[Elecraft] I'm Stuck!- counter hints

[Martin AC6RM]

Thanks Stuart.  The frequency counter definitely worked out.  I had to
stretch my budget a little bit, but I've wanted to have one for quite some
time and at this point I'm sort of hell-bent on getting the K2 done and as
accurate as I possibly can.  I tell ya, I am just so excited about this
project.  The help from you, Elecraft and the rest of the list has just
been outstanding, so thank you.

73, Martin

ps: cc'ing the list because the data you give is great.

> Hi,
> 15 dBm is 15 db above a milliwatt reference, (0.001 watt).  But that is
> a pretty strong signal.  If you do a google.com search on "dBm" as
> keyword, I bet you can find all sorts of references, and conversion to
> volts, etc.  The classic dBm is referenced to 600 ohms, and I believe is
> about 0.7 plus volts.
> Since 20 dB is a factor of 10 increase in voltage, and that would be 7
> volts, 15 dBm is less than 7 volts rms.
>
> You may need a foot of wire on the hot end of the coax to pick up enough
> signal from an oscillator.  Direct connection thru the cap is less
> desirable for you might "pull" (change) the oscillator frequency by the
> loading of the counter and its cable effects.  Coax is more than 32 pf
> per foot, so use the shortest cable that will work.  I find a 1 meg
> probe very handy with counters, and since that is equivalent to a X1
> Scope probe, I just borrow a scope probe to hook up counters direct here
> in the lab.  The rule of thumb is actually that you need to have a
> series isolation impedance of X10 times the circuit impedance to avoid
> pulling and loading effects.  But higher gives more isolation at the
> cost of voltage attenuation, so it is a trade off sometimes.  A very low
> power oscillator might require you to hook to it directly to verify
> operation.  You then go to an indirect hook up, like the coupling wire,
> to read the frequency accurately, as you did with the 2m radio sig.
>
> You probably are dealing with less than 1 dBm sigs on the board, except
> for the final driver and finals of a transmitter.
>
> The type counter you got is a handy ham counter for its wide frequency
> coverage and ability to work off an antenna whip plug in.  It was a good
> buy.
>
> Some instruments which are needed for accuracy you want to spend more
> on, and some for general trouble shooting can be less accurate if they
> are portable and convenient like a pocket DVM.
> 72,
> Stuart K5KVH