Hi Ravi, Copied to the group for instruction purposes.
If the scope is fully calibrated, perhaps. My scope does not have the precision needed for aligning a PLL. Especially true when it’s a voltage-controlled oscillator. The first tuning point is to have 8V at 110MHz using both a frequency counter and a RF voltmeter. The next alignment is for 170mV at 36 MHz. Here’s my bench meter: https://www.rigolna.com/products/digital-multimeters/dm3000/dm3068/ <https://www.rigolna.com/products/digital-multimeters/dm3000/dm3068/> As great as it is, it does not work at RF and here’s why: All digital meters perform INTEGRATION over time to determine the voltage. The Rigol or an Agilent or Fluke - all have really fast and solid integration circuits. But the limit is imposed by the speed of the signal. 100MHz is operating faster than the digital integrators can react. Also, most DMMs have relatively LOW impedance and load the circuit under test. You garden variety DMMs are in the 100k-ohm range. At these frequencies, the test leads themselves have reactance that affects measurement. An Analog meter integrates as a function of the electronics themselves. I have an analog meter, but it is powered by the circuit under test and loads the circuit. It rectifies the AC into DC for the meter movement. Again, the impedance is low. A vacuum tube voltmeter presents a very high impedance to the circuit under test - usually > 10Meg Ohms. It uses a bridge circuit of vacuum tubes that is very sensitive and a good unit can read accurately at fractions of a volt. They have a test probe just like an oscilloscope. * My workaround may be to use a Schottky diode with a small cap - 10nanofarad to rectify the RF for the DMM. Alternatively, I could try to get a 170mv reference voltage using an adjustable source to calibrate the oscilloscope trace and then do the measurement. I hope everyone learned something from this thread <grin> And yes, a $5000 test monitor would do it all… 73!!! B > On Jun 19, 2016, at 11:00 AM, Ravi Patrick Ratnala via BVARC > <[email protected]> wrote: > > Hey Bill, > > Wouldn't an oscilloscope be the best tool for the job here? > > Alternatively - I just picked up a couple of these to serve as backups for my > Fluke 117 - only $20, and just horsing around, I've already measured > frequencies up to 144 MHz: https://smile.amazon.com/gp/product/B015OFMUYO > <https://smile.amazon.com/gp/product/B015OFMUYO> > > 73! > > Ravi > > Sent from my iPhone > > On Jun 19, 2016, at 10:44 AM, Bill Crowell, N4HPG via BVARC <[email protected] > <mailto:[email protected]>> wrote: > >> Howdy All, >> >> Does anyone have a vacuum tube voltmeter that can be borrowed/purchased? >> Including the probe. >> >> Doing PLL alignment and I need to measure at 100MHz. Your normal AC >> voltmeter does NOT work at these frequencies. >> >> 73 >> Bill Crowell, N4HPG >> Pearland, TX >> [email protected] <mailto:[email protected]> >> I prefer to live a life of galvanic isolation. >> >> _______________________________________________ >> BVARC mailing list >> [email protected] <mailto:[email protected]> >> http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org >> <http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org> > _______________________________________________ > BVARC mailing list > [email protected] > http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org Bill Crowell, N4HPG Pearland, TX [email protected] <mailto:[email protected]> I prefer to live a life of galvanic isolation.
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