Hardware to convert from 10.7MHz to 2.5MHz
The local oscillatorThe local oscillator uses quartz crystals to provide very low noise sidebands. The X-tals are at the third overtone since the Q is higher compared to X-tals that operate at the fundamental. (More quartz, 3 times thicker resonators, give narrower resonances.)
Click here for details of the local oscillator
The mixerThe A simple CMOS mixer configuration with 74HC4053 is used. To get the dynamic range required to fit the 2.5MHz to audio converter it is necessary to connect four IC's in parallel so the mixer operates with 12 switches in parallel - there are three in each 74HC4053.
The impedance of the mixer is very low, some extra toroids take care of impedance matching. The mixer also includes an amplifier with 10dB gain followed by a 4dB attenuator for a total gain of 6dB with a well defined output impedance.
The RF amplifier and filterThe local oscillator is around 13MHz so the mirror frequency is around 15.7MHz. The buffer amplifier and filter attenuate the mirror frequency by something like 90dB. This may seem inadequate, but since this is an IF unit one has to add the selectivity of the preceeding unit(s).
Click here for details of the 10.7MHz amplifier and filter
The complete 10.7 to 2.5MHz converterThe schematic diagrams contained in the links above as well as the DC supply schematics are laid on a double sided PCB. The unit has two channels so there are two RF amplifier/filter units and two mixer units.
All the layout files for the EDWIN CAD system as well as all PCB production files can be down loaded here CAD files for 10.7 MHz unit There are also some photos of the assembled unit.
The 10.7 MHz to 2.5 MHz converter described here is a free design. All the information on this page is free and may be used by anyone for any purpose
The 10.7 MHz to 2.5 MHz converter was sold by Antennspecialisten under the name WSE RX10700
Testing and tuningThe RX10700 10.7 to 2.5MHz converter can be tested and tuned by means of simple standard instruments if the unit is connected to a RX2500 unit and a computer with a Delta44 board installed running Linrad. Look here for details: Testing and tuning the RX10700
Sensitivity and noise figureThe NF (noise figure) of the RX10700 itself is 7.4 dB, but when the unit is used together with a RX2500 and a modified Delta44, the system noise figure is 14 dB at the 10.7 MHz inputs when the Delta44 is run at minimum gain. Table 1 illustrates how the system noise figure (sensitivity) and inband dynamic range is affected by the A/D converter performance.
Table 1. The system performance depends on the A/D converter used. This table shows typical data for the dynamic range within the visible passband.
For the RX10700 unit, the 1dB compression point is at +18dBm.
Two signal dynamic rangeA weak and a strong signal are combined in a 3dB hybrid and the result is sent to channel 1 of the RX10700 + RX2500 + Delta44 (mod) signal processing chain. The weak signal is placed at 10.682 MHz and a notch filter at the same frequency is inserted between the strong signal and the hybrid.
The level of the strong signal is adjusted until the S/N of the weak signal is degraded by 3dB. Table 2 shows the result of this test with the Delta44 board in maximum and in minimum gain mode.
Table 2. Two signal dynamic range of a RX10700 + RX2500 + Delta44 combination.
An inspection of table 2 shows that the dynamic range is better by 8dB for offending signals within the visible passband if the Delta44 is operated in minimum gain mode. One can also see that the dynamic range is better by about 3dB for signals outside the visible passband if the Delta44 is operated in minimum gain mode. A better soundcard than the Delta44 may provide even better results than combining the best values of the two columns of table 2.
All frequency mixers produce spurious responses at very high signal levels. Because of the very high dynamic range of the RX10700 + RX2500 + Delta44 combination such spurs become more visible than in conventional receivers having about 30dB higher noise floor in the local oscillators. Table 3 shows the spurs observed in the frequency range 9.0 to 12.5MHz when tuned to 10.682MHz. The Delta44 is run at minimum gain.
Table 3. Mixer spurs of the RX10700 unit. 10.682 MHz is the desired signal.
The noise figure is 14dB so the noise floor in 500Hz bandwidth is at -133dBm. From table 3 it is clear that the worst spur originating from a signal at 11.941 MHz will be close to the noise floor when the offending signal is 130dB above the noise floor.
Three signal dynamic rangeTable 4 shows the level of the third order intermodulation signal at different signal levels. Two signals of equal power at 10.582 and 10.482 MHz are sent into the RX10700. The received signal or intermodulation product is at 10.682 MHz.
Table 4.Typical third order intermodulation data for the RX10700 + RX2500 + Delta44(mod) at 100kHz separation between the two equally strong signals at the input power level.
A third order intercept point of 36dBm for a receiver with a noise figure of 7.4dB may seem overkill, but when used in a wideband system as described above, the system noise figure becomes 14 dB. For a no compromise 144 EME receiver one has to add about 35dB gain which will reduce the third order intercept point to about 0dBm for signals well outside the visible passband.