Thursday, October 28, 2010

2304 MHz Transverter - A Little Bit About the TX Mixer

While I'm waiting for the parts to come to repair the ABPM, I thought I'd write a little something about the mixer that I'll be using with this rig.  The mixer I'm going to use is an old Vari-L DBM-184 flatpack double balanced mixer.  The specs on this device, although it is a rather old device shows some promise according to the datasheet.  It will accept up to a +20 dBm LO, the LO to RF isolation is about 20 dB, and is good to 2500 MHz on the RF port.  The conversion loss is about 8 dB, which is typical for a DBM.  Its 3 dB compression point is +6 dBm, which I would assume is the output power, which corresponds to an IF drive level of +14 dBm when  used with a +20 dBm LO.  I will not be driving this device that hard though, figuring on driving the device with about 0 dBm, and between +10 - +13 dBm on the LO port.

I picked up 2 of these mixers at the 2010 Packrat (Mt. Airy VHF Club) VHF Conference.

I am in the process of packaging the mixer in a brass box, just slightly larger than the mixer itself (3/4 inch square), with SMA connectors connected to all 3 ports.  This will make the unit a "module" which is how I like to build things, especially when working with the microwave bands.  This method of construction allows one to substitute different components, so one can experiment to find the best performing module for this particular combination of parts.

Basically, the planned signal path will be something like this:
From the LO tap in the CalAmp RX converter, LO will be amplified using a MSA-0386 in a homebrew amplifier run at saturation, feeding the DBM-184 mixer.  The IF will be an attenuated 144 MHz TX signal from the FT-817nd, feeding 0 dBm into the IF port of the mixer.  The RF port will feed a 3 resonator BPF designed by G0ORY, details of which appear here.  From the filter, the RF will be amplified by the already constructed ERA-2 mmic amplifier.  From that point, I'm unsure as to what the final power output will be, but I am hoping I will find an amplifier that will provide a couple of watts on the band.  What I use will depend on what the pocketbook will allow, as well as what power supply I have available.

Once again, stay tuned, as work progresses, I will post again.

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