# How to test for Q of LC circuit unloaded

Discussion in 'Electronic Basics' started by Bill Bowden, Oct 9, 2007.

1. ### Bill BowdenGuest

I have a ferrite rod antenna that measures a Q of around 200 unloaded
using a Q meter. This was done by someone else and I don't have a Q
meter, so I tried to measure the Q using a generator and monitoring
the BW on a scope. The generator is coupled to the tuned circuit using
a single turn of wire in series with 50 ohms. This setup seems to
present a load to the circuit since I only measure an 8KC bandwidth at
600KHz which is a Q of less than 100.

Question is, how can I couple the generator to the LC circuit without
presenting any load and lowering the Q?

-Bill

2. ### John LarkinGuest

Just dangle the generator output, on a clip lead maybe, near the rod,
and crank the generator output to max.

Or make a bigger loop, a few inches in diameter, but keep it pretty
far away. "Loose coupling."

Use a 10:1 scope probe; 1:1 probes are pretty lossy.

John

3. ### mikeGuest

I have even put a 100k to 1meg resistor in series with the probe, helps to
isolate the 15pf of probe capacitance.
For my measurements I adjust coupling and scope gain to get 7 units
peak to peak at resonance. then adjust frequency until I have 5 units
peak to peak. I do this above and below resonance.
Note that .707 times 7(units) equals 4.949(units) Close enough to
5 units for my eyes. (The .707 point is the 3db point.)
Then upper 3db frequency minus lower 3db frequency equals difference
frequency.
So then resonant frequency / difference frequency=Q
Example: Resonant frequency = 1,000,000
Upper freq - lower freq --- 1,001,500 - 997,500 = 4000
1,000,000 / 4000 = 250 So Q= 250
I know you knew this but I added it for those who didn't.
Mike

4. ### john jardineGuest

Gives a shunt dynamic resistance about 250kohm at Q=200. For 5% error that's
a signal source + meter loading in the >5Mohm area which is difficult.
For a repeatable setup (like a Q meter), maybe series resonate the coil and
capacitor.
For Q=200 at 600kHz the series coil loss is say 7ohm. If you short the
generator output connector with 0.03ohms and drive the coil from that, then
it's just a question of measuring the developed voltage across the
capacitor, using a 10Mohm scope probe .
(Say) "100mV" on generator dial, would give 70uV to the coil and 40mVpp
across the cap at resonance, which gives a just about visible, one 'scope
division at 50mV per div.
(myself I'd drive from a function generator at say 10Vpp )

.------------. The Coil 10:1 10Mohm probe
| ___ | ___ .----------.
| ,-|___|--o---o---------o--UUU-o-o---o<-------o .-----. |
| | 50ohm | | | | | | |
| / \ | .-. | | | ~~~ | |
| ( ) | | | --- | '-----' |
| \ / | | |0.03ohm --- | 50mV-div |
| | | '-' | ,---o |
| | | | | | | |
| ===0V | ===0V ===0V ===0V| |
'------------' '----------'
600kHz Sig source 350pF tuning cap' Scope

(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

5. ### JamieGuest

The last time I did something like that I used my old (OLD),
HP R.F. voltage meter. I still have it btw
Any ways, What I did on a case similar to what you're doing is
use a 50 ohm R from the output of the generator to drive a
series LC circuit with one end to the R and the other to common.
Place the probe from your scope or an RF Vm like I did and
find the lowest peak reading when moving the generator frequency.

from there, do your calculations to determine the effective
R in the LC circuit to be used the Q calculations.
----
AT Semco (Capacitor Manuf), I serviced a few capacitor/induction
value and Q meters in the LAB and on the floor. These units were
old but very accurate to the standards.
The theory of operation with these units were to resonate the
tank with out the specimen first to get a center reading and adjust
another dial which was nothing but a variable POT to get the reading
at peak level.
Then you insert the specimen. You then would offset the dial to
give you the exact reading of the component and adjust the POT which
also had an analog dial to get the signal back up to Peak again.
THe offset value of the Pot via the scale would give you the Q of
that component.

6. ### Bill BowdenGuest

Ok, I used a few turns of wire from the generator spaced a few inches
from the loopstick and I get a Q of 133. The turns don't encircle the
stick, they just lay on the table. Scope probe is 10X. It's about
twice a good as before but still a long way from Q=200.

-Bill