Wednesday, November 14, 2018

DBM testing - observations

I put some jacks on the DBM I made a couple of days ago. I used shielded cable to feed the audio amp.
This was the audio amp I used for the test.
With the signal generator connected to the LO port and a 6' lead clipped to the AC register for antenna I turned it on and it hurt my ear. My generator was still set for the test I ran on the DBM. Even with no lead on the RF signal port it sang. I turned the RF signal off and heard voices. The 75 meter ham net was taking roll call. A little down the band I found a round table rag chew. The were discussing the parts and suppliers for a final amp build one was working on. And so went the rest of the evening. I did make a few adjustments and observations. The DBM sings with an LO signal of  0.1 volt to about 0.25 volt. Mini Circuits recommends 0.7 volt drive for their DBM which is the one we copy. I guess the core material makes the difference. Their goes up to 1 Ghz. My core is rated to 50 Mhz.
Anyway it is looking good and working well. The interference from the second generator makes me wonder how many times some one built a perfectly good DBM or NE602 circuit and then it failed because of stray signal bouncing around the room. I put RCA jacks on the DBM and used shielded cables with good results. So as a test I used jumpers to connect a DBM using the same core, AF amp and generator. It would buzz but no reception. I turned on the second signal with a 1Khz offset and it sang. The conclusion I drew was the DBM requires shielding between the ports to prevent overloading. In other words the LO signal is connected to one port by wire and the other port by radiated signal.
The NE602 having a amplification factor would make this more difficult to overcome. So shield those cables and keep the lead short. A good ground plane would help too.
I guess I will try a NE602 test next.

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