Posted by
Jack Smith-6 on
Jul 03, 2007; 6:52pm
URL: http://elecraft.85.s1.nabble.com/Fwd-Re-SSB-Output-Level-Scope-Recommendations-tp449802p449805.html
Don: Following is not intended for you, but rather for someone without
much experience running an oscilloscope.
I would add one cautionary note. A 10X scope probe usually is rated at
250 or 300 volts, so you might think it's safe to use bridged across a
dummy load with a 100 watt transmitter, which only has 70V RMS output
under those conditions.
That's far from the case. Scope probes have a voltage versus frequency
de-rating. By the time you get to 10 meters, for example, a typical
probe (Tektronix P3010, 10X 100 MHz) is only rated at 30V safe working
voltage, corresponding to about 18 watts power with a 50 ohm load. The
same probe is rated at 300 V up to 4 MHz. It *might* be OK to connect
your probe to a 100 watt 28 MHz signal, but I've never cared to make the
experiment with my equipment.
(I generally use a TDS 430 400 MHz digital 'scope with P6138A probes.
They are rated at 50V at 30 MHz.)
To measure power with an oscilloscope, I use a modified procedure.
Transmit into a power attenuator and connect the scope to the
attenuator output, with a 50 ohm termination at the scope input. I
generally use a 100 watt Bird 30 dB attenuator, backed up with a 20 dB
Minicircuits Type N attenuator and switch my scope input impedance from
1 Mohm to 50 ohm via a front panel control. This puts about 220 mV RMS
on the scope input for 100 watts into the first attenuator and thus has
minimal risk of equipment damage. For lower power cases, the second
attenuator may be reduced to 10 dB or even eliminated. You should, of
course, measure the true attenuation of the stack up if you are looking
for maximum accuracy.
Also remember that a typical 10X probe has 10 - 12 pF input capacitance
and at 30 MHz, the probe tip impedance is only a few hundred ohms, not
the 10 MHz it is at DC or low frequencies. You can get very misleading
results if you assume a 10X probe is always a high impedance device. If
you really need high impedance, an FET active probe is the way to go,
but they generally have a rather limited voltage input range, often less
than 10V.
I highly recommend Tektronix's "ABC of Probes" as a tutorial. You can
find it at www.tek.com, search for the title. You may have to provide
some registration information to obtain the download.
Jack K8ZOA
Don Wilhelm wrote:
>
> The best recommendation that I can give for a 'scope is to contact Bob
> Garcia at
[hidden email]
>
> Bob is a Tektronics specialist who often appears at SouthEastern
> hamfests (he is located in GA) and is well known as "Mr. 'Scope". He is
> an extremely fair person to deal with and has good 'scopes at a low
> price.
>
> You will need a couple 10X probes too - I have found that the BUY IT NOW
> new $14.99 probes offered on EBAY work quite well for a good price.
>
> To observe an RF waveform with a 'scope and 10X probe, connect the
> transceiver to a dummy load and connect the 'scope probe across the
> dummy load - if you cannot get to the connections to the dummy load, a
> coax TEE adapter will allow you to gain access to the coax center
> conductor. You can use the same technique with an antenna connected, or
> use an Elecraft CP1 in the coax line and observe the -20 dB output from
> the forward port of the CP1 (terminate both ports at the CP1).
>
> 73,
> Don W3FPR
>
>
[hidden email] wrote:
>>
>> I'm concerned that my SSB output is not matching the set power level.
>> Reviewing the archive on a search by ALC, I found this note. It's
>> been years since I've looked to see what scopes are available. Any
>> recommendations - remembering that cost is a factor.....maybe a
>> Scope that uses the PC.
>>
>> Here's another part to the question....I've never used a scope to
>> look at RF output. I assume that takes some special circuitry.
>>
>> ***from W3FPR note 06/03/07***
>> <<The best instrument I know of to check the peak power output is a
>> 'scope. It can indicate the instantaneous RF voltage easily and it is
>> easy to compare that level to a CW reference level. 'Peak indicating'
>> wattmeters do have a time constant to indicate the peak and can be
>> fooled by the audio waveform. In other words, they are not always
>> correct when indicating the peak output of a SSB signal.>>
>>
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