{"id":2156,"date":"2020-11-18T07:42:39","date_gmt":"2020-11-18T07:42:39","guid":{"rendered":"https:\/\/35.94.173.37\/blog\/?p=2156"},"modified":"2024-11-04T07:45:25","modified_gmt":"2024-11-04T07:45:25","slug":"passive-voltage-probe-measurements","status":"publish","type":"post","link":"https:\/\/www.apexwaves.com\/blog\/passive-voltage-probe-measurements\/","title":{"rendered":"Passive Voltage Probe Measurements"},"content":{"rendered":"

One of the probe types that can be used with an oscilloscope is a passive-voltage probe. The simplest passive-voltage probe is a coaxial cable. But what happens when a coaxial cable is linked to the device under test? A schematic diagram of the test arrangement is given in Fig. 4.1. The device under test is symbolized as a signal source with an internal source resistance\u00a0Rs.<\/em><\/p>\n

 <\/p>\n

For dc voltages, there is no problem as long as R10\u00a0Rs,\u00a0<\/em>because the part of\u00a0E\u00a0<\/em>appearing at the input of the oscilloscope is<\/p>\n

 <\/p>\n\n\n\n
\n\n\n\n
=\u00a0\u00a0\u00a0 +<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Rin<\/u><\/em>\u00a0\u00a0\u00a0\u00a0 E<\/em><\/p>\n

V osc<\/em>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0Rs\u00a0\u00a0\u00a0\u00a0<\/em>R1n\u2022<\/p>\n

 <\/p>\n

Fig. 4.1<\/strong><\/p>\n

\"\"<\/p>\n

However, in applications involving ac voltages, the stray capacitances have an imperative role. Figure 4.1 shows the signal source loaded by\u00a0R, n\u00a0<\/em>in parallel with Cc + C10\u2022\u00a0\u00a0 \u00a0As the impedance Xe of a capacitance is provided by<\/p>\n

1<\/strong><\/p>\n

Xe= 2nfC<\/u><\/em><\/p>\n

in which f is the frequency of the ac voltage, a relatively heavier loading of the signal source will occur at higher frequencies.<\/p>\n

Example\u00a0<\/em><\/strong>C10 is on the order of 14 to 21 pF; for a coaxial cable of 1-m length,\u00a0Cc\u00a0<\/em>is about 100 pF. At 1 MHz, the impedance of\u00a0Cc +\u00a0<\/em>C10 is approximately1.3 kil. Compared with a R10\u00a0of 1 Mil (normally standardized), this means a heavy load.<\/p>\n

From this example it will be realized that it is important to measure the voltage at a point in a circuit under test having a low source impedance, in order not to influence the circuit properties too much by the connection of the oscilloscope.<\/p>\n

Figure 4.2 shows a typical coaxial cable.\u00a0 On one end, a BNC connector\u00a0is mounted for connection to the oscilloscope. The other end is provided with a special measuring clip (the probe tip) and a ground lead connector. This type of probe is a 1:1 probe because the signal is not attenuated by the probe. The cable capacitance of this type of probe may be in the order of magnitude like the example.<\/p>\n

In cases where the cable capacitance of the l:1 probe will be unacceptable, advantage can be taken of the 10:1 passive probe. Now the signal is attenuated l0 times by the probe. Basically, the object is to create a real IO: l divider for both de and high frequencies. This divider will be independent of the frequency for\u00a0RC =<\/em>\u00a0R1nC1n.<\/p>\n

As previously mentioned, C10\u00a0can\u00a0vary from 15 to 25 pF, depending on stray capacitances in the input circuitry of the oscilloscopes, which are different for each instrument. For this reason, C is made variable.<\/p>\n

However, a more realistic schematic diagram would show that the compensation is for 120 pF, giving a value of C = 13 pF. At the probe tip, there should also be a stray capacitance C. of about 2 to 4 pF from the tip itself to its environment. \u00a0Effectively, a complete load capacitance of\u00a0C8\u00a0<\/em>parallel to C and\u00a0Cc\u00a0<\/em>+ C10\u00a0in\u00a0series would be obtained, resulting in about 15 pF. But 15 pF is far less than 120 pF. A disadvantage might be that the signal is attenuated 10 times at the input of the oscilloscope.<\/p>\n","protected":false},"excerpt":{"rendered":"

One of the probe types that can be used with an oscilloscope is a passive-voltage probe. The simplest passive-voltage probe is a coaxial cable. But what happens when a coaxial cable is linked to the device under test? A schematic diagram of the test arrangement is given in Fig. 4.1. The device under test is […]<\/p>\n","protected":false},"author":1,"featured_media":2157,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[127,528,561,921],"class_list":["post-2156","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized","tag-coaxial-cable","tag-passive-voltage-probes","tag-probes","tag-test-equipment"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/posts\/2156","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/comments?post=2156"}],"version-history":[{"count":1,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/posts\/2156\/revisions"}],"predecessor-version":[{"id":2158,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/posts\/2156\/revisions\/2158"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/media\/2157"}],"wp:attachment":[{"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/media?parent=2156"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/categories?post=2156"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.apexwaves.com\/blog\/wp-json\/wp\/v2\/tags?post=2156"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}