[Lowfer] Lowfer transmitter testing

[jrusgrove at comcast.net]

Pat

Been running the scopematch here going back to the beginning days of WD2XNS ... and would never 
consider firing up a high power transmitter without it. At a glance one can see the voltage and 
current waveforms and determine if the antenna is tuned to resonance (zero phase shift between the 
two sine waves) and the resistance match to 50 ohms (amplitudes of the sines are equal). This is 
especially handy when making improvements to the ground system as it is readily apparent whether 
the change made an improvement or not - current will go up indicating a lower antenna R. 
Additionally, if using a tapped ferrite transformer at the base of the loading coil one knows the 
antenna total R based on the turns ratio of the transformer.

As far as your Class D amplifier initial testing ... I typically run into a dummy load and use a 
powerstat on the dc supply to start at low power. Efficiency at low power will be almost as good as 
full power. First look at the gate waveforms with a dual trace scope to make sure there is 
sufficient amplitude (at least 10 vdc at the top of the waveform) and that there is zero overlap. In 
fact with this circuit there should be a small dead zone. Next move onto the drain waveforms with 
the dual trace scope. Again, make sure there is no overlap, the phases are equal in amplitude and 
there is no unexpected FET on/off transitions. Next look at the ringing on the leading and falling 
edges - mainly the leading edge. The voltage waveform on the drains should be twice Vdd and the 
ringing should add only another 10 - 20 volts. This ringing needs to be well controlled - if not, it 
can exceed the FET drain breakdown voltage. If not well controlled look at the transformer centertap 
bypass and snubber components. I've seen on one ocassion where the thick film resistor was open (for 
reasons unknown) and this resulted in a blown FET. An out of resonance antenna (possible when only 
using an SWR bridge) can cause very high ringing voltages due to the reactance reflected through the 
output transformer. This can be a FET killer.

Jay W1VD  WD2XNS  WE2XGR/2 WG2XRS/2




----- Original Message ----- 
From: "Steve" <VE7SL at shaw.ca>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Saturday, August 17, 2013 9:22 PM
Subject: Re: [Lowfer] Lowfer transmitter testing


> Pat - if you haven't yet built the 'scope-match' I would highly recommend that it goes to the top 
> of your project list. It is really one of the most valuable pieces of equipment in the shack and 
> you will never have to worry about impedance mismatch or being off-resonance ever again!
>
> Steve / 73
>
>
>
>> Not sure what happened. Was running cool at about 15 amps at 40 volts  and then one get blew up 
>> in a small ball of fire. May have been the antenna .
>
>
>
>
> WEB - "The VE7SL Radio Notebook":  http://members.shaw.ca/ve7sl
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