Resonator design - Page 1
Forum section: electronics / rf-microwave

author: szoftveres
author_rank: Regular Contributor
timestamp: December 19, 2019, 01:27:34 pm
content: Hey All,I've come across a particular resonator design in old UHF TV tuners in the past, the construction really simple: a piece of stout wire inside an RF shielding box, with one end attached to the RF shield and the other end attached to a tuning capacitor.It looks like this:https://hackaday.com/wp-content/uploads/2016/07/mechanical-tv-tuner.jpgWhat is the exact working principle of this resonator? Does it work as an LC circuit (where the thick wire is the inductor), or is it somehow related to the quarter-wave coaxial resonator, with the tuning capacitor altering the wavelength somehow?From the construction, it's definitely a coaxial resonator (transmission line); if we reduce the tuning capacitance infinitely to zero, then all is left is a piece of coaxial transmission line with an open ending, which is exactly what a quarter wave coaxial resonator looks like. But what confuses me is the tuning capacitor. How does it alter the wavelength? What are the design principles for such resonator? Or do I get the whole thing wrong and this is still a lumped element (LC) filter?Thanks!

author: Gyro
author_rank: Super Contributor
timestamp: December 19, 2019, 01:41:17 pm
content: All you could possibly want to know about them...https://www.maximus-randd.com/tv-tuner-history-pt2.htmlYes, they are designed to be 1/4 wavelength at the midband frequency and then modified by the variable capacitor.These tuners used to be great fun to play with. If you tweaked the capacitor vanes an disabled / bypassed the final filter trough, you could use them as TV sound tuners - down-converting the UHF signal to the FM broadcast band so that you could use with your HiFi FM tuner, without any worries about having to tap into the TV chassis. Better sound quality too.P.S. There is a specific name for those bars, but it doesn't come to mind. Fletcher bars?

author: szoftveres
author_rank: Regular Contributor
timestamp: December 19, 2019, 05:11:51 pm
content: What does the capacitor do to the wavelength?

author: ejeffrey
author_rank: Super Contributor
timestamp: December 19, 2019, 05:26:03 pm
content: The capacitor changes the resonance frequency by altering the boundary conditions.A pure 1/4 wave coaxial resonator has an open circuit boundary at one end and a short circuit boundary at the other. That is what gives you the 1/4 wave resonance -- the distance between a voltage anti-node and a node.When you add a tuning capacitance it is no longer a perfect open-circuit boundary but has some admittance and a resulting phase shift on reflection. That means that you end up less than an full 1/4 wave inside the coaxial stub and tunes the resonance to lower frequency. As a bonus, the tuning capacitor also provides the small coupling to the external feedline that you need to actually use it.

author: Gyro
author_rank: Super Contributor
timestamp: December 19, 2019, 05:38:29 pm
content: Those things were works of art, especially considering the amount of critical hand soldered air wired circuitry, that they had to mass produced consistently.In the earlier / better ones, the bars were actually rectangular section silver plated copper bars, presumably for higher Q and lower microphonics.Another link...https://hackaday.com/2016/07/11/not-quite-101-uses-for-an-analog-uhf-tv-tuner/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2019, 10:32:46 pm
content: Yes, equivalent inductance. The characteristic impedance of that wire in that box is going to be on the order of 200 ohms, and its resonant frequency (without the cap) will be many GHz. So it's loaded down pretty well, to resonate in the 100s of MHz.Tim

author: szoftveres
author_rank: Regular Contributor
timestamp: December 19, 2019, 11:30:39 pm
content: Makes sense now - the capacitor alters the wave velocity by altering the characteristic impedance of (some part of) the transmission line, and consequently the physical length of the wire will correspond to a quarter wavelength of a lower frequency.