Points well-made, and yes, especially since you say you're dealing with a voltage-controlled oscillator (which is necessarily hyper-sensitive/reactive to measurement taps taken off the circuit).
I know I can make use of the high-Z function on my DMMs for higher-freq stuff, but I don't know the practical limit of such offhand. I would think your workaround (or any sort of bridge) would do the job even if the built-in high-Z function wouldn't, and could easily be tuned to whatever frequency is of interest with a little math and an assortment of pico-caps. That's one mean bench meter you've got. I like! 73 Sent from my iPhone > On Jun 19, 2016, at 11:48 AM, Bill Crowell, N4HPG via BVARC <bvarc@bvarc.org> > wrote: > > 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/ > > 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 >> <bvarc@bvarc.org> 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 >> >> 73! >> >> Ravi >> >> Sent from my iPhone >> >>> On Jun 19, 2016, at 10:44 AM, Bill Crowell, N4HPG via BVARC >>> <bvarc@bvarc.org> 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 >>> n4...@comcast.net >>> I prefer to live a life of galvanic isolation. >>> >>> _______________________________________________ >>> BVARC mailing list >>> BVARC@bvarc.org >>> http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org >> _______________________________________________ >> BVARC mailing list >> BVARC@bvarc.org >> http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org > > Bill Crowell, N4HPG > Pearland, TX > n4...@comcast.net > I prefer to live a life of galvanic isolation. > > _______________________________________________ > BVARC mailing list > BVARC@bvarc.org > http://mail.bvarc.org/mailman/listinfo/bvarc_bvarc.org
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