Post by thread:[EE:] RF questions

I have been trying to get an RF application going for a couple of weeks and am having a terrible time. I am sending data from one PIC to another with one of these http://www.parallaxinc.com/html_files/products/RF_Mods/27986-27991.asp at 433 MHz. My results are not consistent. Sometime it works just fine, sometimes it doesn't work at all. I can debug this project just fine till I get to the transmitter. I can not tell if the RF unit is actually transmitting well. I have two questions: #1. Is there some way I can observe RF directly with my scope? Do I need a special probe? #2. I am thinking of buying a field strength meter. The field strength meter I am looking at claims 5 mv sensitivity at 433 MHz. The transmitter I am using claims 1 mw power output. Would this field strength meter be useful with such a tiny transmitter? Jay -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> Jay, Assuming your in the us, 433Mhz is very close to on channel 59 of a TV. Make sure the TV is in "CATV" or cable mode. When you turn it on you should notce some effect on the picture-- at least that will let you know your device is transmitting. Regards, Steve > I have been trying to get an RF application going for a couple of weeks and > am having a terrible time. I am sending data from one PIC to another with > one of these > http://www.parallaxinc.com/html_files/products/RF_Mods/27986-27991.asp at > 433 MHz. My results are not consistent. Sometime it works just fine, > sometimes it doesn't work at all. > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Jay, What kind of antenna setup do you have? At 433Mhz length can be very critical. Metal objects near the antenna can cause reflections, and antenna detuning. 1mw isn't much power even at 433Mhz. At that frequency the receiving antenna is as critical ans the transmitting antenna. To determine the antenna length (assuming a 1/4 wave whip) the formula is 300M Meters/Frequency in Mhz x .25 for the quater wave length, then allowing for the velocity of propogation it that frequency, shorten this by 5 to 7 %, so as a ballpark figure, multiply the result of the last equasion by .93 to .95 for the real antenna length. Vern Jay Hanson wrote: {Quote hidden}> > I have been trying to get an RF application going for a couple of weeks and > am having a terrible time. I am sending data from one PIC to another with > one of these > http://www.parallaxinc.com/html_files/products/RF_Mods/27986-27991.asp at > 433 MHz. My results are not consistent. Sometime it works just fine, > sometimes it doesn't work at all. > > I can debug this project just fine till I get to the transmitter. I can not > tell if the RF unit is actually transmitting well. I have two questions: > > #1. Is there some way I can observe RF directly with my scope? Do I need a > special probe? > > #2. I am thinking of buying a field strength meter. The field strength > meter I am looking at claims 5 mv sensitivity at 433 MHz. The transmitter I > am using claims 1 mw power output. Would this field strength meter be > useful with such a tiny transmitter? > > Jay > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> #1. Is there some way I can observe RF directly with my scope? Do I need a > special probe? Depends on your scope, you need at least 500 MHz bandwidth to play. Your probes also need enough bandwidth. You can make a rectifying probe with a small signal diode and cap, but you loose frequency and waveshape info. Be aware, you are building a device that has to live underneath ham transmitters at <=1500W. January has a major contest weekend coming up too. > #2. I am thinking of buying a field strength meter. The field strength > meter I am looking at claims 5 mv sensitivity at 433 MHz. The transmitter I > am using claims 1 mw power output. Would this field strength meter be > useful with such a tiny transmitter? Hard question to answer, depends on the antennas. I would look at getting at least a decent receiver. An IC-R8500 or similar with AM, FM, CW, internal attenuator(s) and a signal strength meter. At any given impedance, you can calculate the voltage for a given power output. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>What kind of antenna setup do you have? At 433Mhz length can be very >critical. Metal objects near the antenna can cause reflections, and >antenna detuning. 1mw isn't much power even at 433Mhz. At that frequency >the receiving antenna is as critical ans the transmitting antenna. The unit has its own antenna on the circuit board. One of the reasons I got this unit was because I didn't want to have to fool around with antennas. <G> http://www.parallaxinc.com/html_files/products/RF_Mods/27986-27991.asp Jay -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>How? Serial data or just digital data? If digital, how do you synchronize >it? The transmitter accepts serial data inverted from a PIC uart and sends it to the receiver. >If you are transmitting serial RS-232 data from one PIC to the other through >RF, it's better to do a NOT on both ends. RS-232 standard maintains the >line HIGH where there is no data, and those RF modules don't like them. So, >using a NOT gate on both ends make it a default LOW, and it works much [ snip ] Thanks for the rest of these tips. I will look into them. Sending an extra 0x55 on the front sounds especially promising. One of the pecularities I noticed was that some bytes got dropped in a long string of zeroes. Many of these zeroes are just filler bytes and I can change them to 0x55. Jay -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

A simple probe consists of a single 1N4148 (better 1N23) diode with a 5cm wire on one end and a connector mating with the scope probe tip on the other (turned IC socket). Set the probe to 1Meg. You may have to add a smaller resistor between the probe tip and ground to make fast pulses visible. The scope will be used on 10mV/div probably. Do not touch directly with circuit, 1-2cm away from tx antenna or tx booster (-;) should work. The field strength meter will be useful, for the transmitter only probably. This is also true for the probe. Peter -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

That's a really clever little tool Peter, thank you very much. :o) -Roman Peter L. Peres wrote: {Quote hidden}> > A simple probe consists of a single 1N4148 (better 1N23) diode with a 5cm > wire on one end and a connector mating with the scope probe tip on the > other (turned IC socket). Set the probe to 1Meg. You may have to add a > smaller resistor between the probe tip and ground to make fast pulses > visible. The scope will be used on 10mV/div probably. Do not touch > directly with circuit, 1-2cm away from tx antenna or tx booster (-;) > should work. > > The field strength meter will be useful, for the transmitter only > probably. This is also true for the probe. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Will work better with an old dot-size germanium diode or a microwave schottky diode in SOT23 ( small) package. But the measuring results will be pure informative. Vasile On Thu, 10 Jan 2002, Roman Black wrote: {Quote hidden}> That's a really clever little tool Peter, > thank you very much. :o) > -Roman > > Peter L. Peres wrote: > > > > A simple probe consists of a single 1N4148 (better 1N23) diode with a 5cm > > wire on one end and a connector mating with the scope probe tip on the > > other (turned IC socket). Set the probe to 1Meg. You may have to add a > > smaller resistor between the probe tip and ground to make fast pulses > > visible. The scope will be used on 10mV/div probably. Do not touch > > directly with circuit, 1-2cm away from tx antenna or tx booster (-;) > > should work. > > > > The field strength meter will be useful, for the transmitter only > > probably. This is also true for the probe. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

>Will work better with an old dot-size germanium diode or a microwave >schottky diode in SOT23 ( small) package. >But the measuring results will be pure informative. ......... > > A simple probe consists of a single 1N4148 (better 1N23) diode with a 5cm > > wire on one end and a connector mating with the scope probe tip on the If I remember correctly the 1N23 is a microwave germanium diode, and would probably work better at this sort of level than a schottky. But then I could be wrong, it is a long time since I dealt with radar components. :) -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

> my probe using diode on scope probe tip The problem is the very poor impedance match and coupling, not the diode. There is another version of the same probe that uses a closed loop (crocodile clamp to ground of probe. It can have 1 or more turns. This is usefull in testing RFID systems and such (at low frequency). You can test the straight probe quickly using a cellular phone. 1N23 is a microwave germanium diode. The closed loop probe can be tested with a computer monitor or tv (it picks up the field from the deflection yoke 1-2 meter away). The probes are useful for seeing modulation and for snooping for heavy rf leakage. Both probes pick up 50Hz mains hum and requre some care to avoid that. Peter -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu