IN3LBQ on the Hühnerspiel (Amthorspitze) - photo by IN3OTD

Hermes-Lite SDR - frontend v1.3 measurements

Measurements on the frontend v1.3 designed by James Ahlstrom; details on the design are in this discussion thread.

Hermes-Lite with Frontend v1.3 output

The following graphs show the frontend TX output measured when driven by the Hermes-Lite v1.22 board. The firmware FPGA FW version used was the 20160220_txdac/CVA9/, half-duplex. The system clock frequency fs was 73.728 MHz

Output vs. TX frequency

Hermes-Lite Hermes-Lite

Output vs. TX drive level

Changing the TX drive level, which changes the TxDAC gain, see the AD9866 datasheet for details.

TX frequency at 24.895 MHz

Output power at fundamental:

Hermes-Lite

when changing the TxDAC output power (by changing the TxPGA code) the spurs remains adequately suppressed:

Hermes-Lite

Output harmonics and spurs vs. frequency

Graphs showing how some of the TX output spurs and images amplitude changes when the fundamental output amplitude is changed.

TX frequency at 24.985 MHz

The harmonics shown below are generated by the frontend: the ones from the AD9866 TxDAC are filtered out by the frontend filter (above 30 MHz).

Hermes-Lite

The sampling images and spurs are also mostly filtered out by the on-board filter, except of course when they fall in the filter passband. There is also some feedthrough in the filter stopband, probably due to a limited ultimate rejection.

Hermes-Lite Hermes-Lite Hermes-Lite

Standalone tests

The frontend board has also been characterized standalone; input was feed thru a Mini-Circuits ADT1-6T transformer to obtain a differential signal.
Since the frontend is designed to work with the TxDAC output, which is a current source, it does not provide a good match to 50 Ω but this should not change much its distortion.
The frontend was powered using a 12 V regulated supply.

The fundamental and harmonics output levels versus input power, with a 10 MHz sinewave input, are shown in the graph below:

the output power at 1 dB of compression is about 66 mW (18.2 dBm).

RF output and DC current drawn from the supply versus input power (10 MHz sinewave) are shown in the following graph:

and the resulting efficiency is shown in the graphs below (data are the same for the two graphs, in the first the input and output power are in dBm and in the second they are in watt):

The third-order intermodulation products amplitude w.r.t the output power per-tone is shown in the graph below (to obtain the actual [average] output power multiply the value by two, to have the PEP, multiply by 4):

IMD levels vs. output power IMD levels vs. output power

at an IM3 of -30 dBc the output power is around 17 mW per tone.

The graph below shows the relative amplitude of the IMD products (separately for upper and lower products) for a two-tone test with varying tone spacing at 70 mW PEP output:

IMD vs. tone spacing

there is an increase in the intermodulation levels around 100 kHz, probably due to some resonance in the power supply or input filter circuitry.

By plotting the third-order intermodulation products power vs. the input power we can find the intercept point by extrapolating the results. At low input levels the amplifier third-order intermodulation products follow the theoretical 3:1 slope w.r.t. the input power up to about -5 dBm input, as can be seen from the graph below:

Computed IP3

The input IP3 turns out to be 21.6 dBm and the corresponding output IP3 is 30.7 dBm.