating correctly or not. ANY JUMPING, WILDLY VARYING, OR ERRONEOUS READINGS THAT ARE WAY OFF WHAT IS EXPECTED ARE A SIGN OF THE PRESENCE OF INTERFERING SIGNALS, INSUFFICIENT SIGNAL LEVEL FOR A RELIABLE MEASUREMENT, OR IMPROPER TECHNIQUE AND SHOULD BE IGNORED. A proper reading is generally steady and repeatable. Note that especially at VHF and UHF, normal frequency instability as encountered in free running oscillator circuits may cause a slow, steady drifting in the last few digits. These will be somewhat predictable after watching the change for a minute or so. Random jumping is a sign of interference or insufficient signal, or the presence of excessive phase noise. However, in crystal controlled situations this should not be occuring except in the last digit or so, due to normal uncertainty in the least significant bit.

Without a Spectrum analyzer, it is difficult to see just what is present at the counter input. In this case, it is helpful to use a method of coupling to the counter that will give some control of what is being picked up. There are several methods:

WHIP ANTENNA: Useful for sampling the near field of a transmitter antenna. While supplied with many counters as an included accessory, the antenna is almost useless for any other purpose. Again, whips are only useful in cases where it is definitely known that the antenna is in an area that is dominated by one large strong signal that is at least 10 to 20 dB stronger than anything else. Most counters have as short whips and these act as high pass filters that cut off below 30 to 50 MHz and hence the whip is best used for VHF and UHF measurements, as weaker VHF and UHF signals may dominate strong signals in the lower HF range (below 10 MHz)

PROBE or CLIP LEADS: Good for general work in the HF range up to about 50 MHz or so. If connections are kept short is a fairly reliable fool proof method, especially at audio and lower RF frequencies. May cause loading down and detuning of sensitive circuits such as oscillators, and the frequency read with the counter may not be the actual frequency due to this detuning. Poor to useless when several frequencies may be present. Also remember that many counters have low input impedances at the higher frequencies (above 50 MHz) or separate inputs that are 50 ohms for high and very high frequencies.

ACTIVE PREAMP PROBE: Used so as not to cause excessive loading at high frequencies or in high impedance circuits, but have same limitations as a scope probe, the input impedance still being a few picofarads, which can cause severe detuning of VHF and UHF circuitry.

COUPLING LOOP: Very good for RF work. Consists of one to several turns of preferably insulated wire large enough to hold its shape (#18 AWG or larger) connected to a BNC female connector. Provides a low impedance to the counter and a length of 50 or 75 ohm coax fitted with suitable connectors at each end can be used to connect it to the counter. If made small (1-2 CM diameter) it has a small pickup area and is useful in RF work where several frequencies are present in a small area, as the loop can be positioned near the circuit to be measured. Can be held near a tuned circuit and coupling adjusted by positioning the loop. The tuned circuit acts as a bandpass filter and mainly whatever energy is present in the tuned circuit is sampled. We find this method of coupling very useful and reliable, and no direct connection to the circuit is needed, and only enough coupling sufficient to get a reading is needed. Properly used, this technique has much merit and causes almost negligible loading of the circuit being measured. Unwieldy at lower frequencies (<5 MHz) unless a large loop with a number of turns is used. Not generally useful for audio or very low (<100 KHz) frequency work.

TUNED ANTENNA AND PRESELECTORS: These act as variable frequency filters and are useful where several frequencies are present, as in off the air measurements. Excellent as long as you know the approximate frequency to be measured in advance. There are a few older counters on the surplus market such as the HP 5245 models, available to experimenters at reasonable prices, that have plug ins with this feature.

Keeping these facts in mind may save hours of frustrating labor in the efforts to find circuit problems that do not exist. The correct coupling technique is the one that produces a steady reliable reading in the expected frequency range you are trying to measure. Most of the time, in frequency measurement, we know closely in advance what readings to expect. This is why wildly different readings should be suspected as being in error.


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