Tech Slot
Reported by G4DDK
The DX'er 50 Part 2
In the second part of the article on the DX'er 50
I shall be describing some changes made to the receive converter since last month, as well
as describing the transmit converter. First the changes.
I found, when testing the receive converter, that
the level of 22MHz local oscillator feedthrough to the IF output was too great for the
noise figure meter (and therefore probably the IF receiver), so I decided to dispense with
the diplexer altogether and follow the MSA1104 IF amplifier with a 28 to 30MHz bandpass
filter. Unfortunately the 28MHz band pass filter described in reference [1] proved to have
too much insertion loss, so I designed an alternative one with much lower loss and a
bandwidth of 2MHz at the 1dB points of 28 and 30MHz. Using AppCad, the design showed an
attenuation of 25dB at 22MHz, whereas I measured 26dB on the network analyser. This filter
has proved adequate with the IC735, but a better one would be more desirable if the 22MHz
feedthrough proves troublesome, as it might with some other receivers
The figure below shows the schematic of the bandpass filter. This should be connected between IC3 and the IF output connector. C13 is not required, since the filter now provides DC blocking.
Now to the transmit converter. You will recall that I used relays to provide the switching around the mixer and filter. My experience with switching diodes is not too favourable!
Using the Tektronix spectrum analyser to measure
the level of transmit signal for various IF input levels at 28MHz, I found that I could
get -14dBm out at 50MHz, after the filter, with 0dBm input at 28MHz. Allowing 5dB for the
band pass filter, this gives a conversion loss of 9dB in the mixer. This is a little
higher than expected, but may be due to the reactive termination provided by the filter.
Adding a matching attenuator between the mixer and filter would have led to even higher
loss, so I decided to accept the loss and provide some extra gain to make-up the loss. It
is interesting to note that the equivalent level in the Suffolk was about -21dBm, when the
input level was adjusted for maximum acceptable 5th order output. The TAK1H is therefore
providing some improvement in level over the SRA1 used previously and is in-line with the
7dB difference in local oscillator drive level!
The next problem was the IC735 driver. Maximum IF
output from my transceiver was just -8dBm. Attempts to get more resulted in a considerable
degradation in spectrum purity. I overcame this problem by using an MSA0204 amplifier at
the transmit IF input to increase the drive to provide a clean, uncompressed +4dBm. With a
470R potentiometer in front of it, the level could then be adjusted down to give the
required level of 0dBm. It also provides some extra gain to allow for some insertion loss
when I eventually fit a current controlled ALC attenuator.
Following the relay contacts on the filter side of
the mixer I have provided two further Modamp gain stages. These provide about 20dB of
gain. The first uses an MSA 0304 and this feeds a MSA1104. At the MSA1104 output the level
is about +6dBm. A second bandpass filter, with just two stages provides further
attenuation of the unwanted mixing products. The insertion loss of this filter is 2.5dB,
bringing the output level to approximately +3dBm. Another MSA1104 then amplifies the
signal to approximately 30mW (+15dBm), although by increasing the transmit IF drive, an
output of approximately 80mW can be obtained. At this level the output MSA1104 is in
compression and the required drive is beyond the "green" environmental limit!
A level of 30mW is sufficient to drive the
following Mitsubishi module to over 10W output, so no further amplifier stage should be
required between the MSA1104 and the power amplifier module.
Next month I will describe the Mitsubishi module
and its bias supply, transmit/receive switching and the transmittter output low pass
filter.
73 de Sam