[Fwd: Re: SSB Output Level & Scope Recommendations]

[Don Wilhelm-3]

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.>>
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Jack Smith-6]

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.>>
>>
> _______________________________________________
> Elecraft mailing list
> Post to: [hidden email]
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
> http://mailman.qth.net/mailman/listinfo/elecraft   
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Jack Smith-6]

I said "the probe tip impedance is only a few hundred ohms, not the 10
MHz it is at DC or low frequencies."

Of course, that should be "... the 10 Mohms it is ..."

Jack K8ZOA
www.cliftonlaboratories.com
_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Jack Smith-6]

I've mentioned before that crystal filters are not perfectly linear
passive devices and can in fact generate intermodulation products. The
reason is that a crystal's motional parameters are not constants, but
rather are a function of the applied drive, something called "drive
level dependency" in the trade.

I've just completed a quick IMD measurements for a 4-pole 21.4 MHz
monolithic crystal filter (ECS 21K7.5) and posted data at
http://www.cliftonlaboratories.com/Updates.htm under the 03 July
updates. This is a long update, in several stages, so read it all,
should you find the first part interesting. The ECS 21K7.5 is a $10 part
in single lots, so it's certainly not an expensive filter.

Monolithic crystal filters are known for IMD, and the test was run with
a rather high drive level, around +9 dBm. However, the data is quite
instructive and should convince anyone who doubted that crystal filters
could themselves be a source of IMD. (These measurements are quick and
dirty and I will refine them as I move towards a final filter design. )

Bringing the discussion back to the K3, it shows the importance of
getting the roofing filter correct, as intermodulation generated in it
will come in "down the pipe" in the DSP stage.

And, it's not just crystals that are non-linear passive devices.
Capacitors and inductors also have the same problem. In the case of a
capacitor, some dielectrics have a dielectric constant that is a
function of applied voltage. Ferrite and powdered iron (but to lesser
degree than ferrites) are also non-linear. Hence any component in the RF
path must be suspect and carefully evaluated for IMD performance. (I've
written about non-linear capacitance. See
http://www.cliftonlaboratories.com/capacitor_voltage_change.htm, but
have yet to make serious measurements on non-linear inductors.)



Jack K8ZOA
www.cliftonlaboratories.com
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com

[Jack Smith-6]

I've re-run the crystal filter intermodulaton tests this morning, with 2
KHz generator spacing. This allows the IMD products to fall within the
filter passband, and gives a more useful picture of the total filter
performance.

Details and plots are at http://www.cliftonlaboratories.com/Updates.htm.
Look for the 04 July 2007 entry.

The revised data shows IP3 of +23 dBm over an input signal range of +8
dBm to -12 dBm. The manufacturer's recommended maximum input level is
-10 dBm.

Jack K8ZOA
www.cliftonlaboratories.com
>

_______________________________________________
Elecraft mailing list
Post to: [hidden email]
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
 http://mailman.qth.net/mailman/listinfo/elecraft   

Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com