High Frequency Electronics

Direct Carrier Derivation for a 16-QAM Data Transmission System

Usually the carrier signal for a 16-QAM (Quadrature Amplitude Modulation) system is derived, at the receiver, by using PLL circuits. This investigation here was to demonstrate that the carrier signal for a 16-QAM system could also be derived directly from the input signal.

For this the input signal was quadrupled to yield a 560 MHz component of the 140 Mbps data signal (I'll create a diagram later on how this works). Then a filter of very high quality (Q > 3000) is used to filter the 560 MHz component. The filter mainly consists of a cavity resonator that can be tuned and that can be adjusted regarding the input and output coupling factors. After the filter, amplifier stages follow and then a 4:1 frequency divider to regain the 140 MHz carrier.

I apologise for the quality of the scans, they are directly from prints taken with an Agfamatic Pocket 4000 camera (using the Pocketfilm 110).

This shows the frequency quadrupler followed by two tuning circuits and an amplifier stage.
The background shows the massive cavity resonator made of brass.

This shows the output stages consisting of two output stages with tuning circuits (for 560 MHz) and then an IC broadband amplifier to feed the 4:1 divider (two ECL flip-flops). Following that is another amplifier to deliver enough output power.

Showing the open case, on the left the input stages, in the centre the cavity resonator (approx. 100mm diameter), on the right the output stages. Some power supply circuitry is located in front of the 'pot' (in the German language the cavity resonator is called 'Topfkreis' as it looks a bit like a pot).

The closed box from the front. 16-QAM input on the right at 0dbm, 140MHz carrier signal on the left at +8dbm.
External power supplies required are +5.2V (for the ECL circuitry) and +15V for the RF circuitry.

Some Details of the Massive Cavity Resonator

The cavity resonator for 560 MHz was made from brass and was later silver plated inside to further improve the quality. It is approx. 120mm long and about 100mm in diameter and it weighs a ton :-). It achieved an unloaded Q-factor of around 3800 at 560 MHz.

Cavity Resonator's front. You can see the tunable capacitor.

Cavity Resonator's back. You can see the large bolt in the centre that holds the inner conductor. You can also see the two adjustable coupling loops with SMA connectors. These loops could be turned so they pick more or less of the field inside the resonator and hence achieve an adjustable coupling factor.

On the left one of the coupling loops. On the right the adjustable capacitor that sits at the end of the Lambda/4 circuit inside the resonator. The capacitor has a disc for coarse adjustment plus a smaller bolt in the centre for fine-tuning.