linrad support: High performance hardware for linrad: RXHFA, the lower HF bands to 70 MHz converter.
(March 16 2004)

Hardware to convert from lower HF bands to 70 MHz

The local oscillator

The local oscillator uses quartz crystals to provide very low noise sidebands. Click here for details of the local oscillator

The mixer

The A balanced mixer with J310 junction-FET transistors running as switches is used. To get the dynamic range required to fit the 70 MHz input of the RX70 unit it is necessary to use several transistors in parallel.

The RF amplifier, filter and attenuators.

The RF amplifier and attenuator section can provide gain or attenuation in 5 dB steps from -15 dB to + 10 dB. The filters add about 3 dB losses, their purpose is to improve the second order intermodulation characteristics. Click here for design information and the complete schematic diagram for the RXHFA front end. The link contains information about intermodulation in filters and various types of RF switches. The input switches use J310 junction FET transistors. The level of second order intermodulation they generate is very sensitive to the layout. To minimise the second order intermodulation in the input switches the gate resistors connecting to the J310 transistors are mounted in the air and not on the PCB. The reduced stray capacitance improves IP2 by 20 dB. This link second order intermodulation in the RXHFA unit gives details.

The complete HF to 70 converter

The 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 RXHFA unit There is also a photo of the assembled unit.

RXHFA, the HF to 70 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

Testing and tuning

The RXHFA, the HF to 70 MHz converter can be tested and tuned by means of simple standard instruments if the unit is connected to a RX70 unit, in turn connected to a RX10700, a RX2500 a computer with a Delta44 board installed running Linrad. Look here for details: Testing and tuning the RXHFA

Sensitivity and noise figure

The NF (noise figure) of the RXHFA itself is 6 dB, but when the unit is used together with a RX70, a RX10700, a RX2500 and a modified Delta44, the system noise figure is 11 dB at the HFA inputs when the Delta44 is run at minimum gain and the RXHFA at maximum gain (+10dB).

Saturation and third order intercept point

For the RXHFA unit, the 1dB input compression point is at about +11 dBm when the gain is set to +10 dB and +21dBm when the gain is set to 0dB or below. The third order input intercept point, IP3 depends on the gain setting. Typical values are shown below.

Gain      IP3
(dB)     (dBm)
-15       +41
-10       +46
-5        +45
 0        +37
+5        +32
+10       +27

When the RXHFA is used together with the other WSE units, the input IP3 of the entire system is typically a little lower because third order intermodulation of similar magnitude is generated in the other units. The intermodulation added by the rest of the WSE converter chain seems to vary in an unpredictable way and the effect on IP3 seems to be in the range 0 to 5 dB.