Quantcast Frequency Limitations of Conventional Tubes - Continued - 14183_84

Share on Google+Share on FacebookShare on LinkedInShare on TwitterShare on DiggShare on Stumble Upon
Custom Search
2-4 A third limitation caused by tube construction is TRANSIT TIME. Transit time is the time required for electrons to travel from the cathode to the plate. While some small amount of transit time is required for electrons to travel from the cathode to the plate, the time is insignificant at low frequencies. In fact, the transit time is so insignificant at low frequencies that it is generally not considered to be a hindering factor. However, at high frequencies, transit time becomes an appreciable portion of a signal cycle and begins to hinder efficiency. For example, a transit time of 1 nanosecond, which is not unusual, is only 0.001 cycle at a frequency of 1 megahertz. The same transit time becomes equal to the time required for an entire cycle at 1,000 megahertz. Transit time depends on electrode spacing and existing voltage potentials. Transit times in excess of 0.1 cycle cause a significant decrease in tube efficiency. This decrease in efficiency is caused, in part, by a phase shift between plate current and grid voltage. If the tube is to operate efficiently, the plate current must be in phase with the grid-signal voltage and 180 degrees out of phase with the plate voltage. When transit time approaches 1/4 cycle, this phase relationship between the elements does not hold true. A positive swing of a high-frequency grid signal causes electrons to leave the cathode and flow to the plate. Initially this current is in phase with the grid voltage. However, since transit time is an appreciable part of a cycle, the current arriving at the plate now lags the grid-signal voltage. As a result, the power output of the tube decreases and the plate power dissipation increases. Another loss of power occurs because of ELECTROSTATIC INDUCTION. The electrons forming the plate current also electrostatically induce potentials in the grid as they move past it. This electrostatic induction in the grid causes currents of positive charges to move back and forth in the grid structure. This back and forth action is similar to the action of hole current in semiconductor devices. When transit-time effect is not a factor (as in low frequencies), the current induced in one side of the grid by the approaching electrons is equal to the current induced on the other side by the receding electrons. The net effect is zero since the currents are in opposite directions and cancel each other. However, when transit time is an appreciable part of a cycle, the number of electrons approaching the grid is not always equal to the number going away. As a result, the induced currents do not cancel. This uncancelled current produces a power loss in the grid that is considered resistive in nature. In other words, the tube acts as if a resistor were connected between the grid and the cathode. The resistance of this imaginary resistor decreases rapidly as the frequency increases. The resistance may become so low that the grid is essentially short-circuited to the cathode, preventing proper operation of the tube. Several methods are available to reduce the limitations of conventional tubes, but none work well when frequency increases beyond 1,000 megahertz. Interelectrode capacitance can be reduced by moving the electrodes further apart or by reducing the size of the tube and its electrodes. Moving the electrodes apart increases the problems associated with transit time, and reducing the size of the tube lowers the power-handling capability. You can see that efforts to reduce certain limitations in conventional tubes are compromises that are often in direct opposition to each other. The net effect is an upper limit of approximately 1,000 megahertz, beyond which conventional tubes are not practical. Q-1.   What happens to the impedance of interelectrode capacitance as frequency increases? Q-2.   What undesirable effect is caused by the inductance of the cathode lead? Q-3.   How does transit time affect the relationship of the grid voltage and the plate current at high frequencies? Q-4.   Moving tube electrodes apart to decrease interelectrode capacitance causes an increase in the effect of what property?

Electrical News
Data Centers May Ride on ASICs
Firebox, a research prototype in the works at UC Berkeley,...
Product Compatibility Is a Constant Dilemma
Because most new products are based on previous ones, engineers...
White Goods Consolidation Drives Power Deals
The power semiconductor industry has seen two key announcements this...
How I Hacked My Home, IoT Style
It didn't take long to find a score of vulnerabilities...
The Internet of Overhyped Things
Vendors and analysts would have us believe that the Internet...
Apple Preps Jumbo iPad
Apple is preparing a larger version of its iPad for...
Apple's iPhone 6: What to Expect
What will the iPhone 6 look like? And when will...
Windows Threshold May Merge Windows Phone, RT
Microsoft's upcoming Windows Threshold operating system reportedly will merge Windows...
Forget iPhone: 4 Megatrends in China's Smartphone Market
Touch Taiwan is exposing some of Asia's techno-political tensions, focused...
Google's Project Ara Is Science Fiction, Says Critic
Google's Project Ara to develop a modular mobile smartphone is...
Aging Brass: Cow Poop vs. Horse Doo-Doo
There are myriad ways to artificially age brass, including the...
Google Preps Virtual Network
Google described Andromeda, a central server-based controller users will be...
EEVblog #656 – Pacemaker Monitor Teardown
What’s inside a dial-in pacemaker monitor system? Datasheets: LMV824 MAX4330...
Getting Your First Sales Pick Right
A company needs to create a repeatable sales process to...
The People Who Created Our Jobs
Without the likes of Ohm, Faraday, Henry, and many others,...
Mobile Engineering: 10 Cheap, Mostly Android Apps
Whether Android- or Apple-based, your mobile phone can be an...
Brazil's Path to Hardware Manufacturing at Home
John Hall, president of the Linux Foundation, lays out a...
Fairchild to Close 2 Manufacturing Facilities
Fairchild is eliminating its internal 5-in wafer fabrication lines and...
JEDEC Unveils Low-Power Memory Standard
JEDEC published its LPDDR4 specification, which, with a redesigned architecture,...
SanDisk Bullish on SSD Adoption
Ultra II is aimed at consumers looking to upgrade HDD...

Privacy Statement - Copyright Information. - Contact Us

comments powered by Disqus

Integrated Publishing, Inc.
9438 US Hwy 19N #311 Port Richey, FL 34668

Phone For Parts Inquiries: (727) 755-3260
Google +