[Amps] Vacuum Relay

[Paul Christensen]

Pete,

On a single or multi channel scope, connect an RF sample from a loop, sense 
wire, or capacitive bridge to the vertical input (or Channel A on a 
multi-channel scope).  However, don't be tempted to connect the transmitter 
or amplifier through a coaxial "T" section direct to the scope input.  At 
the 100W level (70 Vrms) , most any scope can likely handle it fine into a 
50-ohm resistive system.  But, start adding power and VSWR from a 
non-resistive line and you can quickly exceed the scope's maximum input 
level.

I use two types of sampling.  The first is a Bird variable coupler.  A lot 
of money that's really not needed for this purpose.  The second type of 
sample comes from a W8ZR SationPro II switching device.  W8ZR designed the 
RF switching unit with capacitive pre and post amp samples.  Very 
convenient.  But, to get you started, you can use a test lead looped around 
the transmission line.  Increase turns to get more level into the scope as 
needed as you only need enough coupling to give a clear scope image. 
Absolute value in vertical amplitude is of no concern when using the scope 
for relative wavform measurements.

The paddle/key input contacts on most transceivers normally rest at a high 
logic level (say +3.3V or +5V).  The input is normally pulled to circuit 
ground with keying action from the paddle, bug, or straight key.  Next, you 
can parallel the transceiver's key jack (either dit or dah side) and connect 
it to the External Trigger port on the scope.  The scope's trigger function 
can be set to pulse the sweep on a negative-going change in key line logic 
(e.g., +3.3V to 0V).  Set the scope time base to 5 ms/div.  You now have the 
ability to view the transmitted waveform and in the case of CW, you can 
precisely measure the time it takes from paddle contact to RF output.  The 
result looks exactly like the keyed waveform figures shown in the QST 
Product Reviews.

There are other ways so trigger a scope.  I often trigger from Channel B, 
omitting the External Trigger connection.  Most newer scopes have a menu 
(push button or software) to trigger from several sources, including any of 
the scope input channels.

Paul, W9AC

----- Original Message ----- 
From: "Pete Smith N4ZR" <n4zr at contesting.com>
To: <amps at contesting.com>
Sent: Saturday, December 15, 2012 7:03 AM
Subject: Re: [Amps] Vacuum Relay


>I could sure use asimple-minded explanation of how to connect my dual-trace 
>scope in order to get a display of system timing.  The description below, 
>while I'm sure it is correct, doesn't get this history major far enough.
>
> 73, Pete N4ZR
> Check out the Reverse Beacon Network at
> http://reversebeacon.net,
> blog at reversebeacon.blogspot.com.
> For spots, please go to your favorite
> ARC V6 or VE7CC DX cluster node.
>
> On 12/13/2012 2:15 PM, Paul Christensen wrote:
>>> A scope isn't going to tell all but it
>>> tells a lot about a signal.  They're not difficult to use
>>> and don't have to cost a lot.
>>
>> It doesn't take long to find some incredibly good buys on used Tektronix 
>> scopes -- and at a price about half of the popular high-end wattmeters. 
>> Folks have no problem paying $600 for a wattmeter with unnecessary high 
>> accuracy, but seem to find no need for a scope.  Is it the number of 
>> controls that's so intimidating?
>>
>> The dual trace function isn't required to determine if hot-switching 
>> occurs, but is required to measure system timing. For station monitoring 
>> here, the scope's Channel A is set to trigger on the keyer's output while 
>> Channel B samples RF.  This setup makes it easy to replicate the same CW 
>> waveform measurement as shown in the QST Product Reviews.  By triggering 
>> the scope from the keyer, the waveform stays in place for timing 
>> measurements without having to chase it with the scope's variable 
>> time-base control.
>>
>> Paul, W9AC
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