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Old 01-19-2011, 11:26 AM
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Tonight I am looking at showing you all how to convert a cobra 29 into a stable signal generator.
I will put what I can together and try to get this layed out. The generator will need to be 50 ohms, and produce a clean 100uv signal. This will be unmodulated. You will also have the option to drop the 100uv at 10 increment levels. this will be done with a simple divide by 10 network of resistors.

What we need.
A clean stable 100 microvolt signal. ( 100 uv is enough to set a S-meter to S9)
A way to drop the signal for testing the sensitivity and alignment of the receiver. (peak the stages)

The idea of the signal generator is that when you have a radio on nearby, and you turn the generator on with out being connected to the radio you want to test, you should not hear anything coming from the generator. This is going to take a bit of work and a lot of shielding to stop this from happening. The one I built years ago from the RCA co-pilot was mounted inside a steel chassis. The tuned circuits you build will also have to have there on fabricated boxes. These can be made of very thin tin sheets. As long as you can bed it and solder to it. No aluminum. You can also fabricate the boxes from double sided circuit board. This is easy to work with and has good sheilding.

So how do we start?
Looking at the cobra 29 schematic find the TX mixer. This is IC 3. Pin 9 of the mixer chip is your rf out. This goes to L20, the first tuned circuit in the output stage. From L20 we go to R63 then to C65. This is as far as we need in the circuit. Everything from the antenna side of C65 towards the antenna is useless. This is the section we will pick our signal off at and convert it to a usable signal.

At this point in the radio we are going to have around a 1000 ohms impeadance @ about 1 milivolt. As you can see we have to go a long way to get this to 50 ohms @ 100 microvolts (uv). The idea is to get this signal through a cable and to the radio under test and not through residual radiation from a poorly built leaky generator. So what we will do is take our 1 milivolt 1000 ohm signal and force it to 50 ohms @ 500 uv via a 1/2 divider network. To do this we need 3 47 ohm 1/4 watt resistors, 2 100 ohm 1/4 watt resitors.

If anyone see's error in what I am saying please jump in and let me know..
More later
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