On a sunny day (Mon, 22 Oct 2012 10:01:59 GMT) it happened Jan Panteltje wrote in :
PS, you can actually poke around with this on the ground plane and find the hot spots, where the RF goes so to speak, few cm makes all the difference. Find bad capacitators.. etc etc etc
Is the coaxial shield of the coax and the 3n3 capacitor supposed to create some virtual RF ground on the PCB, since there seems to be no real RF ground connection ?
What is the purpose of the 1/4 wavelength tip, is it some kin of 1/4 impedance transformer, transforming an open circuit to short circuit and visa versa ? With 1 Mohm scope input, the load at the tip would be nearly 0 ohms.
On a sunny day (Mon, 22 Oct 2012 16:11:46 +0300) it happened snipped-for-privacy@downunder.com wrote in :
Yes, at these frequencies, the capacitance of 'the rest' against ground seems to be enough for reliable measurements, as capacitive mains hum (50Hz here) does not pass a lot through the input cap. The 3nF close to the diode makes sure it actually sees a RF short.
It is some experiment with field strength measurement, say 'antenna'.
1/4 wavelength vertical has a base impedance of about 70 Ohm, (cable forms other half of a half wave dipole, if not that a ground plane, say
50 Ohms or there about). good low impedance to drive the diode with:
formatting link
Note that it consists of 2 pieces, soldered somewhere in the middle, I tried 1/8 wavelength too, works just fine. I have been measuring voltages in a 33 Ohm RF attenuator, and the ratios seem to be right on (for several volts to half a volt say for ratio 1/2), comes out fine with the 1/4 tip directly on the resistors.
Main purpose it to be able to adjust things for maximum, and test some phase shift things I am into trying. And at the same time get an idea from radiation emitted by parts of the circuit, and if decoupling caps are actually decoupling.
Reminds me of my old school days with a RF voltmeter, but now at a frequency where a testpoint tip can be a significant part of a wavelength. Fun!
Do any digital scopes have an RF detector function built into their firmware? A 100 or 200 MHz scope wouldn't really need a diode detector... the function could be faked within the scope's bandwidth.
That BAT15 is a pretty nice diode.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
I have used similar circuits (series capacitor and 1N914/1N4148 to ground) to tune RF stages up to 1.3 GHz. At those days the standard OpAmp (741) had far too big bias/offsert currents, making RF probe + amplifier + multimeter useless, so I had to use the quite expensive 25 uA instrument as a signal strength indicator. Removing any active components from the system also removed quite a lot stray capacitance issues.
And dirt cheap, got them from ebay, guy send me a lot more than I payed for :-) The low knee voltage of 200mV at 10uA makes it possible to measure very small signals.
Wow, that would have to be some special cable to have that much capacitance. There is some spiral delay cable that has a lot of capacitance, the center conductor is actually a helix inside the sheath, but typical coax is about
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.