RF Switching Question - Page 1
Forum section: electronics / projects

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: October 12, 2010, 06:36:13 pm
content: I an certainly NOT an RF geek.I have 3 satellite receivers in my house, each with their own TV monitor. I also have another TV monitor on our back deck.I do not want to add another sat receiver for the deck, so I would like to be able to switch that set between the three existing receivers. Note: all of the coax is already in place, two RG6 runs to each room (I went a little nuts when I remodeled a few years ago)Each receiver outputs an RF signal modulated to channel 3 (or 4), so on the deck I'll have three coax cables that each have a different channel 3 signal and I need to switch between them. I also have a boat load of 5vdc DPDT mini relays I'd like to use and avoid having to buy new parts for this. Obviously what I am trying to avoid is having the multiple signals interfere with each other.Soooo ... is the following sketch a decent way to switch between the three signals?

author: sonicj
author_rank: Frequent Contributor
timestamp: October 12, 2010, 07:31:45 pm
content: maximmax4545easy to implement, easy to interface, high quality, free samples.-sj

author: jahonen
author_rank: Super Contributor
timestamp: October 12, 2010, 07:32:28 pm
content: In principle, that relay schematic works just fine for antenna signals. However, the magic is in the construction how good that is in practice. But I guess that you are not after a top of the line isolation or matching figure, and frequency is still relatively low. Go ahead and try it.A microstrip construction on a double-sided PCB should do it if you want to build it on a PCB. With one side as solid contiguous ground plane, 75 ? microstrip trace is approximately as wide as the dielectric thickness.Regards,Janne

author: TechGuy
author_rank: Regular Contributor
timestamp: October 12, 2010, 09:30:02 pm
content: Quote from: JohnS_AZ on October 12, 2010, 06:36:13 pmGo with SonicJ's solution. a Relay is likely to attenate the signal and cause interference since some of the signal will leak out.Perhaps as a completely different solution, there is a DVR that supports multiple TVs, where the SAT connects to a master server and distributes out meda (recorded and perhaps live TV) to multiple TVs. FWIW: I am setting up a Media server to host stored content and distribute it to multiple digital media players (set-top box). The Digital Media players are set-top boxes that play digital media from a server or URL (ie you-tube, hulu, netflix) and play it on your TV. You can also RIP DVDs and play them from a Digital Media Player. Unfortunately, I don't know of commerical digital media player that will also handle Live TV yet.The only system that I know of that can stream live TV is MythTV (Linux), but your set-top box would need to be media PCs since I don't think there is a digital Media player that supports MythTV. MythTV can play live TV using a server with DTV/Analog capture cards, converting live TV to a Stream (MPEG4) and sending to a remove client to play the content. The idea would be to install multiple capture cards in the server (one for each SATTV settop box) and then relay the output to remote MythTV boxes to play. You can use a regular remote to change the channel, On the server side, the server would send IR signals to the SatTV box to change the channel. You would be limited to the number of SatTV boxes for the number of concurrent streams, ie two SatTV boxes = two live streams. You can view recorded material on all systems concurrently, unless you run into network bandwidth performace issues. The data would be streamed using ethernet or WiFi.

author: sonicj
author_rank: Frequent Contributor
timestamp: October 13, 2010, 04:42:33 am
content: i attempted a couple of mythbox builds, but never got it 100% polished. thats about the only thing i like about windows... XP MCE was pretty good as a analog DVR! i do have a couple of old xbox's setup with XBMC. i think they will function as a mythbox front end for sdtv.... controlling them from my droid over wifi is pretty slick!

author: dimlow
author_rank: Frequent Contributor
timestamp: October 13, 2010, 07:08:30 am
content: This looks like an ideal case for a media centre server. I use Windows Home Server with multiple PCs that record and play live TV from Satalite and Cable. Each PC copies their tv programs over to the WHS, then all the recorded tv is available to all the PC's. In the kids rooms i have a linksys boxes, also the xbox 360 plays great tv in HD and i use that for a projector. One of the benefits here is that we always have something to watch. There are around 200 films stored on the WHS, 1000's of pictures, music and TV programs. All this can be streamed to any PC,Xbos,Linksys and even my Phone. The set-up takes a bit of getting used to but once its done i don't think you will go back to normal TV.

author: Zero999
author_rank: Super Contributor
timestamp: October 13, 2010, 09:35:18 am
content: Quote from: jahonen on October 12, 2010, 07:32:28 pmThat's true. Use a ground plane and keep the traces a very short, microstrip would be ideal but I don't know much about it.

author: jahonen
author_rank: Super Contributor
timestamp: October 13, 2010, 03:10:07 pm
content: Out of interest, I measured RF properties of simple power relay (Hongfa JQX-68F), which is not intended to be used in RF application (or even in small signal application). The frequency range was 30 kHz to 1 GHz, and VNA used was HP 8753D, but I did not bother to calibrate the VNA, so data is not ultimately accurate. As you can see, the transmission is surprisingly good. Isolation is however more problematic, but the poor result is probably due that the relay was SPST, thus just single opening contact. With SPDT one could ground the output when relay is off, and thus get better isolation.Regards,Janne

author: TechGuy
author_rank: Regular Contributor
timestamp: October 13, 2010, 09:31:00 pm
content: Quote from: sonicj on October 13, 2010, 04:42:33 amMythTV is definantly a hobbiest tool. its not a commerical quality item like a Tivo. For Set top boxes take a peek at Syabas PopCorn Hour. Its a set top Digital Media player but is hackable/user customizable. Plays just about all media types including DVD-iso with menu functionality, MKV, AVI, MPEG4, Quicktime, etc.http://www.popcornhour.com/Perhaps someone will write an app for the Popcorn-hour that interfaces with Mythtv or MCE so you can watch live TV from it. I don't have an xbox so I am going with the Popcorn hour (just orderd it over the weekend)

author: sonicj
author_rank: Frequent Contributor
timestamp: October 13, 2010, 09:58:35 pm
content: i hope the OP doesn't mind his topic being de-railed by media center chit chat!btw,xbmcis free and is cross platform, no hacked xbox required. very slick interface!-sj

author: joelby
author_rank: Frequent Contributor
timestamp: October 13, 2010, 11:27:39 pm
content: Another thing you could do, with a few orders of magnitude more difficulty, is use RF mixers to shift the incoming signals to different channels (3, 4, 5, etc.) and then combine them all on the same coax.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: October 14, 2010, 04:01:40 am
content: QuoteI don't mind a bit. God knows I've hijacked enough threads in my time.I think I'm going to try my original idea simply because I have everything here I need. If it gives me problems I can always fall back to the Maxim part (thank for the lead!).My original thought was to modulate each source on it's own frequency, but then discovered that even with all TV going digital, frequency agile modulators are still too expensive to suit me.

author: osmosis321
author_rank: Regular Contributor
timestamp: October 14, 2010, 04:56:33 am
content: why not get a small a/v matrix? they're not cheap but the simplicity and convenience more than make up for it.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 14, 2010, 01:35:12 pm
content: I have seen photoresistors being used to switch 2.4GHz.

author: Zad
author_rank: Super Contributor
timestamp: October 14, 2010, 03:07:41 pm
content: PIN diodes are more usual for switching 2.4GHz.http://en.wikipedia.org/wiki/Pin_diodeIsolation isn't as good as RF relays, but with the correct component choice it can be made pretty damn good.

author: kd5jha
author_rank: Contributor
timestamp: October 20, 2010, 04:10:44 am
content: I have a sorta KISS Idea for the TV on the Deck... Why not feed one line out there on CH3, one on CH 4 and the third on CH3 again with the two dupes fed into a good ole A/B switch that you could manually choose which of them to look at. since its an outdoor setup and not your primary unit it would not be too much of an issue, at least for me.If that wont work for you then I suggest ebay for a FA modulator like this one here:http://cgi.ebay.com/S35-BLONDER-TONGUE-AGILE-MODULATOR-AM60-450-AM-SERIES-/180574166050?pt=LH_DefaultDomain_0&hash=item2a0b0f1c22That particular unit will put your other receiver on CH2 so if you already have an incoming CH2 you will have to use a bandpass filter to filter it out...I saw one of those on ebay too for cheap...it was BT as well.RMG

author: sonicj
author_rank: Frequent Contributor
timestamp: October 21, 2010, 06:19:20 pm
content: what a bargain! those things used to be like a grand! you could redistribute cable to the whole neighborhood

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: October 25, 2010, 04:47:10 pm
content: Well this may be total technological overkill, but this weekend I may have found the answer to my RF switching issue. I picked up an old HP 59307A VHF switch at, of all places, a garage sale. Paid $10 for the thing.Haven't played with it yet, but it looks in pretty good shape. Presuming it's functional my next decision will be between making some sort of IEEE-488/HPIB driver (good job for an arduino), or just gut the logic out of the thing and make my own board to drive the relays.

author: kd5jha
author_rank: Contributor
timestamp: November 13, 2010, 05:57:45 pm
content: I never find those kind of cool garage salesI need to move to your neighborhood!