Heathkit HG-10B VFO Impedance

[ko4nrbs]

What characteristic impedance should I use to calculate dBm for my HG-10B.  I know the output is at 5 volts RMS.  I need to make sure I use the correct attenuator before connecting the HG-10B to my frequency counter.  I found that the load impedance is greater than 50k ohms.
Here is the calculator I am using.
https://www.everythingrf.com/rf-calculators/vrms-to-dbm-calculator

Thank you,
Bill

[Tom WA3KLR]

Keep it simple to start with - try just putting a scope probe on the counter input and connect the tip to the VFO output. I think this will work o.k.

[KD6VXI]

Connect a large variable resistors on the output.  Start at max resistance.

Start lowering the resistance and watch the output on the variable side with the scope.

When you start loading it down to where the waveform distorts, youre too low.

It's janky, but I've used this method before.  Then built a buffer or xformer floor the impedance I found.

Used to have a huge 10 meg pot, I'd guess like 20 watts or so.  Probably in the trailer.

--Shane
WP2ASS / ex KD6VXI

[K8DI]

Maybe I'm missing something here?

Five volts rms, that seems like something a frequency counter should be able to deal with.  An old 4000 series CMOS digital circuit could easily run at 18 volts; I don't remember worrying about measuring the clock frequency with whatever counter was available in the lab back in those days, so what is the concern here?

Ed

[KD1SH]

   Good point. My Fluke, Tektronix, and HP counters all have maximum input voltages labeled right at the BNC, and built-in attenuators as well. I don't recall seeing many counters that wouldn't handle 5 volts.
   It's not counters that you need to be concerned with, but spectrum analyzers. With those, you always want to start at maximum attenuation and work your way down until you get the display you want.
   A very handy addition to a radio workbench is a step attenuator; I have a 20dB model mounted right to my workbench, near my spectrum analyzer, and make use of it whenever I'm uncertain. Better yet, do like Tom suggested, and look at the signal with an oscilloscope first. Hard to hurt a 'scope.

Maybe I?m missing something here?

Five volts rms, that seems like something a frequency counter should be able to deal with.  An old 4000 series CMOS digital circuit could easily run at 18 volts; I don?t remember worrying about measuring the clock frequency with whatever counter was available in the lab back in those days?so what is the concern here?

Ed

[WD8BIL]

Most spectrum analyzers can handle 30dBm on the input. At 50 ohms that's a bit more than 7 volts.

[KD1SH]

   True indeed, but HP considers 30dBm "damage level" input for my 8558B, so I always start at maximum attenuation to allow plenty of headroom. For my B&K 2620A, it's 10dBm; my Tektronix 2712, 20dBm, or 2.2vrms.
   Those sensitive linear amplifiers are pretty much the heart and soul of spectrum analyzers, and expensive to replace, so, yes, I'm a big chicken when it comes to applying signals to them. Overcautious, maybe, but better a live chicken than a dead eagle.

Most spectrum analyzers can handle 30dBm on the input. At 50 ohms that's a bit more than 7 volts.

[WD8BIL]

Oh, sure thing! Always protect the input as best you can. Plus, near 30 dBm circuit compressions and internal noises begin to come to life!