EMRFD Message Archive 2536

Message Date From Subject
2536 2008-12-30 09:29:51 rcbuckiii VFO output level
I built the VFO that Leon shows here:
http://www.geocities.com/leon_heller/vfo.html

I have a question about the output level. I do not have the 6-dBm pad
on the output side of the transformer. Instead, I just have a 51 ohm
resistor to ground. When I look at the output signal with a scope I
have a nice clean sinewave. With the scope still connected (1 meg
input) I connect the K3NHI power meter (modified W7ZOI) to the
output. The output level of the sinewave on the scope does not change
at all. And the power meter reads +2.0 dBm.

I thought it strange that the output level did not drop when I
connected the power meter. Since the power meter has a 51 ohm
resistor right on the input connector the effective impedance should
be 25 ohms. I would expect the VFO output level to drop. So I
disconnected the 51 ohm terminating resistor and read the output with
the power meter again. It still reads +2.0 dBm.

What am I missing here?

Ray
AB7HE
2539 2008-12-30 10:51:38 John Kolb Re: VFO output level
Well I'll step in since nobody that knows anything answered :)

My guess is that you have so few turns on the secondary of T1 that
the output impedance is only an ohm or two, so it doesn't care if
it is driving 50 ohms or 25 ohms. If you add more turns, you will
raise the output impedance, and also get a higher output level.

John
KK6IL


At 04:37 PM 12/29/2008, you wrote:

>I built the VFO that Leon shows here:
>http://www.geocities.com/leon_heller/vfo.html
>
>I have a question about the output level. I do not have the 6-dBm pad
>on the output side of the transformer. Instead, I just have a 51 ohm
>resistor to ground. When I look at the output signal with a scope I
>have a nice clean sinewave. With the scope still connected (1 meg
>input) I connect the K3NHI power meter (modified W7ZOI) to the
>output. The output level of the sinewave on the scope does not change
>at all. And the power meter reads +2.0 dBm.
>
>I thought it strange that the output level did not drop when I
>connected the power meter. Since the power meter has a 51 ohm
>resistor right on the input connector the effective impedance should
>be 25 ohms. I would expect the VFO output level to drop. So I
>disconnected the 51 ohm terminating resistor and read the output with
>the power meter again. It still reads +2.0 dBm.
>
>What am I missing here?
>
>Ray
>AB7HE



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2543 2008-12-30 12:13:49 Wes Hayward Re: VFO output level
Hi Ray,

Where do you have the scope attached to the circuit? And generally
you do NOT want to use a 1 Meg scope input to look at such things
unless it is terminated in 50 Ohms or so. The usual scope input is 1
Meg in parallel with 60 to 80 pF, so the input impedance at HF will
transform to something that is quite low. What you really want is to
use a 10X attenuator probe. That will then place a load
2544 2008-12-30 12:13:57 leon Heller Re: VFO output level
----- Original Message -----
2545 2008-12-30 12:14:02 joop_l Re: VFO output level
2554 2008-12-30 20:33:36 rcbuckiii Re: VFO output level