Special signal-tracing probes
Special probes are often needed in land mobile radio servicing for signal tracing in radio transmitters and receivers. The main problem to overcome is the loading effect many probes cause in high-impedance circuits. Certain probes can be used with instrumentation to minimize these loading errors when you are signal tracing.
Circuit loading Resistive and capacitive effects can cause circuit loading when a test probe is applied to a circuit. Generally, the impedance of the probe should be at least 10 times the impedance of the circuit under test. The stray probe capacitance should be a small percentage of the capacitance of the circuit under test. In Figure 1 below, the circuit to the right of the test point is an RLC-tuned circuit. For example, suppose that the impedance of the tuned circuit is 500V. Suppose that: (1) a direct probe were to be touched to the test point (TP1), (2) the input to the instrument (spectrum analyzer) is 50V and (3) the shunt capacitance of the probe is 50% of the capacitance of the tuned circuit under test. Obviously, the loading effect of the probe would be severe, and useful measurements would be impossible.
By using an isolation probe such as shown in Figure 1, the loading effect of the probe can be reduced. Suppose the isolation resistor (Rp) is 450V and the instrument impedance (RI) is 50V. This would represent a probe impedance of 500V as seen by the circuit under test. If the impedance of the circuit under test were 500V, the 500V probe would load the circuit such that the measurement would be useless. In some cases, the circuit under test might even cease to function. If the resistance is placed near the probe tip, it will have the added benefit of reducing the probe’s stray capacitance as seen by the circuit under test. Because this forms a simple 10:1 voltage divider network, the voltage appearing across the input to the instrument will be 10% of the actual voltage at the probe tip. This represents a 20dB isolation, and the instrument should have enough reserve sensitivity to overcome the probe loss. Otherwise, the probe will be of little value.
Similarly, a 100:1 voltage divider could be used to provide even more isolation. This would require a 4,950V isolating resistor, making the probe 5,000V impedance (as seen by the circuit under test). This would provide an isolation of 40dB-barely meeting the 10:1 rule of thumb when connected across the 500V impedance of the circuit previously discussed. The spectrum analyzer or other instrument would have to have high sensitivity to overcome the loss of such a probe.
The previous discussion pertained to resistive loading. The other part of the problem is capacitive loading. Because capacitive reactance is frequency-dependent, the probe input impedance will drop as the frequency increases. Thus, the loading effect increases dramatically as the frequency increases.
Active probes One way to reduce probe loading is to place an amplifier in the probe near the tip. The amplifier would have high input impedance-and an output impedance to match the input impedance of the instrument with which it is used. Low-power, high-impedance FET devices are usually used for such applications. Power to the preamplifier is supplied from the instrument-or externally for generic devices.Many spectrum analyzer manufacturers provide special connections, designed specifically for their instruments, for powering an active probe.
Photo 1 above shows an instrument that can be used in conjunction with a spectrum analyzer to signal trace or to examine signals in high-impedance circuits. This particular device is the preamplified probe, model PP-1, made by Ramsey Electronics of Victor, NY. Figure 2 on page 30 is a block diagram of a typical use for the active probe. Although Figure 2 shows the probe used with a spectrum analyzer, it can be used with a frequency counter, oscilloscope or other instrumentation as well. The specifications are listed in Table 1 at the top of page 30.
Grounding hints The PP-1 preamplifier probe comes with various probe attachments: two grounding adapters-a gold wire spring clip and a clip-on alligator clip, plus a BNC probe tip adapter. The following information is from the instruction manual for the PP-1 probe.
When probing on a PC board, use the gold spring clip. At higher frequencies, it is important to keep the probe’s ground connection short. Errors will be introduced by a long ground wire draping over the circuit board under test. The best technique is to touch the probe tip to the desired measuring point while simultaneously touching the ground spring wire to a nearby ground point. This technique provides a short path to ground, but it does require some manual dexterity.
Low-frequency measurements allow the use of the standard alligator clip ground. Simply slip the alligator clip wire on the probe body and clip the alligator to a convenient ground point. Above about 10MHz, the gold spring clip ground should be used for best results.
You can even use your body as a ground when measuring some high-frequency signals. Simply hold your finger firmly against the metal ground sleeve near the probe tip. Sometimes this technique works well when measuring frequencies in the 50MHz-150MHz range.
Ramsey Electronics also makes a special RF detector probe (model RF-1), which it calls the “Sniff-it.” It is very broadband (100kHz to over 1GHz) and can be used to measure low-level signals in conjunction with any multimeter. The probe uses microwave low-barrier Schottky diodes to provide a sensitive detector reaching 1GHz with a logarithmic output. See Photo 2 above.
The current price of the preamplified probe (PP-1), fully wired and tested, is $195.95. The price of the model RF-1 “Sniff-it” detector probe is $22.95, wired and tested. Contact Ramsey Electronics, 793 Canning Parkway, Victor, NY 14564; 716-924-4560. Web site: www.ramseyelectronics.com. Until next time-stay tuned! n
