# Which test lead for HP 53131a frequency counter?

I want to analyze the frequency (and drift) for simple LC resonant, hartley and colpitts oscillator, and VFO circuits under 30MHz in general, built from discrete components. I’ve just acquired a HP 53131a frequency counter but it has no test probe and I’m wondering which probe would be appropriate to measure these tuned circuits without disturbing them. My counter’s manual describes the following frequency range:

dc coupled -----> dc to 225 MHz
ac coupled -----> 1 MHz to 225 MHz (50$\Omega$) or 30 Hz to 225 MHz (1 M$\Omega$)

Would a 10:1 probe with 10 M$\Omega$ input resistance be best?

Even a 10:1 probe has a significant capacitance, probably in the 10 to 15pF range.

In short, no probe will do if you plonk it onto a sensitive part of your oscillator, and expect it to read the correct frequency.

Any probe will do if you connect it to a buffered output, or a very insensitive part of your oscillator.

My favourite probe is a 470ohm resistor in series with a small capacitor, connected directly to a piece of 50ohm coax that plugs into the 50 ohm input. This way you have an AC coupled, more or less resistive, 20dB attenuation down to your counter, the price is right too. It presents a load of more or less 520 ohms to the circuit under test, which applied to a 50 ohm source, barely loads it. It's also good for 'scope inputs and spectrum analysers.

• Could a buffered output mean placing the oscillator circuit in front of an emitter follower stage and taking the output from there? Commented Sep 8, 2016 at 21:31
• A buffer stage (like your emitter-follower) is often used to lightly load an oscillator so that frequency is disturbed minimally, while maintaining amplitude. Commented Sep 9, 2016 at 17:46
• @Neil_UK: Your probe might be used in this case in an optimal way: Decide which end of the oscillator's coil (L) is the "cold" end. Start probing the coil's turns close to this end, moving up toward the "hot" end until the counter give a stable, sane display. Probing the "hot" end affects frequency the most. Could you suggest value for a "small capacitor"? (perhaps 5-10pf)? Commented Sep 9, 2016 at 18:00
• @glen_geek: I've just come across this write-up for rolling one's own inductive pick-up...very interesting. Commented Sep 9, 2016 at 18:13
• @echobase: Beauty! (I have built a few as well). The problem with this probe is holding it steady near the oscillator inductor, if you're trying to see oscillator stability. As with any probe, try to couple in as lightly as possible that yields a stable counter display. Commented Sep 9, 2016 at 23:09

Generally if you can see it well on a scope, you can count it easily with a HP 53131a using either 1: or 10:1 ac coupled for any square wave and DC coupled for a pulse at mid-range on trigger.

This assumes you are aware of the probe capacitance and not to connect to the collector, but rather any buffered output.

Certainly using 30pF / ft coax with 470 Ohm in series and 50 Ohm or 1M terminate works well to.

For > 30MHz 50 Ohm drivers , coax and 50 termination is best, but probably not needed up to 100MHz in logic levels if the ground lead is removed and using only tip and barrel.

Remember Scope probes have noisy ground straps > 20MHz so use shortest ones possible to avoid ringing. ( improved by 50 termination along but degraded with attenuation )

side notes:

• If you want improved tempco on LC look for P250 caps otherwise use COG/NP0

If it's HP... you can count on it ....working well... ;)