/Built/222-28.doc 8 Rev. H 10/11/2005
the system. This signal passes through a 2 pole helical filter that is quite selective and
filters out the out of band signals that may have been amplified by the preamplifier. The
second stage is a MMIC with a 1dB compression point of≅+18dBm, which amplifies the
filtered signal. In this section, some assembly options may be installed. If you wish to use
a mast-mounted preamplifier, the GaAs-FET (Q4) should be by-passed. There is a
provision on the circuit board for the RX signal to be routed directly to the 2 pole helical
filter (F4) then the MMIC amplifier. Other line-ups may be used such as the GaAs-FET
being the only gain stage in the system before the mixer. With the standard line-up, the
signal then passes through a 3 pole, 5 dB insertion loss, helical filter,(F2) to further
eliminate out of band signals that would cause inter modulation products in the mixer. The
high level mixer (M1) has a +17dBm local oscillator input, which is supplied after passing
through a 2-pole helical band pass filter (F1). This mixer has a 1 dB compression point of
+14dBm and an IP3 (3rd order intercept point) of approximately +29dBm. The mixer is
then terminated into a diplexer band pass filter combination to reduce reflections back into
the mixer further reducing intermod (This is the circuitry between C70 and C76). The IF
signal then enters the IF amplifier stage (IC8) and a adjustable attenuator (R38). This IF
amplifier stage is optional and would only be used if your 28 MHz. receiver can tolerate
signals as high as +15dBm. If a cascade analysis were done with the standard line up, the
math would show that weakest point in the converter system would be the MMIC in the RF
stage (IC6). This means a -10 dBm (71mV) input signal would just start to compress the
converter resulting a system with a IP3 of ≅+25 dBm and a 1dB compression point of ≅
+15 after the IF gain stage. This is a large signal for anything but some of the newest
and/or best receivers on the market. This level for some of the latter day transceivers
could be as much as 35 dB into compression! The adjustable attenuator has about 25db
of range and could be used to attenuate the IF signal, but could still fall 10 dB short. If the
IF gain stage is not used, there would be maximum of ≅+2 dBm signal at the input of a 28
MHz. receiver, which is still a large signal!
On transmit, as little as 0dBm will produce a minimum of 10 watts out. If your
transceiver has a lower drive level than that, an optional gain stage may be installed on the
transmit side (IC4). The signal proceeds through an adjustable attenuator, (R35) then
through the same filter diplexer combination as the receive signal. This is done with a pin
diode switch which is biased in the transmit position only (circuitry between C81 and C77).
Although the mixer can handle up to +14dbm before compressing, that level is never
needed or approached. The transmit signal also shares the mixer (M1), 3 - pole helical
filter (F2), and pin diode switch with the receive side (circuitry between C35 and C67). It
then proceeds to a 2-stage amplifier consisting of MMICs (IC4 and IC8). The two stages
have approximately 22 dB gain. The signal is then filtered with a 2-pole helical band pass
filter (F3) with about 3 dB insertion loss. With an +0 dBm IF level entering the transverter
there should be ≅50 mW driving the Hybrid module (IC5). The output of the hybrid then
enters a low pass filter to eliminate the 2nd harmonic and above spurious. The signal then
enters the TR switch (K2) or exits the transverter using its own TX port.
All switching functions are controlled by a Signal to Ground or a +1.5-15VDC that is
provided by a transceiver on transmit. The transverter is in RX mode during standby.
Isolated auxiliary contacts are provided for switching external equipment such as mast
mounted pre-amps, power-amps, or T/R switches.
