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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 13th Jun 2019, 4:29 pm   #1
kkyahoo
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Default Microwave Modules 70cms Converter to 28 MHz

Does anyone know what the 404 MHz LO port measures into 50 ohms or open circuit like a high Z scope?

Thanks

Kevin
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Old 14th Jun 2019, 7:54 am   #2
Martin G7MRV
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Default Re: Microwave Modules 70cms Converter to 28 MHz

What is the mixer device used?

If it's a sealed DBM like an SBL-1 etc, then they typically specify 5-7dBm LO input level.

I suspect though it might be a discrete device?
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Old 14th Jun 2019, 7:59 am   #3
Martin G7MRV
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Default Re: Microwave Modules 70cms Converter to 28 MHz

There looks to be the circuit diagram available on the website of G4KFK
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Old 14th Jun 2019, 9:07 am   #4
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Default Re: Microwave Modules 70cms Converter to 28 MHz

Isn't that port an output? Useful for incorporating the converter into a transverter?
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Old 14th Jun 2019, 11:25 am   #5
kkyahoo
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Default Re: Microwave Modules 70cms Converter to 28 MHz

LO port is at 404 MHz, it is an additional output.

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Old 14th Jun 2019, 11:31 am   #6
kkyahoo
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Default Re: Microwave Modules 70cms Converter to 28 MHz

Mixer is a 3N204
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Old 14th Jun 2019, 4:30 pm   #7
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Default Re: Microwave Modules 70cms Converter to 28 MHz

Can't find the circuit of my particular unit, MM documentation is a bit random!

LO Output is -1 to-3 dBm depending on supply voltage, probably ok. But the overall conversion gain is only 15 dB and noise figure too high. Compared to another/later 70 cms MM converter, sensitivity is -120 dBm for 10db s/n against -126 dBm for the good one, which has a gain of 25db. This one has a better front end, a BFR34 transistor but it doesn't sound like the BFY90 front end of the faulty one is the source of the fault.

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Old 15th Jun 2019, 9:14 pm   #8
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Default Re: Microwave Modules 70cms Converter to 28 MHz

Maybe expecting too much for a BFY90? They beat a BF180 in a PYE PF1RX but they were not that good.

Conversion gain of 15dB is OK for anything except a deaf 28MHz IF.
If the IF is not that sensitive it will appear that the NF is high instead - even if it isn't in reality.

If this is the die-cast box RX only version, rather than a transverter, then I have one too.
It hasn't been on for years but maybe I could still find it.
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Old 16th Jun 2019, 6:40 am   #9
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Default Re: Microwave Modules 70cms Converter to 28 MHz

I recall the BFY90 has a NF of 5dB or so at 70cms, so not great. I would expect the sensitivity was better maybe -123dBm not -120. There were at least 2 versions of 70cms converter, the LO one and the one with 2 switchable crystals. Latter measures better at 2.5 dB NF, 25 dB conversion gain.

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