The DEM 219-29 is a 219 MHz to 29 MHz transverter designed specifically to operate with the PacComm 56kb RF modem (GRAPES Modem). Separate mixers and filters are used in the receive and transmit sections of the transverter to provide additional isolation enabling the transverter to perform the fast switching functions required by the 56kb modem. The 219-29 output power is approximately 35 watts. On receive, a GaAsFET preamplifier, a Doubly balanced mixer, and two helical filters provide a sensitive, yet over-load proof front end with superior out of band rejection. The DEM 219-29 uses solid state switching for the DC and RF controls. IF connections are via matching BNC connectors and the RF connector may be either UHF or type "N". The control and power connections are RCA connectors. The 219-29 is housed in a 7.4" x 4.7" x 2.2" aluminum die cast box with an external 7" x 4.5" x 3/4" heat sink to provide cool operation under any condition. A complete kit version is supplied with all hardware, connectors, machined enclosure and heat sink.

Receive signals enter through a connector of choice (BNC, UHF, or type "N") and either pass through the solid state TR switch or by-passes it and enters the RX gain stage directly. The RX gain stage is a GaAs-FET designed for approximately 18 dB gain. The FET has a tuned input circuit and is biased to utilize its high 3rd order intercept point. Being that the FET is designed for microwave frequencies, it has an inherent low noise figure at the frequency used in this transverter but has it's noise figure established by the solid state PIN diode switch and results a nominal 1.5 dBNF for the system. This signal passes through a 2 pole helical band pass filter and is then amplified 10 dB by a second stage MMIC gain block. This signal then passes through another 2 pole helical band pass filter to further eliminate out of band signals that would cause inter modulation products in the mixer. The standard level mixer has a +7dBm local oscillator input which is supplied after passing through a 2 pole helical band pass filter. The mixer IF port is then terminated into a diplexer band pass filter combination to reduce reflections back into the mixer. The IF signal then exits the transverter via a BNC. What does all of this mean? It means that the transverter is more than sensitive enough and able to handle moderately strong out of band signals without submitting them to the RF modem. It is also designed for a minimum amount of gain so that the modem will not be subjected to any unnecessary overload.

On transmit, the standard output of the modem will produce a full 35 watts of output. The signal proceeds through a separate 28 MHz bandpass filter, mixer, 219 MHz helical band pass filter and gain stages from the receive section. This signal is then amplified by a Hybrid power module. The out put of the hybrid then enters a low pass filter to eliminate the 2nd harmonic and above spurious. This signal then enters the pin diode TR switch or exits the transverter using its own port.

All switching functions are solid state and are controlled by a grounded input (TTL-low ) on transmit which is provided by the modem. The transverter is normally in RX mode during stand-by. The transverter can not be damaged if it is off on the modem is used. The transverter also can not be damaged if any of the connections are crossed or not connected properly.