oa91 equivalent

hello

i have constructed diode wave meter shown on
http://www.ajpotts.fsnet.co.uk/Circuits.html.

it is not working.it is like a field strength meter to peak up the output power. it contains 10 LED's , which indicate the signal strength. i placed a 200mW transmitter and the LED's are not illuminating. but, when i touch the 9v battery or the potentiometer ,the LED's are illuminating. please tell me what the fault is. i am enclosing the circuit.

Seems like you are collecting bad performing and erroneous circuits?

The original circuit has a dubious and most likely non-operational diode bias, in your circuit, at least the 22M variable resistor (hard to get, I guess?) is a completely meaningless circuit detail. For higher sensitivity, a detector bias seems advisable. I'm pretty sure, that you can find operational RF field detectors at many HAM radio sites.

could some one give me a FIELD STRENGTH METER CIRCUIT. i want it for peaking the output power of my homebrew transmitters.

I don't see anything really wrong with the circuit shown. Realize, it is only good for realatively strong signals, certainly above a power level of -10 dBm or so. But there are improvements to be made!

Pin 3 is the VCC input. You need a capacitor to ground there. Try a 0.1 uF ceramic in parallel with a 4.7 uF tantalum. Will keep all those op amps from oscillating.

I would change the 470 pf to a 0.01 uf ceramic cap.

I would replace the 22 Mohm pot with a 100K ohm pot, and switch the leads so that one end goes to ground, one end goes to the diode, and the wiper goes to the IC pin 5. 22 Mohm might be way too big considering the input impedance of the IC.

After that it is simply a matter of balancing the resistors properly. Put in a strong RF signal, and use a voltmeter to measure the DC voltage across where the 470 pF was. This is your detected signal voltage. Lets say it is 0.15 volts with a strong RF signal present. That means you want the voltage on pin6 of the IC to be around ~0.15 volts also! Adjust the 2.2 K between pin 6 and 7 accordingly.

Should work!

Added after 12 minutes:

Oh, there is one more fundamental issue with the diode voltage multiplier circuit! If indeed you have an antenna connected as shown, it is ok to go. If, however, you replace the antenna with a coaxial connector and start connecting stuff to it, you will absolutely NEED a series capacitor. Try a series 0.01 uF or 1000 pF between the antenna and diodes . without the series cap the voltage multiplier will not multiply!

thanku mr.biff44! but, i have the same problem. when i touch the anodes of the LED'S the LED'S are glowing. otherwise no. actually the first LED indicated the POWER ON/OFF. it is not glowing when i apply power. an more tips? please help.

did you put the bypass cap in at pin 3? Sounds like something is oscillating and the capacitance of your finger changes it. Try probing with an oscilloscope.

yes, i used a 0.1 uF ceramic in parallel with a 4.7 uF tantalum, like you said. but,the problems remains same. any more tips?

I would start with fundamentals. Disconnect the detector diodes, and wire in a variable DC source and a 10 K series limiting resistor in their place as an input to the IC. Vary the voltage from zero to positive and see if you can get the IC to behave in a stable fashion. Let me know what happens.

hello biff

sorry for the delay. i got an RPS. i tested the circuit, and its very stable. must be the diodes.i did not use the OA91 diodes. i used another germanium diode. what is the equivalent diode for OA91? what do you want me to do now?

thank you very much for your support.

Germanium diodes? What do you do, shop for electronics parts in an antiques store? I suppose that is a step up from a cat whisker diode!

Try these:
http://cgi.ebay.com/ws/eBayISAPI.dll...2BSI%26otn%3D4

Added after 7 minutes:

You should be able to test the diodes all by themselves, now that you know the IC is operating and stable. Using an oscilloscope or high impedance voltmeter, measure the detected DC volts coming out of the diodes as the RF power is applied. You will need some sort of DC voltage return to ground, like a 10 K resistor, in place of the IC. Leave out the potentiometer.

Apply RF and read the DC. Vary the RF power, and see how the detected DC voltage responds. If no DC, then something is broken, and you will have to screw around until you find it. Remember to AC couple the RF into the diodes.