Low-band AM Tx: RF Deck

‘RF deck’ is a term that describes the RF power modules in a transmitter or linear amplifier. In this 200 watt 160 and 80m AM transmitter, the RF deck consists of two identical FET RF modules each with its own driver, an RF power combiner, the band switched Low Pass Filter module, a Transmit/Receive switching relay, and an SWR and RF power sampling module.

Schematic

160-80-transmitter_Page4-RFPowerDownload

RF power modules

RF power is delivered by a pair of identical H-bridge RF switching modules, capable of delivering up to 200 watts carrier power into a 50 ohm load. This H-Bridge is a modification of the widely used circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. In this configuration the H-Bridge simply switches alternate half-cycles of power through a transformer load, generating an approximation of an RF square wave at the drive frequency, in this case, 1.8 or 3.6MHz.

The two identical H-bridge RF modules are built side by side to allow both to share a common 6 inch heatsink (rear view).

Excitation is from a square wave clock (running directly on the signal frequency, no division necessary), from an si5351 PLL followed by a high speed logic gate which squares the waveform up to TTL level. The driver is an IXDD614 single-sided gate driver IC which drives the primaries of two pairs of gate transformers arranged in anti-phase. The H-bridge load is designed for around a 12 ohm impedance at the frequency of operation and the 1:4 impedance step-up output transformer matches the switch to a 50 ohm load.

W8JI RF power combiner

A W8JI Magic Tee power combiner takes the RF carriers from the two H-bridge RF modules and combines these using a transformer with a 25 ohm secondary impedance, which is followed by a 25:50 ohm impedance step-up transformer (1:1.4 turns ratio) to match to the Low Pass Filter input. W8JI’s pages describe the operation of a number of RF power combiners and splitters in detail. The large beads are FB-43-1020.

W8JI ‘Magic Tee’ two port RF power combiner.

Two series 50 ohm ceramic power resistors dissipate residual mismatched power.

The RF power combiner has some useful characteristics. Firstly, any power loss due to any impedance mismatch between the power source and either of the combiner’s 50 ohm input ports is dissipated in the 100 ohm 250 watt ceramic power resistors. Secondly, if either power source fails, the power combiner passes the remaining port’s power to its output, meaning the tranbsmitter can still be used, at half power — a useful reliability feature and one that I have seen occur whilst in a QSO. As long as the power balance resistors are adequarely heatsinked, there is in fact no rush to fix the faulty module.

Low Pass Filters

Low Pass Filters (and some large ex-surplus store relays) were designed based on the W3NQN filters, then tweaked slightly for the VK 160 and 80m band allocations. Thanks to Paul VK3KHZ from PK- Loops for this RF design work, kindly offered whilst I was sourcing the T106-2 toroids and high voltage glass mica capacitors.

1.8MHz LPF (top) and 3.5MHz LPF (bottom) with chunky band switching relays. The toroids are T106-2 and the capacitors are glass mica high voltage pieces.

LPF simulations and sweeps.

SWR bridge

A SWR bridge is used to monitor forward and reflected power and therefore SWR. The module drives a digital front panel SWR meter, as described in a separate post. It also includes a ‘high SWR interlock’ back to the enable line on the switching regulator in the HT power supply. This DC control line reduces the PWM duty cycle in the presence of high SWR, which in turn drops the HT with a corresponding drop in transmitter output power. This feedback mechanism is entirely analogue and can be adjusted to back power off, or inhibit it completely.

Transmit/ Receive switch

A final module hosts separate SO239 sockets for 160 and 80m antennas (an operating convenience), and a third socket for antenna pass through to the station receiver. T/R switching is done by the Arduino allowing software-controlled sequencing.

T/R switch board. The left-hand relay selects the 160 or 80m antenna socket, while the right-hand relay switches the selected antenna between receiver and transmitter.

What’s next?

These modules complete the AM transmitter. Check out the other posts and videos in this project. Thanks for reading and please leave a comment.